FUNDAMENTAL PRINCIPLES
FOR THE ILLUMINATION
OF A PICTURE GALLERY

TOGETHER WITH THEIR APPLICATION TO
THE ILLUMINATION OF THE MUNICIPAL
MUSEUM AT THE HAGUE

DOOR

J. G. EYMERS

'S-GRAVENHAGE

MARTINUS NIJHOFF
1935

8IBU0TH£EK DER
RIJKSUNIVERSITEIT
UTRECHT.

►'îî

FUNDAMENTAL PRINCIPLES
FOR THE ILLUMINATION OF A PICTURE GALLERY

FUNDAMENTAL PRINCIPLES
FOR THE ILLUMINATION
OF A PICTURE GALLERY

TOGETHER WITH THEIR APPLICATION TO
THE ILLUMINATION OF THE MUNICIPAL
MUSEUM AT THE HAGUE

PROEFSCHRIFT TER VERKRIJGING VAN DEN
GRAAD VAN DOCTOR IN DE WIS- EN NATUUR-
KUNDE AAN DE RIJKS UNIVERSITEIT TE
UTRECHT OP GEZAG VAN DEN RECTOR MAG-
NIFICUS DR. C. W. VOLLGRAFF, HOOGLEE-
RAAR IN DE FACULTEIT DER LETTEREN
EN WIJSBEGEERTE, VOLGENS BESLUIT VAN
DEN SENAAT DER UNIVERSITEIT TEGEN DE
BEDENKINGEN VAN DE FACULTEIT DER WIS-
EN NATUURKUNDE TE VERDEDIGEN OP
MAANDAG 2 DECEMBER 1935, DES NAMIDDAGS
TE 4 UUR

DOOR

JOHANNA GEERTRUIDA EYMERS

GEBOREN TE VELP (GLD.)

PRINTED IN THE NETHERLANDS

AAN MIJN OUDERS

ra^ 17

De voltooiing van dit proefschrift biedt mij een welkome gelegen-
heid mijn oprechte dank te betuigen aan allen die tot mijn weten-
schappelijke vorming hebben bijgedragen.

Hooggeleerde Ornstein, Hooggeachte Promotor, ik acht het mij
een voorrecht dat ik zovele jaren in verschillende functie's onder Uwe
bekwame en bezielende leiding werkzaam heb mogen zijn. Dat ik in
die tijd aan zo sterk uiteenlopende problemen heb mogen werken is
voor mij van veel belang geweest. Ik verheug mij er op dat ik nog
enkele jaren onder Uw leiding mag werken aan een toepassing der
natuurkunde, welke voor mij grote aantrekkingskracht bezit.

Hooggeleerde Moll, Zeergeleerde Burger, ik zal steeds een
prettige herinnering bewaren aan de tijd dat ik als hoofdassistente
op uw practicum heb gewerkt.

Beste Vermeulen, ik wil je bij deze gelegenheid gaarne zeggen dat
ik dankbaar ben voor onze jarenlange prettige samenwerking. Dat ik
bij onze werkverdeling als onderwerp voor mijn proefschrift datgene
van onze gezamenlijke problemen mocht kiezen, wat mij het naaste
stond, daarvoor ben ik je ten zeerste erkentelijk, te meer daar het mij
bekend is, hoezeer dit onderwerp ook jouw belangsteUing heeft.

Van der Held, ook jij van harte bedankt voor de bereidwilligheid
waarmee je mij toeliet een gezamenlijk probleem tot een proefschrift
uit te werken.

Het was mij een grote eer bij dit werk in contact te komen met den
onvergetelijken kunstenaar Berlage.

CONTENTS

Page

Introduction.................... xi

Chapter I. General conditions necessary for the illumination

of a picture-room................ 1

Chapter II. The investigation of the colour and the intensity
of daylight and the optical properties of various kinds

of glass..................... 8

Chapter III. A further sepcification of the conditions neces-
sary for the illumination of a picture-room..... 17

Chapter IV. The various types of constructed rooms .... 24

Chapter V. Artificial illumination............ 35

Appendix...................... 47

§ 1. Investigation of the spectral constitution of day-
light .................... 47

§ 2. Investigationof the diffusing properties of specimen

of glass................... 57

§ 3. Inquiring into the reflective power of the surfaces

of pictures................. 65

m'

im^

w VI

m

INTRODUCTION

In November 1932 Prof. 0 r n s t e i n was requested to assist in
designing a system of artificial lighting for the New Municipal Mu-
seum at the Hague, then already in course of being built. He entered
upon this request and began to study the problems involved, in
colaboration with Dr. E. F. M. v. d. H e 1 d, Dr. D. V e r m e u-
1 e n and the present writer. From the beginning it was clear, how-
ever that one cannot inquire into the matter in question, without
including in one's considerations the very closely related problem
of the lighting in daytime. The commission from the Hague town-
council led, therefore, to directions, concerning the lighting, both in
daytime and at night. In the following pages the various investi-
gations carried out on this subject are expounded. In doing so I
tried, on the one hand to treat the problem in as wide as possible,
while on the other hand the particular solutions applied to the
Hague Museum are also given. These solutions are not in every case
the most advantageous ones, from a lightingengineering point of
view. The latter gives definite directions as to the preferable di-
mensions of the rooms. Since, however, the plans of the Museum
were already decided upon, and the actual building was already
begun, it was too late to alter the dimensions of the rooms, though,
for a number of them even, so slight a change as ^ M. would have
meant a considerable improvement. The lighting engineering
directions given, extend also to the sculpture room, the glass cases
and a few items of miner importance. In the following, however, I
have confined myself to the chief problem: the illumination of the
picture rooms.

To begin with, the question was asked: what is the most advisable
illumination of a picture ? One might feel inclined to answer: that
illumination, prevailing while the picture was being painted. In
many cases, however, this illumination cannot be realised in a
museum. In all those cases, for example, in which pictures are painted

from nature, in the open air, it is impossible and not desirable to repro-
duce the conditions as regards the intensity and the direction of
incidence. It was, therefore, necessary to find another critérium. To
this we were led by the consideration that a painter will, without
exception, judge his work in his own studio too, so that one may assu-
me the lighting conditions, there prevailing, to be the most advan-
tageous, since they are, probably, also the conditions that can be
reproduced the best in a gallery. We took them to form the right
criteria.

A systematical inquiry into the factors, governing the illumination
of a gallery, leads to definite demands as to that illumination. Once
these are formulated, the question becomes, how far they can be
met with and to what constructive directions they lead. The treat-
ment, given below, will make it clear, that one cannot dispense in
this connection with a number of lighting-engineering data. These
however were not available at the time, and, therefore, various in-
vestigations had to be carried out of a physical-technical nature,
for example those, concerning the colour and intensity of daylight,
the relevant properties of various kinds of glass and the reflective
power of pictures. The methods themselves apphed in these in-
vestigations are a matter, apart from the hghting system, finally
constructed in the museum. They are, however, in themselves of
lighting-engineering interest. For convenience, these methods
together with the systematical treatment of the measurements, form
therefore the subject matter of the appendix, while the results of
these investigations and their direct application of the lighting
of the museum in question are given in Chapter I to V inclusive.
Thanks to this procedure, these chapters contain therefore the dis-
cussion of the problems, purely from a museum-technical point of

view, without any interruptions concerning the measuring technique,

while those, who are more specially interested in the physical
and technical details of the measurements, will find them, all
together, in the appendix.

FIRST CHAPTER

general conditions necessary for the illumination of
a picture-room

Any project for the illumination, daylight and artificial of a
picture gallery must necessarily be based in the first place on the
knowledge of the most appropriate illumination of a picture.

Now, as a rule, a painter while at work on a picture cannot and,
therefore, does not take into consideration the surroundings in
which thkt picture will ultimately be hung. These are unknown to
him but for a few exceptions where the pictures are being painted by
special order, in which case, however, they seldom find their way
into a picture gallery. Let us start therefore, from the assumption
that the illumination of the picture while it was being judged in the
studio, was as favourable as possible. We are then confronted by the
problem how to reproduce that illumination. In order to solve this,
we must know by which factors the nature of that illumination is
determined. These factors turn out to be:

1.nbsp;The colour of the incident light.

In the majority of cases the painter uses north light for his work.

2.nbsp;The intensity of the incident light.

As an average for the illuminating intensity in a studio, durmg
the painting, one may take 150 Lux.

3.nbsp;The direction of the incident light.

Most right-handed painters prefer overhead light slanting
from the left in widely diverging rays on to the picture.

4.nbsp;Reflections.

On the canvas in a studio, these are restricted to a minimum.

The question is therefore: to what extent can we reproduce the
above conditions in a picture-gallery? Here, at the outset compli-
cations arise from the fact that the pictures in a gallery hang in
rooms. For, beside the problem of the most appropriate illumination,
required by the picture as such, one must now also reckon with the

Eymers, Illuminationnbsp;^

influence of the room on the picture and of the room on the
spectator, both these factors demanding a further detailed exami-
nation.

As regards, namely, the question what illumination is required
by the spectator of the surroundings, for the picture and for himself,
so as to obtain the most favourable conditions for viewing it, one
must bear in mind that it is partly physiological and partly psycho-
logical, whereas the other factors are physical.

A further complication arising from the arrangement of a picture-
gallery is that one can only enter more closely into an examination
of these factors after having decided whether each of the pictures
shall be illuminated in the way speciaUy required for it (here
„illuminatedquot; is used in its widest sense, meaning the illumination of
the picture together with that of its surroundings) or whether the
illumination shall be such that any of the pictures can hang any-
where in the rooms of the gaUery. When one has to deal with very
large and very special pictures, the first mentioned way of illumi-
nating is obviously to be chosen but as regards the great majority of
pictures, one can but choose the second solution, the more so with a
view to the possibility of a varying grouping of the pieces, temporary
exhibitions etc.

Our general considerations will, therefore, deal with this second
way of illuminating. This does not, however, involve a restruction of
our problem; indeed, from our treatment of various possible con-
structions, the best way to obtain the solution for any special case
will become at once apparent.

It will be clear from the above that, in order to obtain the right
colour and direction of the light one would preferably illuminate the
rooms by means of windows in the upper part of their northern
walls. But in the first place an awkward consequence of this solution
would be that only a very narrow margin would be left as regards the
possible orientation of the building, and secondly by this position of
the windows only the opposite walls would be illuminated adequate-
ly. For these two reasons this fenestration can only be applied in a
few exceptional cases f.i. Special Gallery at Millbank; in general it is
not sufficiently economical, and one must therefore look for a
compromise. To this, one is led by the following considerations,
starting from the questions :

1 What is the specific nature of the north light ?

Would it be feasable, perhaps, to use the light from the whole sky

indiscriminately ?

From an investigation of the dependence of light from the sky on
wavelength we know that, as far as the northern sky is concerned, it
is given for the visible region on the average by an equi-energy
spectrum, that is to say, that the energy is practically the same at all
wavelengths. (Here we have added „on the averagequot; because the
dependence is partly also influenced by the type and degree of
cloudiness). When one determines, however, the dependence of the
light from the whole sky on wavelength, it appears that in the direct
radiation of the sun the red and yellow parts of the spectrum pre-
dominate over the blue part. Now the intensity of the direct sunlight
varies widely. The diffuse light from the sky is not subject to strong
fluctuations. Whereas, relatively speaking, the direct sunlight varies
considerably. Consequently at a spot illuminated by both the light
from the sky and the direct sunlight, the intensity, as well as the
colour of the resulting illumination, will also vary strongly. Taken
separately — the intensity fluctuations are perhaps the more im-
portant but that does not concern us here. For, whenever these
variations are directly observed —, i.e. whenever, owing to the
construction of the room, or to some other cause, a shadow pattern
of some kind is projected on the wall (the shadowed parts receiving
the scattered light from the sky only, and the parts immediately
next to them the light from the sky and the sun), we have to deal
with the combined effect of these intensity- and colour variations,
an effect, which by many people is thought particularly annoying.

If, however, proper care is taken that the direct sunlight shall
never fall on the canvas, or, what comes to the same, if the direct
sunlight is mixed completely with the scattered Hght from the sky,
the drawback arising from the strong variation of the former light is
for the greater part overcome.

It is true that when the sun shines, the colour of the incident light
will still be influenced by it, but thanks to the mixing, to a much
smaller extent that would be the case if it fell directly on the canvas.
This change of colour is, moreover, the same all over the surround-
ings. If, therefore, one is prepared to accept this change of colour,
one can indeed use the hght from the whole sky, so that by this
concession one is free as regards the orientation of the building. The
intensity fluctuations will be discussed in detail later on.

2°. Since, as mentioned above, the illumination must satisfy the
condition that all the walls of a picture-gallery shall be equally
suitable for the pictures, one must give up the idea of letting the
light fall through windows in the upper part of the walls. The
substitute for it is to use a horizontally placed skylight, provided the
latter serves at the same time to bring about the right direction of
the incident light. As regards this direction, I may remark the
following:

If the incident light consists of pencils of small opening the strong
shadows of the frames, as well as of the roughness of the paint (the
latter shadows being of course strongly dependent on the painting-
technique applied) will become prominent, whereas, in the case of a
completely diffuse illumination, no shadows at all will be formed.
Now both these effects are undesirable, but the latter is much more
objectionable than the former. Indeed, it is not exclusively by the
colour, but also by the direction and thickness of a brush-stroke,
that a certain effect is aimed at, so that shadows, if not too strong,
can by no means be dispensed with. For this reason the opening of
the light-pencils incident on the picture may not be too small.

3°. We shall now pass on to the consideration of the intensity of
the incident light in a room or gallery. It is well known that in a
studio the adequate intensity is 100 to 150 Lux. But now the question
arises whether this intensity will also be the most appropriate for a
gallery. In order to settle this question numerous experiments have
been carried out. To obtain information from different sources in our
country professionals (painters, directors of picture galleries, art-
critics) and non-professionals have been consulted. A number of
pictures, of widely varying styles, were examined, taking good care
that the colour of the light, which obviously plays a very important
part in this matter, was the same in every case. As was to be expected
the opinions were divided according to the nature of the pictures
(modern and ancient, of a light and of a dark hue, etc.), but one may
state as a general result that an illuminating intensity of about 100
Lux was considered to be the most appropriate.

The same question was also the subject of an enquity in Japan, (1),
where the nature of the pictures as well as the techniques are so
completely different from ours. In this case also, the opinion was
asked of professionals and non-professionals, 40 persons in all; the
result for the most suitable illuminating intensity turned out to be

140 Lux for Japanese pictures and 87 Lux for ordinary oil-paintings

and watercolours.

In the case of the experiments carried out m Holland there ap-
peared besides, to exist a small group of persons, who did not wish
for a stronger illumination than 20 Lux.

Since, however, this intensity is within the region, where the
Purkinje-effect is very prominent, and where, consequently, all
colours are distorted, one can for the time being, safely discard
their opinion ; it is, indeed, straining the point too far to assume that
this colour distortion was the effect aimed at by the painter.

Summarising, we may draw the conclusion that daylight illumi-
nation can be apphed to a picture-room by means of a glass roof,
provided the latter satisfies certain conditions. The principle of this
way of illuminating is aheady old. It was often put into practice by
simply making the roof of transparent glass. This solution has
however, various serious disadvantages ; a wall of a room illuminated
with this arrangement receives at one time the radiation of the blue
sky, at another time the light from white clouds or perhaps even
the direct light of the sun. This involves very strong changes as
regards the colour as well as the brightness and the direction of the
incident hght. Moreover, these changes are not only dependent on
the time but also on the place in the room. A picture on one of the
walls may at one and the same time be easily subject to an illumi-
nation differing widely from that of a picture hanging on another
wall of that same room. Finally, at instants when the direct sunlight
falls upon a picture the intensity of the illumination can be many
times too strong, as regards the most favourable conditions for viewing
it, as well as the detrimental effect the of light on the picture itself.

In many cases these difficulties have been overcome more or
less by making the roof of frosted glass or by stretching a linen cloth
underneath the roof. For both these methods it is true that the light
is to a certain extent scattered and absorbed, but if the incident
light has a certain direction of preference this will still continue
to predominate considerably. The drawbacks, mentioned above
will therefore be weakened but by no means removed. The only
real way out is to use for the glass roof a material suitable for bringing
about a complete mingling of the directed sunlight and the scattered
light from the sky.

This condition determines to a great extent the construction of the
picture-rooms. This condition determines the construction of the
rooms to a great extent, but not entirely since, in this connection
another condition plays also an important part, namely, that the
distribution of the intensity shall be sufficiently uniform.

As regards this second condition, not only the evenness over the
height of the picture itself is necessary but also the evenness over
the length and height of the Myalls. As regards the former let us
suppose that a light cloudy sky of a picture representing a landscape
is illuminated about 2 or 3 times stronger than the dark scenery
underneath. The „point of gravityquot; will then be displaced appreci-
able, which will have a slightly irritating effect and may spoil the
impression of the picture. As regards the homogeneousness of the
illumination over the height of the walls, when their top-part is
darker, it will make the room look gloomy; if, on the other hand,
it were too light, the attention of the visitor would be diverted in an
upward direction. Experience has shown that the best solution
in this respect is to make the brightness of the top part of the
walls not more than twice that of the walls at the level of the
pictures.

Finally, as regards the evenness of the lighting in the length
direction of the walls, it has turned out that a gradual decrease in
brightness down to half its value over a distance of 4 or 5 meter is
already realised as annoying. In planning the building the architect
must, therefore, take good care not to let the corners of the rooms
and galleries be too dark. This can be attained either by slanting off
the corners, or by some other special lighting-engineering device.
The „awarenessquot; of a given unevenness is, besides, dependent on
the brightness itself. Between the limits of an adequate lighting of a
picture-gallery, the less the lighting, therefore, the sooner a definite
unevenness will be noticed. This is one reason the more, why also on
dark days the illumination may not be less than the 100 Lux agreed
upon.

Before, entering into the question as to what extent the physiolo-
gical and psychological factors determine the further details of the
construction we must first ascertain whether it is at all possible
to satisfy those conditions as regards colour, brightness and homo-
geneousness. To this end it is, in the first place indispensable to have
at one's disposal data on the colour and intensity of daylight at

various hours of the day for various days of the year. These data
were not apparently available m Holland. In the second place,
one must know the lighting-engineering properties of substances
likely to serve as diffusing media. These properties turned out to be

only insufficiently known to dealers in these materials.

These two facts made it therefore necessary to set about investi-
gating on our own account the spectral constitution of daylight as
well as the properties of various substances.

SECOND CHAPTER

the investigation of the colour and intensity of daylight
and the optical properties of various kinds of glass

§ 1. The colour and intensity of daylight

In order to obtain the necessary data concerning the colour and
the intensity of daylight at various times of the day and for various
days in the year the following investigations were carried out:

for two years, daily, at about 9, 12, 14 and 17 o'clock the bright-
ness of a horizontal white surface exposed to the radiation of the
entire hemisphere of the sky was measured as a function of the
wavelength. The times, mentioned, were chosen so as to cover the
usual visiting hours of a picture gallery. As for using the light from
the entire hemipshere, considering that this investigation was begun
on behalf of the planned construction of the new Municipal Museum
at the Hague of which the roof, owing, to its favourable situation,
receives the light from the whole hemisphere of the sky, we were
bound to carry out our measurings under similar conditions.

For an account of the measuring method applied and of the way
the results were obtained from those measurements the reader is
referred to Appendix § 1. All measurements and the way to bring
them into workable form, are to be found in (2). The following
statements may be sufficient here. The results were obtained from
the combination of our observations with metereological data
referring to the same times. These data were kindly furnished by the
Royal Dutch Met. Inst, at De Bilt. First we can calculate the
colourvariations of the daylight. Secondly, as regards the intensity
variations of daylight, it is possible with the ?id of the sensitivity
curve of the eye and of the mechanical lightequivalent to compute
from the known functional connection between the intensity at a
given moment and the wavelength the value of the illumination
in Lux. It appeared that this value can be expressed as a

function of the sun's altitude and the degree of cloudiness. Now, the
average number of days on which the various degrees of cloudiness
in our country in different months of the year is known from
statistical data which have been collected at De Bilt for years. One
can, therefore, determine the intensity of the daylight to be expected

on the average in each month.

From the measurements mentioned above one can also deduce
the very considerable fluctuations occuring occasionally in the
course of one day or even within a few minutes. A change of intensity
by a factor 10, within a few hours is by no means an exception!

The following examples may serve to illustrate this. Table A
gives the intensity per cm^ of the white surface as a function of the
wavelenght on an arbitrarily chosen day. Table B gives the intensity
variations at one and the same wavelength on a day with strongly
varying cloudiness. The amounts of energy are expressed m
ergs/A.cm^.sec.

differences is out of the question and the necessity, therefore, arises
for an arrangement by which the illumination in a picture-room
can be regulated. A regulating system was therefore indeed plan
ned and subsequently applied in every room. This will be discussed
more in detail when we come to deal with the actual construction of
the building.

The question now arises: to what value of the intensity of the
daylight must the construction of the rooms be adapted ? Not to the
average value, because in that case on a great number of days either
artificial illumination will be necessary or the light will not be
sufficient. Since, however, as already stated, a regulating system
for the intensity must be apphed in any case there is no fear for
excessive illumination. The most efficient way will therefore be to
adapt the construction to that daylight intensity, for which the
number of days with insufficient light inside is a minimum.

The intensity of daylight once being known we have to decide
which material is the most appropriate for the skylight. This ma-
terial is subject to the condition that it shall be colourless and that
it shall mix the diffuse light from the sky and the directed sunlight
so completely that in the emerging light no trace of a direction of
preference is left. Moreover, its transparancy must be such as to
allow the daylight after passing through it to possess the illumi-
nation required for the walls of the picture-room.

Materials or substances likely to satisfy these conditions are :

a.nbsp;Cloth:

This has the great advantage of being inbreakable and light, easy
to handle and, owing to its texture, pleasant to look at. Its very
serious disadvantages are its great inflammability and liability to
decay.

b.nbsp;Bakelite:

This, too, is unbreakable and light but the kinds introduced on
the market up to now are liable to warp quickly and to change
colour in course of time, and, besides they are not quite fireproof,
so that, for the time being at least, they can be left out of consider-
ation.
• c. Glass:

This has the disadvantages of being heavy and breakable but as
it is absolutely fireproof, not subject to decay and constant as re-

gards colour and shape, it is to be preferred above the others for use
in a picture-gallery.

§ 2. Investigation of the optical properties of samples of glass

Since glass, as already stated, is the most suitable material for the
construction of skylights there remains to find out the right kind of
glass. To this end we examined sampels of of aline glass, opal-sheet
glass, and frosted glass. We measured more in particular:

1)nbsp;the dependence of the vertically emerging light on the wave-
length in the case of vertically incident light. This enables us to
check the colour of the glass.

2)nbsp;At one and the same wavelength the intensity of the light
emerging at various angles in the case of incident light in a few
definite directions (analogous to the position of the sun).

For a detailed account of the measuring method and of the results
obtained we refer the reader to appendix § 2.

It may suffice here to observe that practically speaking all kinds
of opaline glass and a few kinds of opal-sheet-glass act as completely
diffusing media (see fig. 1); only, opahne glass transmits a much
smaller amount of light than opal-sheet-glass.

In figure 1. we plotted for a few samples of glass the power of
transmission against the angle of refraction in the case of vertical
incidence. The value of the angle between the emerging light and the
normal are read along the abscis, the ordinate gives the corresponding
intensities incident on 1 cm^ of a surface at 1 m distance from the
glass, on the understanding that 1 cm^ of the glass radiates on to the
surface and that the intensity of the light incident on the glass is
put arbitrarily equal to 1.

The curves a, b and c refer to samples of opaline glass, completely
diffusing opal-sheet-glass and non-completely diffusing opal-sheet-
glass respectively. In the case of frosted glass the dependence of the
transmission power on the angle of refraction is strongly onfluenced
by the method of frosting applied. It turned out, however, that the
direction of the incident light is always more or less prominent in
the emerging light, whatever the method of frosting.

In order to decide between the possible kinds of opal-sheet- or
opaline glass the dimensions of the rooms or galleries must be taken
into account. Usually these dimensions from an illuminating engi-
neering point of view they are of necessity rather vague.

Now, there are two ways of proceeding under these conditions,
namely either by computation or by making a reduced model of the
room to be constructed. The latter method may only be applied,
however, when one has ascertained that the principle of similarity
holds for the case in question. Since we have to deal here with
completely diffusing media, the method may be applied without
hesitation.

In our case the length and breadth of the rooms were given as

^fOO SOOO fSOO 6000 biOO

Fig. 1.

was also the height of the glass roof. The height of the skylight was
still left more or less indeterminate. For this reason lighting-
engineering computations were carried out for a number of these
heights and widths and from the results the most suitable values
could be chosen.

If, however, as is here the case, reduced models may be used, this
way of proceeding is to be preferred.

Let us for example construct a model of the room in question in
the proportion, say, of 1 to 10. We can do this by simply taking a

box of whicli the horizontal cross section is somewhat larger than or
equal 1/100 of the one of the room to be built. The box is covered
with diffusing glass illuminated from underneath. With the help
of easily adjustable strips of black paper or card board certain
dimensions are then readily given to the model. Finally, the distri-
bution of the light over the walls is measured, for example by
means of a rectifier-cell in connection with a galvanometer. By
moving the paper strips and by taking in turn for the glass layer the
different kinds of glass likely to prove suitable it is possible to measure
rapidly a number of cases. The advantage of working with reduced
models as compared with the computing method as a quick means of
obtaining final results is greater according as fewer dimensions are
fixed definitely beforehand or according as one has to deal with
more unknown quantities.

Now in the case of our model a simple measuring gives the ratio
between the intensity of the light on the glass, (this, of course,
must be illuminated homogeneously) and on the „wallsquot;. The actual
intensity of the light to be expected on the walls inside the building
is then obtained from the intensity of the light which will in reality
fall on the glass roof by reducing the latter in the same ratio.

The following example may serve to illustrate the use of reduced
models, but it is in this connection, necessary first to explain the
meaning of the „copingquot;. From computations it appeared that when
the opal-sheet-glass occupies the whole of the ceiling the lighting is
uniform at the same height all round the room, but that the toppart
of the walls receives, in this case, much more light than the lower
part. It is, therefore, impossible to use this construction, but it
must be altered so as to reduce the excessive illumination at the
top. This is achieved by making the border of the ceiling up to a
certain width of an opaque material. This opaque border is called
the „copingquot;. Computations showed that the optimum effect of this
contrivance is obtained when it is applied somewhat lower than the
opal glass itself .

As regards the most suitable width of the coping, this might have
been determined also by computation, but it is here that our reduced
model comes in. It represented a room of the following dimensions:
length 11m., breadth 8 m., height of the opal glass 6 m., height of
the coping 4,80 m. (The effective distance between opal-sheet-glass
and coping was therefore 1,20 m.) By means of this model we could

readily measure the distribution of the light in such a room, first
without a coping, then with a coping of m width and finally with
one of 1 m width. The results are given in fig. 2a, b and c respectively.
Since the distribution appeared to be symmetrical with respect to
the middle of the walls only one quarter of the room is represented
in the figures and the walls are drawn flattened out into the plane of
the floor. The illumination was measured for every 1,50 m round
the room, at heights of 1, 2, 3 and 4 m and the numbers give the
relative values, obtained at the corresponding parts of the walls.

Ho

'JL___X

e_S.

Zl_i

ia_z-

t£_Z

From the results here given, it will be particularly clear, that a
coping is necessary. Without one, the illumination at the height of
4 m is three times stronger than at the height of I m; with a coping
of ^ m width, the former is reduced already apprecially and finally
with a coping of 1 m width the intensity at the height of 2 and 3 m
is even stronger than at a height of 4 m. As was shown when the
uniformity of the intensity over the walls was discussed it is the
latter distribution that contributes to the right effect of the pictures.

The arrangement must be such that an intensity of 100 Lux

Mi 97 92 lt;?

0_-UO_It_£?

65_i

r_^t_^

it_10_-

12_9

'tirCcUX

0 ?

lt;£_t'l.

Fig. 2b.

7n lijnbsp;_iJ___-

amp;i_-

2_it

J J___i.0_i

ilt;L_a

O_6 0nbsp;/

^_a

Fig. 2c.

shall fall on the walls of the room. This condition led us by the
following considerations to choose a special opal-sheet-glass for the
material in question. In the first place it diffuses the incident light
completely and in the second place its power of transmission is such
that for the inside intensity just mentioned an outside illumination
of 4000 Lux is necessary. As matters stand, however, it appears
from statistical data that in our country on about 10% of the days
in the course of a year the daylight intensity falls short of 4000 Lux.
In the case of opal-sheet-glass, therefore, the light on the central
parts of the walls will be insufficient on such days and one will have
to make use of, if at all feasible, artificial illumination. Since, however,
the power of transmission of opal-sheet-glass is the highest of all
(see fig. lb) any other material would require a stronger outside
illumination, and consequently there would be a greater number of
days on which artificial illumination would be necessary. This is the
reason for our choice of that opal -sheet-glass.

Later on, in the chapter on artificial illumination we shall treat
the question in further detail.

'h

THIRD CHAPTER

A FURTHER SPECIFICATION OF THE CONDITIONS NECESSARY FOR THE
ILLUMINATION OF A PICTURE-ROOM

In order to specify tlie illuminating system in further detail, we
must carefully consider which factors, depending on the illumination
influence the impression of a picture exhibited in a picture-room.

These factors were :

1.nbsp;The purely physical ones, i.e. those referring to the illumination

of the picture itself and

2.nbsp;the physiological-psychological ones, i.e. those referring to the
illumination of the surroundings of the picture and of the spectator
himself, required by him, so as to provide the best conditions for
looking at the picture.

Now, the physical factors could be specified (see page 1) as colour,
illumination, and direction of incident light, and as reflexions on the
picture. The three first of these factors were already discussed in
detail in the above so that we can now pass on to the remainmg
question concerning the reflexions.

Every picture has a more or less reflecting surface, glass or veneer,
so that one might ask what it is that the spectator actually sees;
the images formed by the surface of parts of the room or of objects in
the room are, so far as this reflected light enters his eyes, superposed,
as it were, on the picture itself. These reflexions are not necessarily
disagreeable as is proved by the fact that many painters even prefer
to cover their pictures by glass. One may say that these reflexions
may never strike the spectator by their colour nor by their intensity;
neither may they possess any considerable intensity- or colour-
gradients. They must be whitish, and the combined effect of these
grey reflexions which, as it were, cover the picture, is that all the
colours in it will be slightly less saturated and give the impression of
flowing more harmoniously into each other. But if this grey image

Eymers, Illuminationnbsp;^

is not to be felt as an annoyance, its brightness may not be more
than a certain fraction of the brightness of the picture itself. An
immediate consequence of this fact is, that direct light from the
light source (window or skylight) may not by any means reach the
eye of the spectator after being reflected in the picturesurface,
because the intensity of such a reflected image would be far too
strong 1).

Since it is impossible to avoid these reflexions when the room is
illuminated by windows at the same height as the pictures, this is
one of the arguments against this fenestration.

In order to prevent any possible reflexion from the skylight
windows in the upper part of the walls the pictures are sometimes
given a slightly overhanging position. In the case of very large pieces
however, this is apt to give rise to difficulties and it is with these
pieces more than with the others, that the chance of reflexions m the
top part, which can still catch the eye, is the greatest.

Besides, notwithstanding all the measures taken to prevent it,
the chance will always remain that the surface of the picture under
consideration will show reflexions of pictures hanging on the other
walls or of the wall-covering. This is one of the reasons for choosing
white or light grey for the colour of the wall-covering; another
reason is that thanks that colour the walls will add a weak general
side-illumination to the illumination of the pictures. The chief
reason, however, will be mentioned later on.

The great influence of the homogeniousness has been discussed

in Chapter I.

Let us now pass on to the physiological-psychological factors.

1. The influence of the brightness and colour of the surroundings
(the walls) on the appreciation of a picture.

As is well-known, this influence is very strong, and may easily
spoil the impression of a picture. The primary condition, which the
surroundings must satisfy, for the right impression is that they shall
intensify to the utmost our power for receiving colour-differences
and intensity-differences.

1) The power of reflection of glass amounts to about 8 %, that of veneer to about 7 %
in the chief direction of the reflection. For the determination of this value and also for the
determination of the power of reflexion in the case of various directions of incidence and
for the age and treatment of veneer see appendix § 3.

As regards colour-differences, our power of perception is greatest
in white or grey surroundings i).

As regards the intensity, the lighting is obviously the same on a
picture and on the wall next to it. Now the wall may not divert our
attention from the picture; or, what comes to the same, the
brightness of the former must be such as to make our power of
perception an optimum.

This w ill be the case here when the brightness of the surroundings
is equal to that of the picture. Now the power of reflexion of light
paints is as high as 60%, but of the dark hues, frequently present in
older pictures, it often amounts to only a few percentages. If, there-
fore, the wallcovering were white (power of reflexion 60% to 80%)
our power of perception in the case of dark-toned pictures would
be considerably reduced by the large difference in brightness; light-
toned (many modern) pictures, on the other hand, can stand white
surroundings very well. As it happens, however, grey wall-covering,
which, as is clear from the above, is the most appropriate for dark-
toned pictures, has a favourable influence on the appreciation of
light-toned pictures also.

2. The influence of the illumination of the spectator himself, on
his appreciation of a picture.

Here we are concerned with the question: What illumination does
the spectator require for himself so as to acquire the best conditions
for looking at the picture.

In order to answer this question, one must bear in mind that our
power of discrimination is not only dependent on the colour and
brightness of the immediate surroundings of the object under
consideration. It is also, and very strongly, influenced by any extra
hght, for example the light from „the lightsourcequot;, that may eventu-
aUy enter out eye. In this connection it is not only the pupil of the
eye that comes into play, but the greater part of the eyeball. It has
turned out(3),namely, that the transmission power of the sclerotic is
so great, that the light which enters our eye through it, can play an
important part in our way of perceiving.

The questions confronting us are, therefore: may the skylight,
which illuminates the pictures, be seen by the visitor? If so, what

1) This is the reason why a greay wall-covering is chosen for sortingrooms, where the
eye must be able to register small colour-differences, as, for example, in the case of the
sorting of tobacco, corn etc.

must be the brightness of the skyhght as seen from the average
position 1) of the visitor? If not, to what extent is it advisable to

screen it from his view by special arrangements ?

As will appear from experiments, presently to be mentioned, it
is impossible to answer these questions unambiguously. One has
simply to admit the fact that some persons like to have an illumi-
nated surface overhead, while others dislike it strongly. The ex-
periments in question, were carried out also with a view to throwing
light on this difference. (We realise quite well, that they can not

claim to be more than a rough approxi-
jf mation to what is required by our problem ;

they were, indeed, only meant as a first
orientation) They were carried out as
follows: fifteen persons were examined as
regards their power of discrimination
between spectral (saturated) and non-
saturated colours, first, while light entered
their eyes in a slanting direction from
overhead, (for the illuminated surface a
plate of opahne glass was used) and
secondly, while this surface was screened
from their eyes.

They were also checked as to a possible
difference in colour-perception for both
cases.

In order to secure the necessary data,
the following arrangement was used (see
fig. 3). The person to be examined fixed
his eye which in the fig. 3 is at 0 on a small
white surface V at the same level as the
eye. On this surface two fields were
proj ected one above the other, which could
be made to differ in colour; the coloured

fields were surrounded by a grey boder-field. Over this arrangement

1 ) By „average positionquot; is here meant the whole of the room, reckened from a small
distance from the walls. When one enters a room, namely, one is at that moment oneself
part of the wall and receives then the same illumination as the pictures; one is, therefore,
bound to see the illuminating surface, unless some special lighting-engineering measures
are taken at the place of the doors, a sloping roof, for example, over the door-openmg or
ome appliance in the skylight.

a opaline glass plate M was applied. In the case of pure spectral
colours the surface V was screened from the light radiated by M ; in
the case of non-saturated colours the screen was removed. The whole
of the arrangement was mounted in a small room with white walls ;
by the illumination applied above M the room was at the same time
dimly-lighted by diffuse hght.

By means of a shade the light from M could be screened at will
from the eye of the observer. The numbers given in the fig. represent
the distances (in meters) in the actual experiments. The brightness
of M amounted to about 0,01 K/cm^

The result obtained from these observations can be stated as
follows: there appeared to be chiefly two groups of persons; for the
first group the power of perception of colour-differences is less when
the plate of opaHneglass is visible than when it is not; i.e. a larger
wavelength difference is necessary in the former case than in the
latter to make them aware of a colour-difference. For the second
group the power of perceiving colour-differences is greater in the case
of a visible illuminating surface, than without it; especially in the
green and the orange parts of the spectrum, whereas it is less in the
red and the blue-green parts. For both types of persons the colour-
perception changes also slightly. In the case of saturated colours, the
red and green made a slightly more yellow impression when the
skyhght was visible than when it was not.

The persons examined were also asked whether they thought the
visible skylight trying or disageable.

We investigated also the value, for various persons, of the angle
between the horizontal line joining the eye with the point at its own
level, on which the eye is fixed, and the line joining the eye and a
lightsource, overhead, when it is first seen entering the field of view.
As could be expected this angle turned out to depend on the depth
of the eyesockets of the various persons examined, but in our
applications we can safely use the average value, which aws 77°. Be-
sides, we have in reality to deal not with a lightsource of very
limited extent, but with a luminous surface of which the effect of its
visibility is much less disturbing than that of the lightsource. We
can therefore, take 77° as a safe upper limit of the angle with the
horizontal direction, at which the luminous opalglass surface will
first become visible, when we look at pictures hanging at the level
of our eyes. We can conclude from this the value for example, that.

when the opal-sheet-glass is at a height of 4,80 M. above the floor
and the coping has a width of 1 M., we can stand away from the
walls at a distance of 1,70 M. before the luminous surface can have
any disturbing effect.

It results also impossible to draw general conclusions regarding
this matter and one can but ascribe equal importance to the ob-
served facts. The result, already referred to above, was that there
are two groups of persons; one that prefers a visible skylight, provid-
ed there are no parts in it possessing a too strong brightness, and
another that strongly prefers a screened skylight. Since the number
of persons of the two groups was about equal, our problem is how
to meet the demands of both of them. This is obviously only possible
by constructing two types of rooms with skylight illumination :

1°. rooms where the visitors can see the skylight directly. The
visibihty is dependent on the dimensions of the room. We shall
enter into further detail as regards this point, when we come to the
discussion of the construction of the rooms.

2°. velum rooms, i.e. rooms in which the skylight is screened

from the eye of the visitor.

As regards sub. 1, a skylight of opal-sheet-glass looks rather severe
and cold; besides, it scatters the light evenly in all directions, where-
as the visitor would prefer a stronger illumination on the walls
than on the floor or himself. In order to obtain this, it is advisable
to apply under the diffusing glasslayer a second layer, thereby
bringing about the most favourable distribution of the light and
at the same time improving the aspect of the roof.

As regards sub. 2. For reasons of a lighting engineering nature,
the dimensions of the velum and the level it may be applied are
subject to conditions, arising from the construction of the room.
For, in order to illuminate the pictures by means of light-pencils of
maximum opening, and also to obtain the required evenness, the
wall must receive on a level with the lower part of the pictures (i.e.
about 1 M. above the floor) as much light from the glass skylight

as on a level with the toppart.

It follows from this that a velum, if it is not to spoil the lighting,
may nowhere be higher than the lines AS and SB (see fig. 4). In
planning the part so demarcated, the architect must, besides, satisfy
certain aesthetic and psychological conditions. A velum may not be

too low, for example, because that gives an oppressive feeling; it
must not be too dark either, for the same reason. The material for
such a velum must, therefore, be more or less transparent, and this
leads, once more, to the choice of glass for the same reasons as were

mentioned in the beginning. It remains only to decide upon the
kind of glass possessing the right transparency.

For the second glasslayer in the skylightrooms various kinds of
figured glass are hkely to serve the purpose. Since, however, the
glass-dealers could not provide the data necessary for a decision,
we had to set about furnishing them on our own account.

FOURTH CHAPTER

THE VARIOUS TYPES OF THE CONSTRUCTED ROOMS

In the preceding chapters we deduced from physical and psycho-
logical data the conditions, which the rooms of a picturegallery
must satisfy. It was found that the application of an opal-sheet-
glass skylight to rooms lighted from above, meets the various
demands to a great extent, that, however, a further improvement of
the lighting can only be obtained by either applying a second sky-
light or a velum. It was also made clear that for this second skylight
figured glass must be used. Before entering into a discussion about
the construction of the rooms, it will, therefore, be advisable to deal
with the relevant properties of figured glass a little longer. Since
this kind of glass will always be used underneath the opal-sheet-
glass, its colour and the angular distribution of the emergent light,
only in the case of diffuse incident light, were investigated. A
detailed description of the experiments and their results is given in
Appendix § 2. Here the following statements will be sufficient.

It appears to make a difference for this kind of glass whether the
smooth or the uneven side is turned towards the opal-sheet-glass. In
the latter case the distribution of the light suits our purpose better;
indeed the proportion of the amount of light, emerging at great
angles to the normal to that in the direction of the normal is then
greater, than when the glass is placed the other way round and this
proportion governs the amount of light on the walls, compared with
that on the floor or the visitor. This difference is particularly
pronounced for the various types of prismatic glass; this glass must
therefore in the majority of cases, be applied with the prism's turned
upwards. Since, however, in this po;-'tion, any dust entering from
outside, is apt to gather in the furrows of the glass, it must be
protected by an extra glass-layer. For the common types of figured
glass the difference, mentioned above, is much less pronounced.

They can therefore, in all cases be apphed with the smooth side up,
which greatly facilitates the cleaning.

By way of illustration, fig. 5A and B show a few transmission-
curves of the various types of figured glass. Along the abscis the
angle is measured between the direction of the emerging light, and
the normal, along the ordinate the intensity of the emerging light is
expressed in percentages of the incident light. The curves a and b in
fig. 5A give the result for a drop-glass with the smooth side (0)
respectively the figured side ( ) turned towards the opal-sheet-
glass. In fig. 5B the results for a symmetrical prismatic glass are
given in both situations (see 0 and ) while the curve c in the same

To a visitor, a prismatic glass syklight will consequently appear
less bright than one of drop-glass. It would seem, therefore, that by
the application of the former kind of glass the same hghting-engi-
neering effect could be obtained as one tries to realise by a velum-
construction. As a matter of fact however, the ways these two con-
structions affect the visitor, is not nearly the same, owing to the low
position of the velum as the outstanding difference.

Fig. 6 shows by way of illustration photographs of a few kinds of
glass used by us. These were obtained as follows: the sample was
put on a background half of which was white and the other half
black. It was then photographed from above and sideways in a
slanting direction (in the latter case the cross section of the glass
was made black, so as to make it stand out clearly). The scale is
1: 2, so that one can form an opinion about the real size of the
figuring. The specific action of the various kinds of glass can be
seen from the reproductions, more in jjarticular the action of the
prismatic glass is strikingly reproduced.

Now that the lighting properties of the various kinds of glass are
known, we shall proceed to consider the constructional details of the
rooms. These were studied for the greater part (as already mentioned
in Chapter II) with the aid of reduced models (scale I : 10), which
afford the quickest way to determine for each type of room sepa-
rately the kind of glass required and also the copingwidth, the
height and dimension of the velum etc. By these models we were
led to the following constructions:

a.nbsp;skyhght rooms.

b.nbsp;rooms with high fenestration,

while the skylight rooms are further subdivided into

1 rooms with a secondary skylight of drop-glass

2°. „ „ „ „nbsp;„ „ prismatic glass

3°. velum rooms

Fig. 7 shows a cross section of part of the first floor of the Mu-
nicipal Museum at The Hague. Three types of rooms are here re-
presented, namely (from left to right) a skylightroom, a velumroom
and a cabinet, lighted by windows in the upper part of the waU.
(Next to this there is still a passage with glasscupboards, receiving
its light from an inner court.)

We shall discus the various types of rooms one by one, and point
out their specific advantages and disadvantages.

Pl. I, p. 26

Pl. II

H

a
w

c
gt;

»
►H

O

Cl

c/)
H

w
W

o

quot;ri
H
K
W
o
O

2;
^

W
C!
O
H
W

Ö
§

O
g

w

Fig. 7.

A. Opal.-sheet-glass. B. Figured glass, C. Velum. D. Screens.

a. Skylight-rooms

Our starting point for tlie lighting construction of the various
rooms was the a shaped roof of transparant wire netting glass,
originally planned by the architect of the building i). From a lighting
engineering point of view this shape of the roof is of no importance,
as the roof itself is not a constructional part of the lighting system
as such. It has, however the quality that, owing to its transparency,
the iron construction of the building can be seen through it ; which
reminds some people unpleasantly of a factory. This difficulty can
be removed by simply making the horizontal opal-sheet-glass layer
i.e. the first essential feature of the lighting construction, play the
part of the roof. On account of the rainwater, however, it may in
that case, not be accurately horizontal; one can either give it a a
shape with very slight inclinations, or make it inchne as a whole at a
very small angle Seen from the street, both constructions will
give the impression of a flatt roof, so that thereby, the drawback of
the visibility of the iron construction is overcome.

In all skylight rooms opal-sheet-glass is used for the diffusing
toplayer. A gangway is applied in this layer from which every part
of the roof-construction is accessible and to which besides the
appliances for artificial illumination can be fastened. All over the
opal-sheet-glass layer movable screens are applied (see fig. 8) which
serve to regulate the illumination in the room. The necessity of
this screens has been commented upon in Chapter II. They are
operated from the rooms so that :

1 in the course of the day the illumination can be kept constant.
2°. if a visitor would prefer — for a short time — a different
illumination, the attendant can adjust the screens accordingly.

3°. so long as the rooms are not used, the light can be kept
screened completely, thereby protecting the pictures from its
damaging influence. It is advisable to do this daily after closingtime
for rooms in which pictures are exhibited, so that the latter need
not be exposed to the light before 10 o'clock in the morning, and

1)nbsp;When the building was nearly finished the effect of the construction of the roof
which could be seen through the glass, was thought ugly and objectionable; in finishing
the building the transparant glass was therefore made white. This entailed an appreciable
loss of light. This might have been avoided if the white roof had been planned from the
beginning.

2)nbsp;The diffusing properties of the used opal-sheet-glass exclude slopes of any ap-
preciable steepness.

also for longer periods at a stretch for rooms in which the pictures
are stored away.

What has been said of the skylight rooms so far, refers equally
to the three kinds of rooms, mentioned sub. 1°, 2° and 3°, because
they have the opal-sheet-glass layer in common. We shall now deal
with the types separately.

1°. rooms with the lower skylight of drop-
glass^)

Owing to the fact that the scattering for these kinds of glass is
symmetrical with respect to the normal and that their transmission
in the direction of the latter is a maximum, the illumination in the
central fart of the room is greater than on the walls (about three times
as much) and seen from anywhere in the room, the glasslayer will
have a considerable brightness. All walls are in this case lighted by
the whole of the skylight.

The distance of the two layers is in the construction about 1,20 M.
This rather large space is also chosen to allow a person to move
about when necessary for repairing or cleaning and besides to
contain the appliances for artificial lighting. For safety the lower
layer must consist of wire-netting glass.

The coping width is fixed for the majority of the rooms at 1 M.
the value for which the illumination at the level of the pictures is
greatest, while at heights of 1 M. and 4 M. it is reduced to 60% a 70%
of it. The attention is therefore, drawn automatically to the right
part of the walls, moreover the distribution of the illumination is
such that the upper part of the walls is not yet so dark as to cause
the room to make a gloomy impression. Some kinds of dropp-glass
possess at greater angles to the normal a higher transmission than
other types of these glasses; in rooms therefore where the lower
skylight consists of the former, the topparts of the walls are lighted
nearly as strongly as the central parts. In the corners of a room,
measuring 8 X 10,5 M. the illumination is about the half that at the
centre of the longer wall. This difference does not affect the visitor
unpleasantly, so long as the central illumination amount to about
100 Lux. For apprecially lower values, however, these differences

1) One type of this kind of glass is called: Large Morocco.

would begin to be disagreable. These darker corners are not an
essential feature of this construction, for by either making the coping
narrower at the corners or by applying there a slightly different kind
of glass, this complication is easely overcome. Besides there are
even people who think it an advantage when the illumination is
not perfectly uniform round the whole room. Applied to long and
narrow rooms, however, this construction would entail a very mark-
ed decrease ot the illumination in the lenghtdirection and the longer
and shorter walls of the room would show a strong difference
between their illumination. It is therefore not advisable to apply it
in such a case, without special arrangements. For smaller, more or
less square rooms however, it is the right construction which, more-
over, recommands itself by its simplicity.

2°. rooms with the lower skylight ofpris-
matic glass

With the prism's turned upwards the maximum transmission of
glass of this description is not in the direction of the normal and the
various kinds have each their own value for the angle of maximum
transmission which differs from the others. By a suitable combi-
nation of a few of these glasses one can therefore arrange that the
diffuse light, emerging from the opal-sheet-glass, is directed any way
one likes, i.e. in our case towards the walls. The kinds of glass to be
used and the way to apply it in the skylight, must, of course be
wholly adapted to the size and the destination of the room in
question. In the Municipal Museum at The Hague the lighting
system of two rooms is constructed in this way, the one room long
and narrow (8 X 15 M.) the other nearly square (11,5 X 14,5 M.).
The lower skylight in the long-narrow room is constructed as follows
(see fig 9): division A consists of symmetrical prismatic glass, which
therefore directs the light towards the two long walls .For the other
divisions a certain kind of asymmetrical prismatic glass is used,
placed in such a way that the projection of the maximum trans-
mission on the plane of the glass is in the direction of the arrows,
while the ridges are at right angles to them. As shown in the figure,
the glass in the corners had to be cut slanting to the ridges, which
can be done without any difficulty. The second, nearly square room,
(see fig. 10) contains very special and very large pictures. Here,
asymmetrical prismatic glass is used throughout and the way it is

PI. Ill, P. 30

Pl. IV

placed in the skylight is shown in the figure. The central part of the
ceiling was not necessary for the illumination of the pictures. The
choice of its material was, therefore, entirely a matter of personal
taste; in this case opaline glass was applied.

The differences between the rooms with prismatic glass and those
with other figured glass in the lower skylight can best be summarized
in the following points:

owing to the fact that prismatic glass shows a certain direction of
preference for its transmission, a wall will receive its light chiefly
from a definite part of the skylight and the light incident on a picture
will, therefore, be more directed.

it follows from that same property that the brightness of the glass
seen from other directions than that of maximum transmission, will
be comparatively small; seen from the room the skylight will there-
fore appear less bright. (The illumination in the central part of the
room is nearly equal to that on the walls).

since the light, emerging from the topskylight is not cast for the
greater part towards the central part of the room, but is used to a
much higher percentage for the illumination of the pictures, one
can, relatively speaking, suffice with less light. This means that on
dark days, the pictures can still be illuminated adequately, while
with the other skylights the intensity would already be insufficient.

by the choise of the pattern according to which the glass is laid
in the skylight and of the kind of glass itself, rooms of all sizes and
shapes can be lighted satisfactorily.

since the only way this kind of glass can here be used is with the
prism's upwards, it must be covered with an extra layer of glass,
fitting hermetically on all sides. This makes the whole of the
construction heavier and more expensive.

prismatic glass ist not to be had reinforced by wirenetting; this
must therefore be applied to the covering glasslayer just mentioned,
so that the chance of breakage for the prismatic glass itself is greater.

finally, in the direction of maximum transmission this glass is,
practically speaking, transparant; consequently, from a place close
to the wall (and therefore also on entering the room) one can see the
constructional parts between the upper and lower skylight, whereas
in 'he other cases these parts are invisible.

The prismatic glass skylights are, properly speaking, an inter-
mediate form between skylightrooms with drop-glass and velum-

rooms Indeed with the former they have the constructional
features in common, with the latter (at least in the way they are
apphed here) the distribution of the hght.

3°. velumrooms

The object of their construction is to screen the skylight from the
visitors eye. When he lookesatthe pictures, at the same time it must
be possible that the visitor can, without strain, read his catalogue.
For that reason the velum may not be quite opaque, apart from the
fact that such a velum would give a strongly oppressive feeling. It
is therefore made of a slightly translucent material, namely of a
certain kind of opalineglass. Our leading idea in fixing the dimen-
sions of the velum was that over the whole height of the pictures
the wall should receive the light from half the breadth of the opal-
sheet-glass skylight. The velumconstruction was actually carried
out in two types of rooms, one of 10 X 11 M. and the other of 8 X 14,5
M. In the nearly square rooms, its application does not meet with
particular difficulties. In our case, we fixed the height of the velum
at 3,90 M. and its dimensions at 6,10 X 7,20 M. (the skylight was at
the usual height of 6 M. above the floor). But the application of the
velum would entail that the corners of the room receive light from a
much greater part of the skylight than the middle of the walls. The
illumination in the corners would accordingly be much too strong.
To prevent this complication, we apphed a few screens on top of
the velum (see fig. 11). With the rooms, equipped with lower sky-
lights of prismatic glass, the velum rooms have in common that the
light incident on the pictures is more directed than in rooms with
drop-glass. Besides in velum rooms, the illumination at the top of
the walls is slightly higher than at the level of the pictures.

The application of the velum in the oblong rooms was less simple.
As already mentioned, the rooms measured 8 X 14,5 M.; now, if the
velum were to hang again at a height of 3,90 M. this would mean
(due to the comparatively narrow room) that its width should not
be more than 3,50 M. From an aesthetic point of view, however, this
width is inadmissible. A smaller height of the velum (which would
make a greater width possible) is not possible, because it would then
make the room look gloomy. We have therefore applied the velum
at the height of 3,90 M. and fixed its width at 4.30 M. This means
that the lower part of the walls, up to about two meters from the

3

Eymers, Illumination

floor, are lighted by less than half the width of the skylight, in other
words: the distribution of the light over the height of the walls is
not homogeneous. In order to put this right again, we once more
applied prismatic glass. This time, however, the object was to di-
minish the light at the top and to increase it, as much as possible,at
the bottom of the walls, while the velum itself may also be illumi-
nated. This involves the condition that the maximum transmission
of the prismatic glass shall make only a small angle with the normal
and that the transmission in directions at larger angles to the normal
shall be considerably less. This condition is satisfied when the pris-
matic glass is laid with the prism's downwards, which we did ac-
cordingly. The opal-sheet-glass layer is this time immediately on
the prismatic glass.

In the nearly square rooms, only the outer border of the glass
velum will contribute to a certain extent to the lighting of the walls;
in the oblong rooms, on the contrary, the part of the velum contri-
buting to the lighting, will not be restructed to the outer border
only. In the latter rooms, therefore, the velum must consist entirely
of white glass, whereas, in the former, it may, except for the outer
border, consist of coloured glass.

b. rooms with high fenestration

This type of lighting is applied to all small cabinets round the
inner court. The windows are made of opal-sheet-glass, thereby
obtaining again an illumination on the walls, which is uniform as
regards colour and intensity. The windows of the rooms not facing
north, must be provided with shutters by means of which the in-
tensity of the lighting can be regulated. The high fenestrating light-
ing has, of course, the serious disadvantage that one of the walls can
not be used for hanging pictures on it, but apart from this it is (at
least according to the tast of the present writer) the most gratifying
way of lighting a picture-gallery.

FIFTH CHAPTER

ARTIFICIAL ILLUMINATION

Before we can proceed to a more detailed specification of an
adequate construction for artificial illumination, we must first
inquire (as in the case of daylight illumination) into the factors
that are relevant to this matter. For the greatest part these factors
are the same as in our former case. We meet again with the intensity
and the direction of the incident light, the possibility of reflections,
the colour and brightness of the walls, the'homogeniousness of the
illumination, and in particular the question whether or not a
„visiblequot; source of illumination is advisable, must be answered a-
gain. Since these factors lead to the same conditions as were found
for daylight iUumination, it follows that the method of artificial
illumination must resemble, as closely as possible, the one of day-
light illumination:

The specifically new problems in this connection are:

a.nbsp;what must be the spectral composition of the artificial light?

b.nbsp;how must it be applied, in order to resemble in its effects,
as nearly as possible, the daylight illumination and to work out at
the same time as economically as possible ?

a. To begin with, let us confine ourselves to the colour of the
artificial light. In order to decide this matter, we must first answer
the questions how the colour of the incident light influences the
impression, made by a picture, and what we do ourselves wish this
impression to be like ?

To answer these questions we must bear in mind, that, apart from
the influence of the surroundings, our impression of a colour in a
picture is dependent on the way in which the power of reflection of
the paint in question is connected with the wavelength, the spectral
composition of the incident light and the sensitivity of the eye. In

order to form an opinion as to what a picture will look like, when a
given artificial illumination is applied, one must, therefore, know
first:

1°. the energy distribution of the various artificial sources of light,

2°. the selectivity of the power of reflection of the paints, used in
pictures,

3°. the changes in colour that are realised as colour distortion.

1°. What is the spectral energy distribution of artificial sources
in comparison with that of the daylight ?

As already stated in Chapter If, one can take the spectral compo-
sition of daylight to be that of an aequienergyspectrum. As arti-
ficial sources of light, suitable to replace daylight, we must mention
in the first place, tungsten lamps. The continuous spectrum of the
light, emitted by these lamps, depends on the colourtemperature
of the particular lamps used. The colourtemperature is in its turn
a function of the power (in Watts). The higher the latter, the higher
also the former. Table C (4) gives a few connected values for these
(and other) quantities for a gas-filled lamp.

TABLE C

By way of illustration, fig. 12 shows the relative intensitydistribu-
tions over the wavelengths for a 50 Watt- (x)and a 1000 Wattlamp (.),
the energy at 5600 A being in both cases put equal to 100. It will be
clear from this figure that with the higher powers the spectral
constitution does indeed approach the one required for our purpose,
but that it still differs strongly from an equienergyspectrum. Ex-
pressed in terms of the colourpoint (see table C under „huequot; and
„saturationquot;) this means that whereas, practically speaking, the
hue does not change, the saturation does change considerable with
the power of the lamp; with increasing power the colour becomes to
an appreciable degree white. Fig. 12 shows also the energycurve for
a Philips daylight lamp. (0)

Besides these hghtsources with continuous spectra, there are
also hghtsources producing discontinuous spectra, which, never-
theless, make an impression on the eye of being white. We mention
for example, the mercurylamp with cadmium, further ordinary
mercurylight, combined with mercurylight in yellow glass (which
absorbs the blue line, so that the light makes an impression of green)
and with neonlight and finally, the light from COa-dischargetubes.
The spectra of all this hghtsources (at least of the specimina brought
on the market up to 1934) show more or less numerous lines, in the

Fig. 12.

majority of cases, widened, between which lie more or less deep
minima.

2°. The selectivity of the power of reflection of the paints in
pictures. This power was measured, for a number of cases (for which
we refer the reader to Appendix § 3). For some of the paints in
ancient as well as in modern pictures, it turned out to show maxima
and minima, restricted to very narrow ranges of wavelength. An
example of this very pronounced selectivity is given in fig. 13, which
shows het reflectioncurve of a flesh coloured paint, used by Jan
Sluyters in one of his pictures.

What changes in the colourimpression are realised as colour-

0/

Fig. 13.

distortion? Before answering, let us put the preliminary question
as to how much the differences in wavelength must at least amount
to, so as to be observed as a colour difference. It is only natural
to think, in this connection, of the numerous investigations, carried

go

V

J

S600 ewo 6100 6000 ssoo i60o JMO IZOO 1000 »JOO veoo iioo

Fig. 14.

out with a view to answering this question. We mention, for example
the doctor's thesis of P. M. L a d e k a r 1 (5), containing an account
of his own observations and, besides, an important survey of the
literature. Most investigators have confined themselves to the
determination for various wavelengths, of the value to which for a
given wavelength X the difference AX can increase, before the colour
belonging to X AX is realised as different from the colour belonging
toX. For normal trichromates, L a d e k a r 1 gives the curve, shown in
fig. 14. The results of earlier investigation showed also minima, lying
at 5800 and 4900 A, occasionally a third maximum was found lying
at 4500 A. We must also mention another investigation, dealing with
the same question, namely the one by G. H a a s e (6). He determined
the AX — X connection for widely different illuminations and also
for various degrees of saturation of the colours. His results show
several minima, which proved

/

io Zo 30 Vo SO 60 ?o do 90 fÖO%

Fig. 15.

pictures contains an appreciable percentage of white. In connection
with H a a s e's results just mentioned, one would be inclined to think
that the risk of colourdistortion cannot be very great. But one must
bear in mind that H a a s e's results do not apply directly to our case.
Indeed, our problem is not such that we have simply to compare the
colourof a paint, illuminated by daylight with that colour of the same
paint, illuminated by artificial light, but it must be stated as follows:
when do we realise the effects of colourchanges, which artificial il-
lumination can bring about in a picture as a distortion to the ef-
fects which we take it that the painter wished to convey to us ? It
is clear from this formulation that it would be more to our point to
show to a great many persons various paints in daylight surround-

ings as well as in artificial lighted surroundings and then to in-
vestigate the differences in colourimpression between the two cases.
But even that would not meet completely the demands of our
problem. Indeed our point is not to find out when a given paint,
illuminated by light of a certain spectral composition makes a differ-
ent impression from that of the same paint, illuminated by light of a
different spectral composition, but when this difference is realised as
a distortion. When, for example, in daylight the colour of an artificial
of dress makes a different impression from that in artificial light, this
difference does not necessary falsify the impression; when, however,
a picture, representing a sunlit snow-landscape looks in artificial
light like a moonlit snowlandscape, the difference does indeed falsify
the impression. The paints of some special details in pictures are
particularly liable to this colourdistortion ; the paint used for ren-
dering the colour of the skin, for example, is very striking in this
respect. Now, those paints turn out, in the majority of cases, to
possess the type of a reflectioncurve with many maxima and minima
as for example is shown in fig. 13. They make it therefore necessary
to illuminate the pictures at night exclusively by light producing a
continuous spectrum. As soon as a minimum in the energy curve
of a lighting source with a discontinuous spectrum covers the same
range of wavelength as a maximum or minimum in the reflection-
curve of the paint in question, an appreciable colourdistortion will be
realised in these specially sensitive hues. For that reason not one of
the daylightlamps, brought on the market up to now, suits our
purpose, owing to a pronounced maximum in the green (see fig. 12).
We are, therefore reduced totheuseof tungsten lamps, and to correct
the spectrum of their light with the aid of special filters, in order
to obtain, as nearly as possible, an aequienergyspectrum. Since,
compared with the latter, the tungsten spectrum contains towards
the longer wavelengths an increasing surplus of light, these filters
should transmit the blue part unreduced, but should possess an
increasing power of absorption towards the red part ; strickly speak-
ing, the transmissioncurve of the filter should be the reciprocal of
the emissioncurve of the lamp.

It will be clear from fig. 12 that the higher the number of Watts
of the lamp used, the more economical the lighting wiU be, for the
amount of blue in the spectrum is then relatively greater so that a
lower percentage of the energy towards the red must be absorbed.

Filters possessing the required curve are not to be had on the
market, we were therefore forced to prepare them ourselves. Once
coated on their base, they should possess of course the right trans-
missionpower, but, besides, their colours should be fast, preferable
(if at all possible) also at high temperatures. In that case namely, the
colouring matter might be applied directly on the bulb of the lamps,
ft turned out, however, to be impossible to meet these various de-
mands at the same time. After many experiments Mr. J. J. Z a a 1-
berg van Zelst succeeded in preparing two coloured filter-
substances for us, which, however, when mixed,appeared not to suit
our purpose. Therefore the two coats must be applied separately,
either each on a plate or on the two sides of one plate. By altering the

Fig. 16.

concentration of the coloured liquids or by choosing a suitable
thickness for the coloured layer, one can prepare filters adapted to
lamps of various powers. In this connection one must not forget
however that the economy of the filters decreases continually with
decreasing power. The filters actually prepared by us for lamps of
500 Watt, have an efficiencyof about 50%. Fig. 16 shows the spectral
composition of the light radiated by such a lamp, with the filter-
combination applied. The colours of the filters are, however, not
fast at high temperatures; the plates must therefore, be used at a
safe distance from the lamp.

b. The lighting system

Here we must try to meet the two demands mentioned above,
namely that the method of lighting shall be as closely as possible
similar to that used for daylight and that it shall be as economical
as possible. We may aid here as a matter of course, that the apphances
for the artificial lighting is on no account to intercept the daylight.

If one confines oneself strictly to the first of these conditions, the
right solution is to mount the lamps above the opal-sheet-glass. This is
accordingly put into practice in the velumrooms. The lamps might,
in this case, have been provided with reflectors secured in the roof
over the revolving screens, but the efficiency of the best reflectors
is not more than 60%. Besides, the purchase and the upkeep of the
armatures is expensive. For this reason we decided on a different
construction. The lamps are now mounted in a horizontal position be-
tween the opal-sheet-glas layer and the screens, as is also shown in fig.7.

The screens which as we know are painted white, are in this system
simply closed at night and so are made to play the part of reflectors.
The gangway is painted white underneath, so that any light re-
flected by the opal-sheet-glass is also sent back for the greater part.
By this solution the screens in the velumrooms are made therefore
to serve a double purpose. Owing to, the fact that only the bare
lamps are mounted between the opal glass and the screens, no day-
light worth mentioning is intercepted. The lamps develop an appreci-
able amount of heat, but since the construction in this system is left
open at the sides a sufficient ventilation can be kept going on. The
white paint on the screens turns out to keep very well at these
temperatures.

As already observed above, high power lamps offer some ad-
vantages over those of low power. In the first place, the light emitted
by the former lamps contains more white and in the second place the
efficiency (i.e. the number of Lumens per Watt) increases with
increasing power. This appears also from the following table, which
gives the various values for gasfilled lamps of 110 Volt.

When very strong lamps are used one can suffice with only a few
of them, but then the brightness of the opal-sheet-glass close to
the lamps will be very high, and since the few lamps are naturely
wide apart, this will entail an inhomogeneous illumination of the
glasslayer, which may make itself felt in an unpleasant way. But
in this connection we must also bear in mind that the artificial
lighting is not only meant to be used at night, but also as an auxiliary
lighting in the daytime, whenever the daylight alone proves unsuffi-
cient. For this purpose however only part of the number of lamps
available, will always be enough. For that reason the connections
are made in such a way that either all the lamps or a certain part of
them can be switched on at a time. In the velumrooms in question
this fraction is Now, if only a few very strong lamps were used for
artificial lighting in these rooms, the number of lamps providing the
auxiliary daytime lighting would be so small that a very incon-
venient inhomogeneousness would be the result. This complication
therefore, limits the power of the lamps to be used. The necessary
compromise between efficiency and homogeneousness led in our case
for example to the choice of 500 Watt lamps.

Generally speaking, however, this method of lighting is not the
most economical; there is indeed stiU a rather appreciable loss of
light. In the majority of the skylightrooms, where a different method
can be readily apphed, the space underneath the gangway is there-
fore used for the lighting appliances, as is also shown in fig. 7. But it
is not feasible simply to mount there the bare lamps, because the
illumination of the figured glass underneath would in that case give
rise to difficulties. Indeed under these conditions the glass would
show little „starsquot;, that is to say one would have to deal, as it were,
with a great many pointsources of light of great brightness m the
illuminating layer and these would have a dazzling effect. To prevent
this it would seem advisable to apply a very slight matting to the
glass: as it happens however this process gives at once a very duU
appearance to it, especially in daylight, so, that we must look in
another direction for the solution. Apart from this complication,
this lighting system would involve the inconsistency that the central
parts of the figured skylight would receive the strongest illumi-
nation, though these parts do not contribute the greatest amount of
light to the lighting of the walls, whereas the light of the lamps
should obviously be directed towards those parts that do. Now in

most skylight rooms the division of the lower skylight is as shown in
fig. 17, the gangway is above the shaded track. Now the glasssheets
of rows 2 and 4 lie nearest the direction from the lamps towards the
walls; the lighting will therefore be the most economical when the
light of the lamps is principally directed towards these panes. This
is achieved by closing off the space underneath the lamps by
symmetrical prismatic glass which indeed directs the light in the
two above mentioned directions. Moreover, the space underneath the
gangway is painted entirely white, to increase the efficiency as much
as possible. Here again, one must take proper care to secure a

sufficient ventilation and also the

construction does not interfere
in the least with the dayhght.

In order to obtain a uniform
illumination along the longer waUs,
the lamps must of course not be
placed at equal distances from each
other, because then the intensity
in the central parts would be
underly high. One must therefore
proceed as follows: to begin with,
one lamp is mounted in the space
underneath the gangway and the
illumination is then measured at
various spots along the walls.
From these data one can readily
compute where the other lamps
must be mounted to secure the
required uniformity. As for the shorter walls, these must chiefly
be lighted from the divisions p and q. Here, therefore, more lamps
must be mounted of which the flux of light is then led in the right
direction by means of prismatic glass. Underneath this glass
the filters for the colour of the light can be apphed.

With a view to an eventual auxiliary lighting, the lamps are in
this case also electrically connected in groups in such a way that
either 1 /3 or 2/3 or all of the lamps can be switched on at the same
time. In order to obtain some idea of the powers, actually used we
mention here for example that in a skylightroom of 8 X 11 M. the
power is 3| K.W. and in a velumroom of 10 X 11 M. it is 12 K.W.

artificial illumination

In rooms with high fenestration in one of the walls it is impracti-
cable to make the artificial illumination enter the room in a way
imitating that of the daylight. Indeed when the artificial light should
at night fall from outside through the windows, this would probably
affect the visitor as most unnatural. For that reason special arma-
tures have been designed for the small cabinets. The principal of
their construction is the following (fig. 18 shows a cross section m
the direction of its length): the lamp is surrounded by the opal-
sheet-glass walls A and right against this opal-sheet-glass the pris-
matic glass B is apphed. The lamp is finished off at the bottom by the
opaUne glass plate C. The di-
mensions and the outward ap-
pearance of the lamp follow more
or less from aesthetical consider-
ations which in their turn are
related to the size of the room
for which the lamp is construc-
ted. The prismatic glass B serves
here to direct the light towards
the walls; the prism's must
therefore be turned once more
towards the opal-sheet-glass.The
glasslayers A and B are here
kept together by the same
dustproof-frame; this offers the
advantage that the prismatic
glass ist at the same time ef-
ficiently protected from dust.
The kind of prismatic glass must,

of course, be chosen in accordance with the demensions of the room.
As regards the transmissionpower of the bottomplate C, one is
quite free. The most pleasant effect is obtained by making the
brightness of C equal to that of the ceiling.

The same type of lamp was used for the small skylightrooms
without a gangway. A few of these lamps of very sober make were in
this case mounted between the upper and the lower skylight prefera-
bly along the sides in order to interfere as httle as possible with the

daylightillumination.Herealso,asforthecabinets,thedimensionsand

shape of the lamps must be adapted to the dimensions of the room.

Now as regards the colour of the artificial illumination!

This remains for the time being an open question. As we have seen
above, the colour and the intensity of the illumination at night must
be the same as of dayhght, if we make it a condition that the picture
shall make the same impression on us at night as in daytime. For
the present, however, an illumination complying with this condition
will certainly not be applied throughout a picturegallery. For, in
the halls, passages and other parts of the building, this equality of
colour at night and in daytime is not really necessary, so that one
shall for economy not apply to these parts an illumination which on
account of the filters is twice as expensive as without them. This
means that one will continually pass from parts with a „yellowquot;
illumination into picturerooms illuminated by „whitequot; light. The
latter will perhaps produce under these conditions a very unnatural
and cold effect .Even if the whole building were illuminated by the
same white light, we should at first, almost certainly get the same
impression of unnaturalness, since we are used in our own surround-
ings to a different colour of the light at night. But this impression
will very soon wear out during a prolonged visit as we ourselves have
experienced. Indeed after a certain lapse of time one is no longer
aware of any unnatural effect of this white illumination at all,
provided the illumination be not too low! It is just the other way
round! Now on passing from the „whitequot; rooms into the „yellowquot;
ones, it is the latter illumination which is apt to affect one as being
unnatural.

At present, this white illumination is applied at night to only one
room in the Municipal Museum at the Hague. It therefore remains to
be seen how long it will, in general, take the visitors to adapt them-
selves to the two different colours of the artificial illumination. In
any case, it is very much to be regretted that this experiment has
not been started on a larger scale, because it will now be much more
difficult to form a definite opinion about this matter. But to the
present writer it would seem to be a safe supposition that if a picture-
gallery were lighted throughout by „whitequot; light, it would not only
not interfere with the visitors from a psychological point of view,
but that, generally speaking, it would have even a beneficial in-
fluence on the appreciation of the pictures.

APPENDIX

§ 1. Investigation of the special constitution of daylight
a. Method of measuring

In order to find out the constitution of daylight, in the visible part
of the spectrum, we measured for about two years daily at fixed
hours the brightness of a horizontal white surface as a function of
the wavelength. To be in accordance with the conditions of our
problem, the surface must be illuminated by the whole of the hemi-
sphere of the sky; this was achieved by placing the measuring ar-
rangement on the roof of the Physical Laboratory at Utrecht, from
which the view is indeed, practically speaking, free on all sides.
The white surface is that of a sufficiently thick layer of magnesmm
oxide, precipitated on a flatt brass plate. When there was no
measuring going on, a cover was laid over it, to keep it clean. In
case of rain during the measuring, it was protected by a bell glass,
because it was desirable not to let the rain interfere with the measur-
ing The magnesium oxide layer was renewed at regular intervals.
The measurings are carried out with the aid of a spectralpyrometer
(7) inside which is applied as the spectral apparatus, a Fuess-
spectroscope of constant deviation. As the white surface is horizon-
tal, it is necessary to adjust the pyrometer at a certam mclmation
with respect to it. By its presence, however, the pyrometer is bound
to screen part of the sky from the surface, but this part is reduced as
much as possible by making the inclination a minimum and by
placing the pyrometer at a large distance from the surface. In our
case the angle between the axis of the pyrometer and the normal to
the white surface was, therefore, made as high as 80°, and the distance
about 3 M. A wooden shed was built round the whole of the arrange-
ment to protect it against atmospheric influences.

The standardising of the apparatus is carried out partly on the
roof in its working condition and partly in a room of the laboratory.
In the latter is performed, firstly, the standardising of the spectros-

Fig. 19.

meterlamp, a support is
applied in front of the first
lens of the pyrometer, in
which various photogra-
phic reducers can be in-
serted. These reducers are
standardised beforehand.

As regards the absolute
standardising, one must
bear in mind that for large
values of the angle of re-
flection, the reflection-
power of MgO has turned
out to be a function of
that angle (see fig. 19), a
function,which is not neces-
sarily the same for aU

wavelengths. Now, owing to the conditions of observation on the
roof, the spectralpyrometer is only used for a large value of this
angle. It will be clear therefore, that the absolute standardising
of the pyrometerlamp may not be carried out in the room in the
laboratory; it was therefore performed on the roof on a dark night.
To this end a projectinglamp was placed perpendicularly over the
white surface at a known distance from it, the amount of energy in
ergs, radiated per sec., per unit of solid angle and per A being
known. The brightness of the white surface was then determined
with the aid of the pyrometer for the same wavelengths as the

daylight, namely: 6800, 6600, 6400, 6200, 6000,5800,5600,5400,
5200, 5000, 4900, 4800, 4700, 4600 and 4500 A. At regular intervals
the standardising was subject to a complete checking.

The measuring took place as nearly as possible at 9, 12, 14 and 17
o'clock, because these times cover the visiting hours of a museum.
At the same time, the degree of cloudiness and the type of clouds
were observed when the sun was shining, the brightness caused by
sky sun, as well as the one due to the scattered light of the sky

only, was measured. The latter measurements were obtained by inter-
cepting the direct rays from the sun by a small screen, taking
care that this should cover as little as possible of the sky, as seen
from the white surface.

b. Results

As already mentioned, the complete list of the observations
together with a discussion of their arrangement, with a view to
drawing conclusions from them are published in (2). In table I are
given, by way of illustration, the observations made on 3 bright
days of constant cloudiness and on 4 days with a completely over-
cast sky, for various times of the year. The intensities are expressed

in erg/A, sec. cm^.

TABLE I

Date........

Time........

Sun's altitude . . . .
Degree of cloudiness . .
Type of clouds . . . .
Total or Indirect . . .

wavelength

6800 A.......

6600 ..........

6400 ..........

6200 .........

6000 .........

5800 .........

5600 .........

5400 .........

5200 .........

5000 .........

4900 .........

4800 „ .......

4700 .........

4600 „ .......

4500 .........

Illumination in Lux.

Eymers, Illumination

The observations are the material from which the colour- and the
intensity-fluctuations of the daylight must be studied. As regards the
former, one must bear in mind that the measurements given under
a definite hour of the day are in reality obtained one after the other,
over an interval of about ten minutes (at each wavelength two or
three pyrometeradjustments were performed). The fluctuations
occuring is such a series, will therefore partly be due to changes in
the cloudiness during the time covered by the measuring. This in-
volves that if one subtrates the observed values, belonging to a
bright hour, due to the scattered light from the sky only, from those,
due to the total light, the results will show fluctuations, which are
certainly influenced by the changes in cloudiness during the times,
covered by the two series. Now observations, for one and the same
wavelength, on a bright day of constant cloudiness have shown that
the intensity-fluctuations on such a day need not amount to more
than a few percentages in the course of half an hour. It will therefore
be permitted in our case to calculate the intensity, due to the direct
solarradiation only, by simple subtracting from the observed inten-
sity, due to sun sky the value due to the scattered light from the
sky only. This is what we did for the bright days, quoted in table I,
and the results are given in Table II.

TABLE II