COLOUR AND ITS PERCEPTION.
It was explained in the last chapter that we see things through the agency
of the light--emitted or reflected--which proceeds from them to the eye,
and is suitably distributed over the retina by the action of a system of
lenses.
Now the "image" thus formed is not generally perceived as a simple
monochromatic one, darker in some parts, lighter in others, like a black
and white engraving. It is, in most cases at least, characterised by a
variety of colours, the light which comes from different objects, or from
different parts of the same object, having the power of exciting different
colour sensations. Light which has the property of exciting the sensation
of any colour is commonly spoken of as coloured light. The light reflected
by a soldier's coat, for example, may be called red light, because when it
falls upon the eye it gives rise to a sensation of redness. But it must be
understood that this mode of expression is only a convenient abbreviation,
for there can, of course, be no objective colour in the light or
"radiation" itself.
Wherein, then, does coloured light differ from white? Why do things appear
to be variously coloured when illuminated by light which is colourless?
And how do coloured lights affect the visual organs so as to evoke
appropriate sensations? These are questions--the first two of a physical
character, the last partly physiological and partly psychological--which
it is now proposed to discuss.
The matter has already been touched upon, though very slightly, in
connection with the spectrum. Let us again turn to the spectrum and
consider it a little more fully.
It is easily seen that the luminous band contains six principal hues or
tones of colour--red, orange, yellow, green, blue, and violet. (See Fig.
1, page 12.) These however merge into one another so gradually that it is
impossible to say exactly where any one colour begins and ends. Look, for
instance, at the somewhat narrow but very conspicuous stripe of yellow.
Towards the right of this stripe the colour gradually becomes
greenish-yellow; a little further on it is yellowish-green, and at
length, by insensible gradations, a full, pure green is reached.
The six most prominent hues of the spectrum are, in fact, supplemented by
an immense multitude of subordinate ones, the total number which the eye
can recognise as distinct being not less than a thousand. All the colours
that we see in nature, with the exception of the purples (about which I
shall say more presently), are here represented, and every single variety
of tone in the prismatic scale corresponds with one, and only one,
definite wave-length of light.
The source of all these colours is, as we know, a beam of white or
colourless light, the constituents of which have been sorted out and
arranged so that they fall side by side upon the screen in the order of
their several wave-lengths. If, then, these coloured constituents were
all mixed together again, it would be reasonable to expect that pure white
light would be reproduced.
The experiment has been performed in a great many different ways, several
of which were devised by Newton himself, and the result admits of no doubt
whatever. The method which I intend to describe is not quite so simple as
some others, but it has great advantages in the way of convenient
manipulation, and affords the means of demonstrating a number of
interesting colour effects in an easily intelligible manner. By the simple
operation of moving aside a lens out of the track of the light, we can
gather up and thoroughly mix together all the variously coloured rays of
the spectrum and cause them to form upon the screen a bright circular
patch, which, though due to a mixture of a thousand different hues, is
absolutely white. When the lens is replaced, which is done in an instant,
the mixture is again analysed into its component parts, and the spectrum
reappears.
The arrangement of the apparatus, which is essentially the same as that
devised by Captain Abney, and called by him the "colour-patch apparatus,"
is shown in the annexed diagram (Fig. 3).
[Illustration: _Fig. 3.--Abney's Colour-patch Apparatus._]
The light of an electric lamp A placed inside the lantern is concentrated
by the condensing lenses B upon a narrow adjustable slit C. The framework
of this slit is attached to one end of a telescope tube, which carries at
the other end an achromatic lens D of about 10 inches focus. The rays
having been rendered parallel by D are refracted by the prism E; they
then pass through a circular opening in the brass plate F to the lens G,
the focal length of which is 7 inches, and form a little bright spectrum
upon a white card held in a grooved support at H. The card being removed,
we place at K a lens having a diameter of 5-1/2 inches and a focal length
of 18 inches or more, and adjust it so that a sharply defined image of the
hole in the brass plate F is formed upon the distant white screen L. If
all the lenses are correctly placed, this image, though formed entirely by
the rays which constituted the little spectrum at H, will be perfectly
free from colour even around the edge.
If we wish to project upon the screen L an enlarged image of the little
spectrum, we have only to use another suitable lens I in conjunction with
K: the diameter of that used by myself is 2-3/4 inches, and its focal
length 6-1/2 inches. When we have once found by trial the position in
which this supplementary lens gives the clearest image[4] it is easy to
arrange a contrivance for removing and replacing it correctly without need
of any further adjustment.
This apparatus shows then that ordinary white light may be regarded as a
mixture of all the variously coloured lights which occur in the spectrum,
the sensation produced when it falls upon the eye being consequently a
compound one.
From these and similar experiments the scientific neophyte is not unlikely
to draw an erroneous conclusion. White light, he is apt to think, is
_always_ due to the combined action of rays of every possible wave-length,
while coloured light consists of rays of one definite wave-length only.
Neither of these inferences would be correct. It is not true that white
light necessarily contains rays of all possible wave-lengths: the
sensation of whiteness may, as will be shown by and bye, be produced quite
as effectively by the combination of only two or three different
wave-lengths. Nor is it true that such colours as we see in nature are
always due to light of a single wave-length; light of this kind is indeed
rarely met with outside laboratories and lecture rooms. Far more commonly
coloured light consists of mixed rays, and like ordinary white light, it
may, and generally does, contain all the colours of the spectrum, but in
different proportions.
This last assertion is easily proved. By means of a slip of card we may
intercept a portion of the little spectrum formed at H (Fig. 3). The dark
shadow of the card in the enlarged spectrum on the screen is shown in Fig.
4. It will be noticed that the shadow cuts off a part only of the red,
orange, and yellow light, allowing the remainder to pass through the
projection lenses. There are still rays of every possible wave-length from
extreme red to extreme violet, but the proportion of those towards the red
end is less than it was before the card was interposed.
[Illustration: _Fig. 4.--Partially intercepted Spectrum._]
If now we remove the lens I (Fig. 3) and so mix the colours of this
mutilated spectrum, the bright round patch where the mixed rays fall upon
the screen will no longer appear white but greenish-blue. If we transfer
the card to the other end of the little spectrum, so as to cause a partial
eclipse of the violet, blue, and green rays, the colour of the patch will
be changed to orange. If we remove the card altogether, the patch will
once more become white.
It follows _a fortiori_ that when any portion of the little spectrum is
eclipsed totally, instead of only partially, the light from the remainder
will appear, when combined, to be coloured. Very beautiful changes of hue
are exhibited by the bright patch when a narrow opaque strip, such as the
small blade of a pocket knife, is slowly moved along the little spectrum
at H, eclipsing different portions of it in succession. The patch first
becomes green, then by imperceptible gradations it changes successively to
blue, purple, scarlet, orange, yellow, and finally, when the knife has
completed its course, all colour disappears and the patch is again white.
We may improve upon this crude experiment, and, after Captain Abney's
plan, prepare a number of small cardboard stencils, with openings
corresponding to any selected parts of the little spectrum. When a card so
prepared is placed at H (Fig. 3) the bright patch upon the screen is
formed by the combination of the selected rays, all the others being
quenched. We shall find that under these conditions the bright patch is
generally, but not always, coloured.
[Illustration: _Fig. 5.--Stencil Cards._]
The first diagram in Fig. 5 represents a blackened card, which allows
only the red and a little of the orange to pass through. When this is
inserted in the grooved holder at H, the bright patch immediately turns
red. The second diagram shows another, which transmits the middle portion
of the spectrum, but blocks the red and the violet at its two ends: with
this card the colour of the patch becomes green. The third card has
openings for the violet and the red rays: this turns the patch a beautiful
purple, a hue which, as already mentioned, is not produced by light of any
single wave-length. The purples are mixtures of red and violet or of red
and blue.
Now I have in my possession three pieces of glass (or, to be strictly
accurate, two pieces of glass and one glass-mounted gelatine film) which,
when placed transversely in the beam of light, either at H (Fig. 3) or
anywhere else, behave exactly like these three cardboard stencils. The
first glass cuts off all the spectrum except the red and part of the
orange, just as the first stencil does, though the line of demarcation is
not quite so sharp. This is in fact a piece of red glass, or in other
words the light that it transmits produces the sensation of red. The
second glass, like the second stencil, allows the whole of the spectral
rays to pass freely except the red and the violet, which disappear as if
they were obstructed by an opaque body. This is a green glass. And the
third (which is really a film of gelatine) cuts out the middle of the
spectrum but transmits the red and violet ends. The colour of the gelatine
is purple.[5]
The glasses and the gelatine in question act like the cardboard stencils
in completely cutting off some of the spectral rays and transmitting
others, and they owe their apparent colours to the combined influence
which the transmitted rays exert upon the eye. Many other coloured glasses
merely weaken some of the rays, without entirely quenching any. A piece of
pale yellow glass, for example, when placed in the path of the beam of
light from which the spectrum on the screen is formed, simply diminishes
the brightness of the blue region and does not wholly quench any of the
rays; and again, a common kind of violet-coloured glass enfeebles, but
does not quite obliterate, the middle portion of the spectrum.
From such observations as these we infer that the glasses derive their
respective colours from the light which falls upon them. The first glass
would not appear red if seen in a light which contained no red rays. This
is easily proved by an experiment with the colour-patch apparatus. The
spectrum being once more combined into a bright white patch (which turns
red if the glass is for a moment interposed), let all the red rays and
part of the orange be cut off with a suitable stencil. The re-combined
light is no longer white but greenish-blue, as is evidenced by the colour
of the patch; and nothing that is illuminated by this light can possibly
appear red. The piece of red glass, if placed in the beam, will now cast a
perfectly black shadow, and a square of bright red paper held in the
middle of the patch will look as black as ink. It will be shown later how
we may obtain light which, although it appears to the eye to differ in no
respect from ordinary white daylight, yet contains no red component, and
is consequently as powerless as this greenish-blue light to reveal any red
colour in the objects which it illuminates.
If we substitute a stencil which admits only red rays, we shall obtain a
beam of light in which no colour but red can be seen. Green and blue
glasses when exposed to this light will cast black shadows, while pieces
of green and blue paper will become either black or dark grey.
We see then that the colours of transparent objects, like the glasses used
in these experiments, are brought out by a process of filtration. Certain
of the coloured ingredients of white light are filtered out and quenched
inside the glass, and it is to the remaining ingredients which pass
through unimpeded that the observed colour is due. The energy of the
absorbed rays is not lost of course, for energy, like matter, is
indestructible. It is transformed into heat. A coloured glass held in a
strong beam of light will in a short time become sensibly warmer than one
that is clear and colourless.
In studying colour effects as produced by coloured glasses, we have at the
same time been learning how the great majority of natural objects--not
only those which are transparent but also those called opaque--become
possessed of their colours. For the truth is that few things are perfectly
opaque. When white light falls upon a coloured body, it generally
penetrates to a small depth below the surface, and in so doing loses by
absorption some of its coloured components, just as it does in passing
through the pieces of glass. But before it has gone very far--generally
much less than a thousandth part of an inch--it has encountered a number
of little reflecting surfaces due to optical irregularities, which turn
the light back again and compel it to pass a second time through the same
thickness of the substance: it thus becomes still more effectively sifted,
and on emerging is imbued with a colour due to such of the components as
have not been quenched in the course of their double journey through a
superficial layer of the substance.
Any coloured rays reflected by an object must necessarily be contained in
the light by which the object is seen. The following is a curious
experiment illustrating this.
A large bright spectrum is projected upon a screen and in the green or
blue portion of it is held a wall poster. The letters and figures upon the
paper are seen to stand out boldly as if printed with the blackest ink.
But if the poster is moved into the red part of the spectrum, the printing
at once disappears as if by magic, and the paper appears perfectly blank.
The explanation is that the letters are printed in red ink--they can
reflect no light but red. Green or blue light falling upon them is
absorbed and quenched, and the letters consequently appear black. On the
other hand when the poster is illuminated by the red rays of the
spectrum, the letters reflect just as much light as the paper itself, and
are therefore indistinguishable from it.
Anything which, when illuminated by a source of white light, reflects all
its various components equally and without absorbing a larger proportion
of some than of others, appears white or grey. Between white and grey
there is no essential difference except in luminosity, or brightness, that
is to say, in the quantity of light reflected to the eye, or--to go a step
further back--in the amplitude of the ether waves. Under different
conditions of illumination any substance which reflects all the rays of
the spectrum equally may appear either white or grey, or even black. A
snowball can easily be made to look blacker than pitch, and a block of
pitch whiter than snow.
It must have struck many of those who have thought about the matter at all
as a most remarkable coincidence that sunlight should be white. White
light, as we have seen, consists of a mixture of variously-coloured rays
in very different and apparently arbitrary proportions, and if these
proportions were a little changed the light would no longer be quite
colourless. No ordinary artificial light is so exactly white as that of
the sun. The light of candles, gas, oil, and electric glow-lamps is
yellow; that of the electric arc (when unaffected by atmospheric
absorption) is blue, and that of the incandescent gas burner green. It is
exceedingly convenient that the light which serves us for the greater
part of our waking lives should happen to be just so constituted that it
is colourless.
But on a little further reflection it will, I think, appear that this is
not the right way to look at the matter. It is precisely because the hue
called white is the one which is associated with the light of our sun that
we regard whiteness as synonymous with absence of colour. We take sunlight
as our standard of neutrality, and anything that reflects it without
altering the proportions of its constituents we consider as being
colourless.
There can be little doubt that if the sun were purple instead of white,
our sentiments as regards these two hues would be interchanged; we should
talk quite naturally of "a pure purple, entirely free from any trace of
colour," or perhaps describe a lady's costume as being of a "gaudy white."
Even as things are, the standard of neutrality is not quite a hard and
fast one. We have a tendency to regard any artificial light which we may
happen to be using, as more free from colour than it would turn out to be
if compared directly with sunlight. If in the middle of the day we go
suddenly into a gas-lit room, we cannot fail to observe how intensely
yellow the illumination at first appears; in a few minutes, however, the
colour loses its obtrusiveness and we cease to take much notice of it.
The effect may be partly a physiological one, depending upon unequal
fatigue of the various perceptive nerves of the retina; but I believe that
it is to a large extent due to mental judgment. The standard of
whiteness, or colour-zero, can apparently be changed within certain limits
in a very short time, and, as we shall see later, this is only one of many
instances in which our organs of vision seem to be incapable of
recognising a constant standard of reference.
And now let us consider how it comes about that each elementary portion of
the retina--at least in its central region--has the power of
distinguishing so many hundreds of different hues. It is incredible that
every little area of microscopic dimensions should be furnished with such
a multitude of independent organs as would be necessary if each of the
many colours met with in nature required a separate organ for its
perception; and it is not necessary to suppose anything of the kind.
Experiment shows that all the various hues of the spectrum, as well as all
(including white) that can be formed from their mixture, may be derived
from no more than three distinct colours. There are, in fact, an
indefinite number of triads of colours which, in suitable combinations,
are capable of producing the sensation of every tone, tint, and shade of
colour which the eye of man has ever beheld.
Old-fashioned books, such as an early edition of Ganot's "Physics," tell
us that the three "primary" colours are red, yellow, and blue, and that
all others are produced by mixtures of these. This was the basis of Sir
David Brewster's theory, which attained a very wide popularity, and even
at the present time is held as an article of faith among the great
majority of intelligent persons who have not paid any special attention to
science. But it is not true. A fatal objection to it is the
well-ascertained fact that no combination of red, yellow, and blue, or of
any two of them, such as blue and yellow, for example, will produce green.
Yet every painter knows that if he mixes blue and yellow pigments together
he gets green. That is one of the first things that a child learns when he
is allowed to play with a box of water-colours, and no doubt Brewster was
misled by the fact.
The truth is, that the colours of all, or almost all, known blue and
yellow pigments happen to be composite. An ordinary blue paint reflects
not only blue light, but a large quantity of green as well; while an
ordinary yellow paint reflects a large quantity of green light in addition
to yellow. When such paints are mixed together, the blue and yellow hues
neutralise one another, and only the green, which is common to both,
remains.
The spectrum apparatus will make this clearer. Hold a piece of bright blue
glass before the slit; the light passing through the glass will be
analysed by the prism, and you will see that it really contains almost as
much green as blue. If a yellow glass is substituted, not only will yellow
light be transmitted, but, as before, a considerable quantity of green. If
now both glasses be placed together before the slit, what will happen? The
yellow glass will stop the blue light transmitted by the blue glass, the
blue glass will stop the yellow light transmitted by the yellow glass, and
only the green light which both glasses have the power of transmitting
will pass through unimpeded, forming a band of pure green colour upon the
screen.
The combination of simple blue and yellow lights of suitable relative
luminosities results in the formation of white or neutral light. If the
blue is a little in excess, the combined light will be of a bluish tint;
if the yellow is in excess, the combination will have a yellowish tint. It
will never contain any trace of green. The combination of simple spectral
blue and yellow is easily effected by the colour-patch apparatus, and the
result will be found to bear out what has been said.
Since, then, no mixture of red, yellow, and blue, or of any two of them,
will produce green, we cannot regard these colours as being, in
Brewster's sense of the term, primary ones.
But it is quite possible to find a group of three different hues--and
indeed many such groups--which when made to act upon the eye
simultaneously and in the right proportions can give rise to the sensation
of any colour whatever. Now this experimental fact is obviously suggestive
of a possible converse, namely, that almost every colour sensation may in
reality be a compound one, the resultant of not more than three simple
sensations. Assuming this to be so, it is evident that if each elementary
area of the retina were provided with only three suitable colour organs,
nothing more would be requisite for the perception of an indefinite number
of distinct colours.
Such a hypothesis was first proposed by Thomas Young at the beginning of
the present century; but it came before its time and met with no attention
until fifty years later, when it was unearthed by the distinguished
physicist and physiologist, Helmholtz, who accorded to it his powerful
support and modified it in one or two important details.
[Illustration: _Fig. 6.--Helmholtz's Curves of Colour Perception._]
According to the Young-Helmholtz theory, as it is now called, there are
three different kinds of nerve-fibres distributed over the retina. The
first, when separately stimulated, produce the sensation of red, the
second that of green, and the third that of violet. Light having the same
wave-length as the extreme red rays of the spectrum stimulates the red
nerve-fibres only; that having the same wave-length as the extreme violet
rays stimulates the violet nerve-fibres only. Light of all intermediate
wave-lengths, corresponding to the orange, yellow, green, and blue of the
spectrum, stimulates all three sets of nerve-fibres at once, but in
different degrees. The proportionate stimulation of the red, green, and
violet nerves throughout the spectrum is indicated in Fig. 6, which is
derived from the rough sketch first given by Helmholtz. The yellow rays of
the spectrum, it will be seen, excite the red and green nerves strongly,
and the violet feebly; green light excites the green nerves strongly, and
the red and violet moderately; while blue light excites the green and
violet nerves strongly, and the red feebly.
[Illustration: _Fig. 7.--König's Curves._]
Fig. 7 shows another set of curves given more recently by Dr. König as the
result of many thousands of experiments made, not only upon persons whose
vision was normal, but also upon some who were colour-blind. König found
that the equations he obtained were best satisfied by assuming as the
normal fundamental sensations a purplish red (not to be found in the
spectrum), a green like that of wave-length 5050, and a blue like that of
wave-length 4700 approximately, the two latter, however, being purer or
more saturated than any actual spectrum colour. But König's curves are not
consistent with every class of vision which he examined, and the question
as to what are the true fundamental colour-sensations, if such really
exist at all, cannot yet be regarded as finally settled.[6]
The Young-Helmholtz theory of colour-vision, whether or not it is destined
in the future to be superseded by some other, has at all events proved an
invaluable guide in experimental work, and there are very few colour
phenomena of which it is not competent to offer a satisfactory
explanation. It has at present only one serious rival--the theory of
Hering, which, although it seems to be curiously attractive to many
physiologists, can hardly be said to present less serious difficulties
than that which it seeks to displace. Neither of these competing theories
has yet had its fundamental assumptions confirmed by any direct evidence,
and the advantage must rest with the one which best accords with the facts
of colour vision. In my judgment the older of the two is to be greatly
preferred as a useful working hypothesis.
Certain curiosities of vision with which I propose to deal in a future
chapter depend upon the properties of what are known as complementary
colours. Two colours are said to be complementary to each other when their
combination in proper proportions results in the formation of white.
[Illustration: _Fig. 8.--Stencil Card for Complementary Colours._]
If we produce a compound hue by mixing together the colours of any portion
of the spectrum, and a second compound hue by mixing the remainder of the
spectrum, it must be evident that these two hues are necessarily
complementary, for when they are united they contain together all the
elements of the entire spectrum, and therefore appear as white. This may
be illustrated with the aid of the colour-patch apparatus. Place at H
(Fig. 3) a cardboard stencil of the form shown in Fig. 8, and focus upon
it a little spectrum, the principal hues of which are indicated by the
letters R O Y G B V (red, orange, yellow, green, blue, violet). The two
oblong apertures in the card should be of exactly the same height, and the
card so placed that one aperture may admit rays extending from the red end
of the spectrum to about the middle of the green, while the other admits
rays from the remainder of the spectrum. If now the lower aperture be
covered, only the red, orange, yellow, and part of the green rays will
pass through the stencil, and these being combined by the lens K (Fig. 3)
will form upon the screen a bright patch, the colour of which will be
yellow. If the upper aperture be covered, and the rest of the green,
together with the blue and violet rays, allowed to pass through the other,
the colour of the patch will become blue; and if both apertures be
uncovered at the same time, rays from the whole length of the spectrum
will pass through the stencil, and the patch will, of course, turn white.
The yellow and the blue which were compounded from the two portions of the
spectrum are, therefore, in accordance with the definition, complementary
colours.
In a similar manner by dividing the spectrum into any two portions
whatever--as, for example, by the complicated stencil shown in Fig. 9--we
can obtain an indefinite number of pairs of complementary colours.
[Illustration: _Fig. 9.--Stencil Card for Complementary Colours._]
But it is by no means indispensable that both or either of a pair of
complementary colours should be compound. To prove this, two strips of
card with narrow vertical openings A and B are prepared as shown in Fig.
10. The cards are placed one above the other and can be slipped in a
horizontal direction, so that the narrow openings can be brought into any
desired part of the spectrum which is indicated in outline by the dotted
oblong.
[Illustration: _Fig. 10.--Slide for mixing any two Spectral Colours._]
Bring the opening A of the upper card into the yellow of the spectrum and
the opening B of the lower card into the blue. The bright patch formed
upon the screen will then be illuminated by simple blue and yellow rays;
yet it will be white--not green, as it would be if Brewster's theory were
correct. If upon the first trial the white should not be absolutely pure,
it can easily be made so by partially covering either A or B--the first if
the white is yellowish, the second if it is bluish. Simple spectral blue
and yellow are therefore no less truly complementary colours than are the
compound hues formed when the spectrum is divided into two parts.
It is noticeable, however, that the white light resulting from the
combination of blue and yellow, though it cannot be distinguished by the
eye from ordinary white light, is yet possessed of very different
properties. Most coloured objects when illuminated by it have their hues
greatly altered; a piece of ribbon, for example, which in common light is
bright red, will appear when held in the blue-yellow light to be of a dark
slate colour, almost black.
If the opening A is placed in any part whatever of the spectrum except the
green, it will always be possible, by moving B backwards or forwards, to
find some other part where the colour is complementary to that at A. To
green there is no simple complementary; a purple is required, which is
not found in the spectrum, but may be formed by combining small portions
of spectral blue and red. For studying mixtures of three simple colours, a
third slide may be added to the two shown in Fig. 10.
The following little table gives the principal pairs of complementary
colours.
TABLE OF COMPLEMENTARY COLOURS.
Red Greenish-blue
Orange Sky-blue
Yellow Blue
Greenish-yellow Violet
Green Purple