With these divergences in brightness are connected certain divergences
in colour, which, physiologically, are caused by the fact that the
scale of sensitiveness is different for different colours. The strength
of the sensation produced by light of a particular colour, and for a
given intensity of light, depends altogether on the special reaction of
that complex of nerves which are set in operation by the action of the
light in question. Now all our sensations of colour are admixtures of
three simple sensations; namely, of red, green, and violet,[17] which,
by a not improbable supposition of Thomas Young, can be apprehended
quite independently of each other by three different systems of
nerve-fibres. To this independence of the different sensations of
colour corresponds their independence in the gradation of intensity.
Recent measurements[18] have shown that the sensitiveness of our eye
for feeble shadows is greatest in the blue and least in the red. A
difference of ¹/₂₀₅ to ¹/₂₆₈ of the intensity can be observed in the
blue, and with an untired eye of ¹/₁₆ in the red; or when the colour is
dimmed by being looked at for a long time, a difference of ¹/₅₀ to ¹/₇₀.
[Footnote 17: Helmholtz’s _Popular Scientific Lectures_, pp. 232-52.]
[Footnote 18: Dobrowolsky in _Graefe’s Archiv für Ophthalmologie_, vol.
xviii. part i. pp. 24-92.]
Red therefore acts as a colour towards whose shades the eye is
relatively less sensitive than towards that of blue. In agreement with
this, the impression of glare, as the intensity increases, is feebler
in red than in blue. According to an observation of Dove, if a blue
and a red paper be chosen which appear of equal brightness under a
mean degree of white light, as the light is made much dimmer the blue
appears brighter, and as the light is much strengthened, the red. I
myself have found that the same differences are seen, and even in a
more striking manner, in the red and violet spectral colours, and, when
their intensity is increased only moderately, by the same fraction for
both.
Now the impression of white is made up of the impressions which the
individual spectral colours make on our eye. If we increase the
brightness of white, the strength of the sensation for the red and
yellow rays will relatively be more increased than that for the blue
and violet. In bright white, therefore, the former will produce a
relatively stronger impression than the latter; in dull white the blue
and bluish colours will have this effect. Very bright white appears
therefore yellowish, and dull white appears bluish. In our ordinary
way of looking at the objects about us, we are not so readily conscious
of this; for the direct comparison of colours of very different shade
is difficult, and we are accustomed to see in this alteration in the
white the result of different illumination of one and the same white
object, so that in judging pigment-colours we have learnt to eliminate
the influence of brightness.
If however to the painter is put the problem of imitating, with faint
colours, white irradiated by the sun, he can attain a high degree of
resemblance; for by an admixture of yellow in his white he makes this
colour preponderate just as it would preponderate in actual bright
light, owing to the impression on the nerves. It is the same impression
as that produced if we look at a clouded landscape through a yellow
glass, and thereby give it the appearance of a sunny light. The artist
will, on the contrary, give a bluish tint to moonlight, that is, a
faint white; for the colours on the picture must, as we have seen, be
far brighter than the colour to be represented. In moonshine scarcely
any other colour can be recognised than blue; the blue starry sky or
blue colours may still appear distinctly coloured, while yellow and red
can only be seen as obscurations of the general bluish white or grey.
I will again remind you that these changes of colour would not be
necessary if the artist had at his disposal colours of the same
brightness, or the same faintness, as are actually shown by the bodies
irradiated by the sun or by the moon.
The change of colour, like the scale of shade, previously discussed, is
a subjective action which the artist must represent objectively on his
canvas, since moderately bright colours cannot produce them.
We observe something quite similar in regard to the phenomena of
_Contrast_. By this term we understand cases in which the colour or
brightness of a surface appears changed by the proximity of a mass
of another colour or shade, and, in such a manner, that the original
colour appears darker by the proximity of a brighter shade, and
brighter by that of a darker shade; while by a colour of a different
kind it tends towards the complementary tint.
The phenomena of contrast are very various, and depend on different
causes. One class, _Chevreul’s simultaneous Contrast_, is independent
of the motions of the eyes, and occurs with surfaces where there are
very slight differences in colour and shade. This contrast appears both
on the picture and in actual objects, and is well known to painters.
Their mixtures of colours on the palette often appear quite different
to what they are on the picture. The changes of colour which are here
met with are often very striking; I will not, however, enter upon them,
for they produce no divergence between the picture and reality.
The second class of phenomena of contrast, and one which, for us, is
more important, is met with in changes of direction of the glance, and
more especially between surfaces in which there are great differences
of shade and of colour. As the eye glides over bright and dark, or
coloured objects and surfaces, the impression of each colour changes,
for it is depicted on portions of the retina which directly before were
struck by other colours and lights, and were therefore changed in their
sensitiveness to an impression. This kind of contrast is therefore
essentially dependent on movements of the eye, and has been called by
Chevreul, ‘_successive Contrast_.’
We have already seen that the retina is more sensitive in the dark to
feeble light than it was before. By strong light, on the contrary,
it is dulled, and is less sensitive to feeble lights which it had
before perceived. This latter process is designated as ‘Fatigue’ of
the retina; an exhaustion of the capability of the retina by its own
activity, just as the muscles by their activity become tired.
I must here remark that the fatigue of the retina by light does not
necessarily extend to the whole surface; but when only a small portion
of this membrane is struck by a minute, defined picture it can also be
locally developed in this part only.
You must all have observed the dark spots which move about in the field
of vision, when we have been looking for only a short time towards the
setting sun, and which physiologists call _negative after-images_ of
the sun. They are due to the fact that only those parts of the retina
which are actually struck by the image of the sun in the eye, have
become insensitive to a new impression of light. If, with an eye which
is thus locally tired, we look towards a uniformly bright surface, such
as the sky, the tired parts of the retina are more feebly and more
darkly affected than the other portions, so that the observer thinks he
sees dark spots in the sky, which move about with his sight. We have
then in juxtaposition, in the bright parts of the sky, the impression
which these make upon the untired parts of the retina, and in the dark
spots their action on the tired portions. Objects, bright like the
sun, produce negative after-images in the most striking manner; but
with a little attention they may be seen even after much more moderate
impressions of light. A longer time is required in order to develop
such an impression, so that it may be distinctly recognised, and a
definite point of the bright object must be fixed, without moving
the eye, so that its image may be distinctly formed on the retina,
and only a limited portion of the retina be excited and tired, just
as in producing sharp photographic portraits, the object must be
stationary during the time of exposure in order that its image may not
be displaced on the sensitive plate. The after-image in the eye is, as
it were, a photograph on the retina, which becomes visible owing to the
altered sensitiveness towards fresh light, but only remains stationary
for a short time; it is longer, the more powerful and durable was the
action of light.
If the object viewed was coloured, for instance red paper, the
after-image is of the complementary colour on a grey ground; in
this case of a bluish-green.[19] Rose-red paper, on the contrary,
gives a pure green after-image, green a rose-red, blue a yellow,
and yellow a blue. These phenomena show that in the retina partial
fatigue is possible for the several colours. According to Thomas
Young’s hypothesis of the existence of three systems of fibres in the
visual nerves,[20] of which one set perceives red whatever the kind
of irritation, the second green, and the third violet, with green
light, only those fibres of the retina which are sensitive to green
are powerfully excited and tired. If this same part of the retina is
afterwards illuminated with white light, the sensation of green is
enfeebled, while that of red and violet is vivid and predominant; their
sum gives the sensation of purple, which mixed with the unchanged white
ground forms rose-red.
[Footnote 19: In order to see this kind of image as distinctly as
possible, it is desirable to avoid all movements of the eye. On a large
sheet of dark grey paper a small black cross is drawn, the centre of
which is steadily viewed, and a quadrangular sheet of paper of that
colour whose after-image is to be observed is slid from the side, so
that one of its corners touches the cross. The sheet is allowed to
remain for a minute or two, the cross being steadily viewed, and it is
then drawn suddenly away, without relaxing the view. In place of the
sheet removed the after-image appears then on the dark ground.]
[Footnote 20: See Helmholtz’s _Popular Lectures_, first series, p. 250.]
In the ordinary way of looking at light and coloured objects, we
are not accustomed to fix continuously one and the same point; for
following with the gaze the play of our attentiveness, we are always
turning it to new parts of the object as they happen to interest us.
This way of looking, in which the eye is continually moving, and
therefore the retinal image is also shifting about on the retina,
has moreover the advantage of avoiding disturbances of sight, which
powerful and continuous after-images would bring with them. Yet here
also, after-images are not wanting; only they are shadowy in their
contours, and of very short duration.
If a red surface be laid upon a grey ground, and if we look from the
red over the edge towards the grey, the edges of the grey will seem as
if struck by such an after-image of red, and will seem to be of a faint
bluish green. But as the after-image rapidly disappears, it is mostly
only those parts of the grey, which are nearest the red, which show the
change in a marked degree.
This also is a phenomenon which is produced more strongly by bright
light and brilliant saturated colours than by fainter light and
duller colours. The artist however, works for the most part with the
latter. He produces most of his tints by mixture; each mixed pigment
is, however, greyer and duller than the pure colour of which it is
mixed, and even the few pigments of a highly saturated shade, which
oil-painting can employ, are comparatively dark. The pigments employed
in water-colours and coloured chalks are again comparatively white.
Hence such bright contrasts, as are observed in strongly coloured and
strongly lighted objects in nature, cannot be expected from their
representation in the picture. If, therefore, with the pigments at his
command, the artist wishes to reproduce the impression which objects
give, as strikingly as possible, he must paint the contrasts which they
produce. If the colours on the picture are as brilliant and luminous as
in the actual objects, the contrasts in the former case would produce
themselves as spontaneously as in the latter. Here, also, subjective
phenomena of the eye must be objectively introduced into the picture,
because the scale of colour and of brightness is different upon the
latter.
With a little attention you will see that painters and draughtsmen
generally make a plain uniformly lighted surface brighter, where it is
close to a dark object, and darker, where it is near a light object.
You will find that uniform grey surfaces are given a yellowish tint
at the edge where there is a background of blue, and a rose-red tint
where they impinge on green, provided that none of the light collected
from the blue or green can fall upon the grey. Where the sun’s rays
passing through the green leafy shade of trees strike against the
ground, they appear to the eye, tired with looking at the predominant
green, of a rose-red tint; the whole daylight, entering through a slit,
appears blue, compared with reddish-yellow candle-light. In this way
they are represented by the painter, since the colours of his pictures
are not bright enough to produce the contrast without such help.
To the series of subjective phenomena, which artists are compelled to
represent objectively in their pictures, must be associated certain
phenomena of _irradiation_. By this is understood cases in which
any bright object in the field spreads its light or colour over the
neighbourhood. The phenomena are the more marked the brighter is
the radiating object, and the halo is brightest in the immediate
neighbourhood of the bright object, but diminishes at a greater
distance. These phenomena of irradiation are most striking around a
very bright light on a dark ground. If the view of the flame itself
is closed by a narrow dark object such as the finger, a bright misty
halo disappears, which covers the whole neighbourhood, and, at the same
time, any objects there may be in the dark part of the field of view
are seen more distinctly. If the flame is partly screened by a ruler,
this appears jagged where the flame projects beyond it. The luminosity
in the neighbourhood of the flame is so intense, that its brightness
can scarcely be distinguished from that of the flame itself; as is the
case with all bright objects, the flame appears magnified, and as if
spreading over towards the adjacent dark objects.
The cause of this phenomenon is quite similar to that of aërial
perspective. It is due to a diffusion of light which arises from the
passage of light through dull media, excepting that for the phenomena
of aërial perspective the turbidity is to be sought in the air in front
of the eye, while for true phenomena of irradiation it is to be sought
in the transparent media of the eye. When even the healthiest human
eye is examined by powerful light, the best being a pencil of sunlight
concentrated on the side by a condensing lens, it is seen that the
sclerotica and crystalline lens are not perfectly clear. If strongly
illuminated, they both appear whitish and as if rendered turbid by a
fine mist. Both are, in fact, tissues of fibrous structure, and are
not therefore so homogeneous as a pure liquid or a pure crystal. Every
inequality, however small, in the structure of a transparent body can,
however, reflect some of the incident light--that is, can diffuse it in
all directions.[21]
[Footnote 21: I disregard here the view that irradiation in the eye
depends on a diffusion of the excitation in the substance of the
nerves, as this appears to me too hypothetical. Moreover, we are here
concerned with the phenomena and not with their cause.]
The phenomena of irradiation also occur with moderate degrees of
brightness. A dark aperture in a sheet of paper illuminated by the sun,
or a small dark object on a coloured glass plate which is held against
the clear sky, appear as if the colour of the adjacent surface were
diffused over them.
Hence the phenomena of irradiation are very similar to those which
produce the opacity of the air. The only essential difference lies
in this, that the opacity by luminous air is stronger before distant
objects which have a greater mass of air in front of them than before
near ones; while irradiation in the eye sheds its halo uniformly over
near and over distant objects.
Irradiation also belongs to the subjective phenomena of the eye which
the artist represents objectively, because painted lights and painted
sunlight are not bright enough to produce a distinct irradiation in the
eye of the observer.
The representation which the painter has to give of the lights and
colours of his object I have described as a translation, and I have
urged that, as a general rule, it cannot give a copy true in all
its details. The altered scale of brightness which the artist must
apply in many cases is opposed to this. It is not the colours of the
objects, but the impression which they have given, or would give,
which is to be imitated, so as to produce as distinct and vivid a
conception as possible of those objects. As the painter must change
the scale of light and colour in which he executes his picture, he
only alters something which is subject to manifold change according
to the lighting, and the degree of fatigue of the eye. He retains the
more essential, that is, the _gradations_ of brightness and tint. Here
present themselves a series of phenomena which are occasioned by the
manner in which the eye replies to an external irritation; and since
they depend upon the intensity of this irritation they are not directly
produced by the varied luminous intensity and colours of the picture.
These objective phenomena, which occur on looking at the object, would
be wanting if the painter did not represent them objectively on his
canvas. The fact that they are represented is particularly significant
for the kind of problem which is to be solved by a pictorial
representation.
Now, in all translations, the individuality of the translator plays a
part. In artistic productions many important points are left to the
choice of the artist, which he can decide according to his individual
taste, or according to the requirements of his subject. Within certain
limits he can freely select the absolute brightness of his colours, as
well as the strength of the shadows. Like Rembrandt, he may exaggerate
them in order to obtain strong relief; or he may diminish them, with
Fra Angelico and his modern imitators, in order to soften earthly
shadows in the representation of sacred objects. Like the Dutch school,
he may represent the varying light of the atmosphere, now bright
and sunny, and now pale, or warm and cold, and thereby evoke in the
observer moods which depend on the illumination and on the state of the
weather; or by means of undisturbed air he may cause his figures to
stand out objectively clear as it were, and uninfluenced by subjective
impressions. By this means, great variety is attained in what artists
call ‘style’ or ‘treatment,’ and indeed in their purely pictorial
elements.