The circumstances which we have hitherto discussed indicate a profound
difference, and one which is exceedingly important for the perception
of solid form, between the visual image which our eyes give, when we
stand before objects, and that which the picture gives. The choice
of the objects to be represented in pictures is thereby at once much
restricted. Artists are well aware that there is much which cannot be
represented by the means at their disposal. Part of their artistic
skill consists in the fact that by a suitable grouping, position,
and turn of the objects, by a suitable choice of the point of view,
and by the mode of lighting, they learn to overcome the unfavourable
conditions which are imposed on them in this respect.
It might at first sight appear that of the requisite truth to nature
of a picture, so much would remain that, seen from the proper point
of view, it would at least produce the same distribution of light,
colour, and shadow in its field of view, and would produce in the
interior of the eye exactly the same image on the retina as the object
represented would do if we had it actually before us, and looked at it
from a definite, fixed point of view. It might seem to be an object of
pictorial skill to aim at producing, under the given limitations, the
_same_ effect as is produced by the object itself.
If we proceed to examine whether, and how far, painting can satisfy
such a condition, we come upon difficulties before which we should
perhaps shrink, if we did not know that they had been already overcome.
Let us begin with the simplest case; with the quantitative relations
between luminous intensities. If the artist is to imitate exactly the
impression which the object produces on our eye, he ought to be able to
dispose of brightness and darkness equal to that which nature offers.
But of this there can be no idea. Let me give a case in point. Let
there be, in a picture-gallery, a desert-scene, in which a procession
of Bedouins, shrouded in white, and of dark negroes, marches under the
burning sunshine; close to it a bluish moonlight scene, where the moon
is reflected in the water, and groups of trees, and human forms, are
seen to be faintly indicated in the darkness. You know from experience
that both pictures, if they are well done, can produce with surprising
vividness the representation of their objects; and yet, in both
pictures, the brightest parts are produced with the same white-lead,
which is but slightly altered by admixtures; while the darkest parts
are produced with the same black. Both, being hung on the same wall,
share the same light, and the brightest as well as the darkest parts of
the two scarcely differ as concerns the degree of their brightness.
How is it, however, with the actual degrees of brightness represented?
The relation between the brightness of the sun’s light, and that of the
moon, was measured by Wollaston, who compared their intensities with
that of the light of candles of the same material. He thus found that
the luminosity of the sun is 800,000 times that of the brightest light
of a full moon.
An opaque body, which is lighted from any source whatever, can, even
in the most favourable case, only emit as much light as falls upon
it. Yet, from Lambert’s observations, even the whitest bodies only
reflect about two fifths of the incident light. The sun’s rays, which
proceed parallel from the sun, whose diameter is 85,000 miles, when
they reach us, are distributed uniformly over a sphere 195 millions
of miles in diameter. Its density and illuminating power is here only
the one forty-thousandth of that with which it left the sun’s surface;
and Lambert’s number leads to the conclusion that even the brightest
white surface on which the sun’s rays fall vertically, has only the one
hundred-thousandth part of the brightness of the sun’s disk. The moon
however is a gray body, whose mean brightness is only about one fifth
of that of the purest white.
And when the moon irradiates a body of the purest white on the earth,
its brightness is only the hundred-thousandth part of the brightness of
the moon itself; hence the sun’s disk is 80,000 million times brighter
than a white which is irradiated by the full moon.
Now pictures which hang in a room are not lighted by the direct light
of the sun, but by that which is reflected from the sky and clouds. I
do not know of any direct measurements of the ordinary brightness of
the light in a picture-gallery, but estimates may be made from known
data. With strong upper light and bright light from the clouds, the
brightest white on a picture has probably 1-20th of the brightness of
white directly lighted by the sun; it will generally be only 1-40th, or
even less.
Hence the painter of the desert, even if he gives up the representation
of the sun’s disk, which is always very imperfect, will have to
represent the glaringly lighted garments of his Bedouins with a white
which, in the most favourable case, shows only the 1-20th part of the
brightness which corresponds to actual fact. If he could bring it,
with its lighting unchanged, into the desert near the white there, it
would seem like a dark grey. I found in fact, by an experiment, that
lamp-black, lighted by the sun, is not less than half as bright, as
shaded white in the brighter part of a room.
On the picture of the moon, the same white which has been used for
depicting the Bedouins’ garments must be used for representing the
moon’s disk, and its reflection in the water; although the real moon
has only one fifth of this brightness, and its reflection in water
still less. Hence white garments in moonlight, or marble surfaces, even
when the artist gives them a grey shade, will always be ten to twenty
times as bright in his picture as they are in reality.
On the other hand, the darkest black which the artist could apply would
be scarcely sufficient to represent the real illumination of a white
object on which the moon shone. For even the deadest black coatings of
lamp-black, black velvet, when powerfully lighted appear grey, as we
often enough know to our cost, when we wish to shut off superfluous
light. I investigated a coating of lamp-black, and found its brightness
to be about ¹/₁₀₀ that of white paper. The brightest colours of a
painter are only about one hundred times as bright as his darkest
shades.
The statements I have made may perhaps appear exaggerated. But they
depend upon measurements, and you can control them by well-known
observations. According to Wollaston, the light of the full moon is
equal to that of a candle burning at a distance of 12 feet. You know
that we cannot read by the light of the full moon, though we can read
at a distance of three or four feet from a candle. Now assume that you
suddenly passed from a room in daylight to a vault perfectly dark, with
the exception of the light of a single candle. You would at first think
you were in absolute darkness, and at most you would only recognise the
candle itself. In any case, you would not recognise the slightest trace
of any objects at a distance of 12 feet from the candle. These however
are the objects whose illumination is the same as that which the
moonlight gives. You would only become accustomed to the darkness after
some time, and you would then find your way about without difficulty.
If, now, you return to the daylight, which before was perfectly
comfortable, it will appear so dazzling that you will perhaps have
to close the eyes, and only be able to gaze round with a painful
glare. You see thus that we are concerned here not with minute, but
with colossal, differences. How now is it possible that, under such
circumstances, we can imagine there is any similarity between the
picture and reality?
Our discussion of what we did not see at first, but could afterwards
see in the vault, points to the most important element in the solution;
it is the varying extent to which our senses are deadened by light;
a process to which we can attach the same name, fatigue, as that
for the corresponding one in the muscle. Any activity of our nervous
system diminishes its power for the time being. The muscle is tired by
work, the brain is tired by thinking, and by mental operations; the
eye is tired by light, and the more so the more powerful the light.
Fatigue makes it dull and insensitive to new impressions, so that it
appreciates strong ones only moderately, and weak ones not at all.
But now you see how different is the aim of the artist when these
circumstances are taken into account. The eye of the traveller in
the desert, who is looking at the caravan, has been dulled to the
last degree by the dazzling sunshine; while that of the wanderer
by moonlight has been raised to the extreme of sensitiveness. The
condition of one who is looking at a picture differs from both the
above cases by possessing a certain mean degree of sensitiveness.
Accordingly, the painter must endeavour to produce by his colours, on
the moderately sensitive eye of the spectator, the same impression as
that which the desert, on the one hand, produces on the deadened, and
the moonlight, on the other hand, creates on the untired eye of its
observer. Hence, along with the actual luminous phenomena of the outer
world, the different physiological conditions of the eye play a most
important part in the work of the artist. What he has to give is not
a mere transcript of the object, but a translation of his impression
into another scale of sensitiveness, which belongs to a different
degree of impressibility of the observing eye, in which the organ
speaks a very different dialect in responding to the impressions of the
outer world.
In order to understand to what conclusions this leads, I must first
of all explain the law which Fechner discovered for the scale of
sensitiveness of the eye, which is a particular case of the more
general _psycho-physical_ law of the relations of the various sensuous
impressions to the irritations which produce them. This law may
be expressed as follows: _Within very wide limits of brightness,
differences in the strength of light are equally distinct or appear
equal in sensation, if they form an equal fraction of the total
quantity of light compared._ Thus, for instance, differences in
intensity of one hundredth of the total amount can be recognised
without great trouble with very different strengths of light, without
exhibiting material differences in the certainty and facility of the
estimate, whether the brightest daylight or the light of a good candle
be used.
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
The easiest method of producing accurately measurable differences in
the brightness of two white surfaces, depends on the use of rapidly
rotating disks. If a disk, like the adjacent one in Fig. 3, is made to
rotate very rapidly (that is, 20 to 30 times in a second), it appears
to the eye to be covered with three grey rings as in Fig. 4. The
reader must, however, figure to himself the grey of these rings, as
it appears on the rotating disk of Fig. 3, as a scarcely perceptible
shade of the ground. When the rotation is rapid each ring of the disk
appears illuminated, as if all the light which fell upon it had been
uniformly distributed over its entire surface. Those rings, in which
are the black bands, have somewhat less light than the quite white
ones, and if the breadth of the marks is compared with the length of
half the circumference of the corresponding ring, we get the fraction
by which the intensity of the light in the white ground of the disk is
diminished in the ring in question. If the bands are all equally broad,
as in Fig. 3, the inner rings appear darker than the outer ones, for in
this latter case the same loss of light is distributed over a larger
area than in the former. In this way extremely delicate shades of
brightness may be obtained, and by this method, when the strength of
the illumination varies, the brightness always diminishes by the _same
proportion_ of its total value. Now it is found, in accordance with
Fechner’s law, that the distinctness of the rings is nearly constant
for very different strengths of light. We exclude, of course, the
cases of too dazzling or of too dim a light. In both cases the finer
distinctions can no longer be perceived by the eye.
The case is quite different when for different strengths of
illumination we produce differences which always correspond to the same
quantity of light. If, for instance, we close the shutter of a room
at daytime, so that it is quite dark, and now light it by a candle,
we can discriminate without difficulty the shadows, such as that of
the hand, thrown by the candle on a sheet of white paper. If, however,
the shutters are again opened, so that daylight enters the room, for
the same position of the hand we can no longer recognise the shadow,
although there falls on that part of the white sheet, which is not
struck by this shadow, the same excess of candle-light as upon the
parts shaded by the hand. But this small quantity of light disappears
in comparison with the newly added daylight, provided that this strikes
all parts of the white sheet uniformly. You see then that, while the
difference between candle-light and darkness can be easily perceived,
the equally great difference between daylight, on the one hand, and
daylight plus candle-light on the other, can be no longer recognised.
This law is of great importance in discriminating between various
degrees of brightness of natural objects. A white body appears white
because it reflects a large fraction, and a grey body appears grey
because it reflects a small fraction, of incident light. For different
intensities of illumination, the difference of brightness between
the two will always correspond to the same fraction of their total
brightness, and hence will be equally perceptible to our eyes, provided
we do not approach too near to the upper or the lower limit of the
brightness, for which Fechner’s law no longer holds. Hence, on the
whole, the painter can produce what appears an equal difference for the
spectator of his picture, notwithstanding the varying strength of light
in the gallery, provided he gives to his colours the same _ratio_ of
brightness as that which actually exists.
For, in fact, in looking at natural objects, the absolute brightness in
which they appear to the eye varies within very wide limits, according
to the intensity of the light, and the sensitiveness of the eye. That
which is constant is only the ratio of the brightness in which surfaces
of various depth of colour appear to us when lighted to the same
amount. But this ratio of brightness is for us the perception, from
which we form our judgment as to the lighter or darker colour of the
bodies we see. Now this ratio can be imitated by the painter without
restraint, and in conformity with nature, to evoke in us the same
conception as to the nature of the bodies seen. A truthful imitation in
this respect would be attained within the limits in which Fechner’s law
holds, if the artist reproduced the fully lighted parts of the objects
which he has to represent with pigments, which, with the same light,
were equal to the colours to be represented. This is approximately the
case. On the whole, the painter chooses coloured pigments which almost
exactly reproduce the colours of the bodies represented, especially
for objects of no great depth, such as portraits, and which are only
darker in the shaded parts. Children begin to paint on this principle;
they imitate one colour by another; and, in like manner also, nations
in which painting has remained in a childish stage. Perfect artistic
painting is only reached when we have succeeded in imitating the action
of light upon the eye, and not merely the pigments; and only when we
look at the object of pictorial representation from this point of view,
will it be possible to understand the variations from nature which
artists have to make in the choice of their scale of colour and of
shade.
These are, in the first case, due to the circumstance that Fechner’s
law only holds for mean degrees of brightness; while, for a brightness
which is too high or too low, appreciable divergences are met with.
At both extremes of luminous intensity the eye is less sensitive for
differences in light than is required by that law. With a very strong
light it is dazzled; that is, its internal activity cannot keep pace
with the external excitations; the nerves are too soon tired. Very
bright objects appear almost always to be equally bright, even when
there are, in fact, material differences in their luminous intensity.
The light at the edge of the sun is only about half as bright as that
at the centre, yet none of you will have noticed that, if you have
not looked through coloured glasses, which reduce the brightness to a
convenient extent. With a weak light the eye is also less sensitive,
but from the opposite reason. If a body is so feebly illuminated that
we scarcely perceive it, we shall not be able to perceive that its
brightness is lessened by a shadow by the one hundredth or even by a
tenth.
It follows from this, that, with moderate illumination, darker objects
become more like the darkest objects, while with greater illumination
brighter objects become more like the brightest than should be the
case in accordance with Fechner’s law, which holds for mean degrees of
illumination. From this results, what, for painting, is an extremely
characteristic difference between the impression of very powerful and
very feeble illumination.
When painters wish to represent glowing sunshine, they make all objects
almost equally bright, and thus produce with their moderately bright
colours the impression which the sun’s glow makes upon the dazzled
eye of the observer. If, on the contrary, they wish to represent
moonshine, they only indicate the very brightest objects, particularly
the reflection of moonlight on shining surfaces, and keep everything so
dark as to be almost unrecognisable; that is to say, they make all dark
objects more like the deepest dark which they can produce with their
colours, than should be the case in accordance with the true ratio
of the luminosities. In both cases they express, by their gradation
of the lights, the _insensitiveness_ of the eye for differences of
too bright or too feeble lights. If they could employ the colour of
the dazzling brightness of full sunshine, or of the actual dimness of
moonlight, they would not need to represent the gradation of light
in their picture other than it is in nature; the picture would then
make the same impression on the eye as is produced by equal degrees
of brightness of actual objects. The alteration in the scale of
shade which has been described is necessary because the colours of
the picture are seen in the mean brightness of a moderately lighted
room, for which Fechner’s law holds; and therewith objects are to be
represented whose brightness is beyond the limits of this law.
We find that the older masters, and pre-eminently Rembrandt, employ the
same deviation, which corresponds to that actually seen in moonlight
landscapes; and this in cases in which it is by no means wished to
produce the impression of moonshine, or of a similar feeble light.
The brightest parts of the objects are given in these pictures in
bright, luminous yellowish colours; but the shades towards the black
are made very marked, so that the darker objects are almost lost in an
impermeable darkness. But this darkness is covered with the yellowish
haze of powerfully lighted aërial masses, so that, notwithstanding
their darkness, these pictures give the impression of sunlight, and the
very marked gradation of the shadows, the contours of the faces and
figures, are made extremely prominent. The deviation from strict truth
to nature is very remarkable in this shading, and yet these pictures
give particularly bright and vivid aspects of the objects. Hence
they are of particular interest for understanding the principles of
pictorial illumination.
In order to explain these actions we must, I think, consider that
while Fechner’s law is approximately correct for those mean lights
which are agreeable to the eye, the deviations which are so marked,
for too high or too low lights, are not without some influence in the
region of the middle lights. We have to observe more closely in order
to perceive this influence. It is found, in fact, that when the very
finest differences of shade are produced on a rotating disk, they are
only visible by a light which about corresponds to the illumination of
a white paper on a bright day, which is lighted by the light of the
sky, but is not directly struck by the sun. With such a light, shades
of ¹/₁₅₀ or ¹/₁₈₀ of the total intensity can be recognised. The light
in which pictures are looked at is, on the contrary, much feebler; and
if we are to retain the same distinctness of the finest shadows and
of the modelling of the contours which it produces, the gradations of
shade in the picture must be somewhat stronger than corresponds to the
exact luminous intensities. The darkest objects of the picture thereby
become unnaturally dark, which is however not detrimental to the object
of the artist if the attention of the observer is to be directed to
the brighter parts. The great artistic effectiveness of this manner
shows us that the chief emphasis is to be laid on imitating difference
of brightness and not on absolute brightness; and that the greatest
differences in this latter respect can be borne without perceptible
incongruity, if only their gradations are imitated with expression.