An independent investigator of this subject was Clerk Maxwell, who
experimented with a “colour-box” of his own design, by which he mixed
the simple colours of the spectrum, and the results he got are really
the first which are founded on measurement. He measured something,
but hardly arrived at the colour sensation. His colour-box took two
forms, both on the same principles, so only one will be here described,
the diagram and description being taken from his classic paper in the
Philosophical Transactions of the Royal Society for 1860.
“The experimental method which I have used consists in forming a
combination of three colours belonging to different portions of the
spectrum, the quantity of each being so adjusted that the mixture shall
be white, and equal in intensity to a given white. Fig. 14 represents
the instrument for making the observations. It consists of two tubes,
or long boxes of deal, of rectangular section, joined together at an
angle of about 100°.
[Illustration: FIG. 14.
Maxwell’s colour-box.]
“The part A K is about five feet long, seven inches broad, and four
deep; K N is about two feet long, five inches broad, and four deep; B D
is a partition parallel to the side of the long box. The whole of the
inside of the instrument is painted black, and the only openings are
at the end A C, and at E. At the angle there is a lid, which is opened
when the optical parts have to be adjusted or cleaned.
“At E is a fine vertical slit, L is a lens; at P there are two
equilateral prisms. The slit E, the lens L, and the prisms P are so
adjusted, that when light is admitted at E, a pure spectrum is formed
at A B, the extremity of the long box. A mirror at M is also adjusted
so as to reflect the light from E, along the narrow compartment of the
box to B C.
“At A B is a rectangular frame of brass, having a rectangular aperture
of six inches by one. On this frame are placed six brass sliders,
X Y Z. Each of these carries a knife-edge of brass in the plane of the
surface of the frame.
“These six movable knife-edges form three slits, X Y Z, which may be so
adjusted as to coincide with any three portions of the pure spectrum
formed by light from E. The intervals behind the sliders are closed by
hinged shutters, which allow the sliders to move without letting light
pass between them.
“The inner edge of the brass frame is graduated to twentieths of an
inch, so that the position of any slit can be read off. The breadth of
the slit is ascertained by means of a wedge-shaped piece of metal, six
inches long, and tapering to a point from a width of half an inch. This
is gently inserted into each slit, and the breadth is determined by the
distance to which it enters, the divisions on the wedge corresponding
to the 200th of an inch difference in breadth, so that the unit of
breadth is ·005 inch.
“Now suppose light to enter at E, to pass through the lens, and to be
refracted by the two prisms at P, a pure spectrum, showing Fraunhofer’s
lines, is formed at A B, but only that part is allowed to pass which
falls on the three slits, X Y Z. The rest is stopped by the shutters.
Suppose that the portion falling on X belongs to the red part of the
spectrum; then, of the white light entering at E, only the red will
come through the slit X. If we were to admit red light at X, it would
be refracted to E, by the principle in optics that the course of the
ray may be reversed.
“If, instead of red light, we were to admit white light at X, still
only red light would come to E; for all other light would be either
more or less refracted, and would not reach the slit at E. Applying the
eye at the slit E, we should see the prism P uniformly illuminated with
red light, of the kind corresponding to the part of the spectrum which
falls on the slit X, when white light is admitted at E.
“Let the slit Y correspond to another portion of the spectrum, say the
green; then if white light is admitted at Y, the prism, as seen by
an eye at E, will be uniformly illuminated with green light; and if
white light be admitted at X and Y simultaneously, the colour seen at
E will be a compound of red and green, the proportions depending on
the breadth of the slits and the intensity of the light which enters
them. The third slit Z, enables us to combine any three kinds of light
in any given proportions, so that an eye at E shall see the face of
the prism at P, uniformly illuminated with the colour resulting from
the combination of the three. The position of these three rays in
the spectrum is found by admitting the light at E, and comparing the
position of the slits with the position of the principal fixed lines;
and the breadth of the slits is determined by means of the wedges.
“At the same time, white light is admitted through B C to the mirror
of black glass at M, whence it is reflected to E, past the edge of
the prism at P, so that the eye at E sees through the lens a field
consisting of two portions, separated by the edge of the prism; that
on the left hand being compounded of three colours of the spectrum
refracted by the prism, while that on the right hand is white light
reflected from the mirror. By adjusting the slits properly, these two
portions of the field may be made equal, both in colour and brightness,
so that the edge of the prism becomes almost invisible.
“In making experiments, the instrument was placed on a table in a room
moderately lighted, with the end A B turned towards a large board
covered with white paper, and placed in the open air, so as to be
uniformly illuminated by the sun. In this way the three slits and the
mirror M were all illuminated with white light of the same intensity,
and all were affected in the same ratio by any change of illumination;
so that if the two halves of the field were rendered equal when the sun
was under a cloud, they were found nearly correct when the sun again
appeared. No experiments, however, were considered good unless the sun
remained uniformly bright during the whole series of experiments.
“After each set of experiments light was admitted at E, and the
position of the fixed lines D and F of the spectrum was read off on the
scale at A B. It was found that after the instrument had been in use
some time these positions were invariable, showing that the eye-hole,
the prisms, and the scale might be considered as rigidly connected.”
[Illustration: FIG. 15.]
With this instrument he made mixtures of three colours, to match with
white. By shifting the slits into various positions and taking as his
three standard colours a red near the C line, a green near E, and a
blue between F and G (see frontispiece), he obtained a variety of
matches, from which he formed equations. After eliminating, or rather
reducing the errors to the most probable value by the method of least
squares, he got from his matches with white a table of colour values
in terms of the three standard colours, from which the diagram of the
spectrum (Fig. 15) was made. (The heights of the dotted curves are
derived from the widths of the slits, and the continuous curve is
the sum of these heights.) Now what appears to be a properly chosen
colour does not necessarily stimulate only one sensation. Indeed the
probabilities are against it, except in the extreme red and extreme
violet. If colours intermediate to the standard colours be matched by a
mixture of the latter, we do not arrive at any solution of the amount
of stimulation of each sensation, since the chosen standard colours
themselves may be due to a stimulation of all three sensations. As a
matter of fact, Clerk Maxwell chose colours which do not best represent
the colour sensations. The red is too near the yellow, as is also the
green. The blue should also be nearer the violet end of the spectrum
than the position which he chose for it. We may take it, then, that
except as a first approximation, Clerk Maxwell’s diagrams need not be
seriously taken into account. The diagram itself shows that the colour
_sensations_ are not represented by the colours he chose. Supposing
any one in whom the sensation of green is absent were examining the
spectrum, there would, according to the diagram, be no light visible
at the green at E. Anticipating for a moment what we shall deal with
in detail shortly, it may be stated that in cases where it is proved
that a green sensation is absent, there is no position in any part of
the spectrum where there is an absence of light. Had he chosen any
other green, the same criticism would have been valid. The diagram as
it stands is really a diagram of _colour mixtures_ in terms of three
arbitrarily chosen colours, and not of colour _sensations_. It merely
indicates what proportions were needed of the three colours, which
he took as standards, to match the intermediate spectrum colours.
The negative sign in some of the equations--given in the appendix,
page 201--may be somewhat puzzling to those who have not made colour
matches, but not to those who have actually made experiments. It
means that where it is present no match of colour by a mixture of
the standard colours is possible; and that it would be only possible
if a certain quantity of the colour to which is attached a negative
sign were to be abstracted--an impossible condition to fulfil, but
one which may often occur in colour-matching experiments. Later you
will find that when colours are chosen as standards so that the
resulting equations give no negative sign for any colour, we have a
criterion as to the colours which give the nearest approach to the
true sensations. The next diagram (Fig. 16) of colour sensations is
due to Kœnig, who investigated the subject with Von Helmholtz. By a
modified method, which perhaps need not be explained in detail here,
he produced them, and they must be apparently not far from the actual
state of things, supposing this theory be proved to be true. For my
own part, I am under the impression that the positions of the colours
which most nearly approach the colour sensations might be slightly
altered in regard to the green and the blue, for reasons that will
subsequently be given when the later experiments of General Festing
and myself come to be described. For the immediate purpose of the
lecture, the curves are sufficiently accurate, and I will ask you to
notice what they tell us. It is presupposed in these diagrams that,
if the three colour-perceiving apparatus are equally stimulated, a
sensation of white will be produced; and the reverse, of course, is
true, in that white will give rise to equal stimulation of the three
apparatus. It follows, then, that in the parts of the spectrum where
all three curves of sensation are seen to take a part in the production
of a colour, such as at the E line, the colour is really due to the
extra stimulation of one or two of the apparatus above that required
to produce a certain amount of white. _The colour in every part of
the spectrum may be represented by not more than two sensations, with
a proportion of white._ In the orange and scarlet there are only two
sensations excited, without any sensible amount of white, as the
amount of violet sensation is extremely small. At the extreme ends of
the spectrum only one sensation--the red or the violet--is excited; but
in the region of the green the colour must be largely diluted with the
sensation of white. As an example, we may take the part of the spectrum
where the red and the violet sensation curves cut each other. At this
point the green sensation curve rises higher than the intersection
of the other curves. The red and the violet sensations have only to
be mixed with an equal amount of the green sensation to make white,
so that the height of the green sensation curve above the point of
intersection represents the amount of pure green sensation which is
stimulated. The colour is therefore caused by the green sensation,
largely diluted with white. A scrutiny of the curves will show that
at no point is the green sensation so free from any other as at this
point, if we regard white by itself as a neutral colour. Looking at
these figures, we can readily see what effect the removal of any one
or two of the three sensations would have upon the colour vision of
the individual. The probabilities, however, against two of the three
sensations being absent must evidently be very much smaller than that
there should be an absence of only one of the sensations, either red,
green, or violet.
[Illustration: FIG. 16.]
It will be well that we should also have before us the theory which is
the only serious rival to that of Young, viz., that of Hering. In the
report of the Colour Vision Committee there is an excellent description
of this theory. As it was furnished by Dr. Michael Foster, we may
be sure that the ideas of its originator are correctly given, and
therefore I will quote it in his words:--
“Another theory, that of Hering, starts from the observation that when
we examine our own sensations of light we find that certain of these
seem to be quite distinct in nature from each other, so that each is
something _sui generis_, whereas we easily recognise all other colour
sensations as various mixtures of these. Thus, the sensation of red and
the sensation of yellow are to us quite distinct; we do not recognise
anything common to the two, but orange is obviously a mixture of red
and yellow. Green and blue are equally distinct from each other and
from red and yellow, but in violet and purple we recognise a mixture
of red and blue. White again is quite distinct from all the colours in
the narrower sense of that word, and black, which we must accept as a
sensation, as an affection of consciousness, even if we regard it as
the absence of sensation from the field of vision, is again distinct
from everything else. Hence the sensations caused by different kinds
of light or by the absence of light, which thus appear to us quite
distinct, and which we may speak of as ‘native’ or ‘fundamental’
sensations, are white, black, red, yellow, green, blue. Each of these
seems to us to have nothing in common with any of the others, whereas
in all other colours we can recognise a mixture of two or more of
these. This result of common experience suggests the idea that these
fundamental sensations are the primary sensations, concerning which
we are enquiring. And Hering’s theory attempts to reconcile, in some
such way as follows, the various facts of colour vision with the
supposition that we possess these six fundamental sensations. The six
sensations readily fall into three pairs, the members of each pair
having analogous relations to each other. In each pair the one colour
is complementary to the other--white to black, red to green, and yellow
to blue. Now, in the chemical changes undergone by living subjects, we
may recognise two main phases, an upward constructive phase, in which
matter previously not living becomes living, and a downward destructive
phase, in which living matter breaks down into dead or less living
matter. Adopting this view, we may, on the one hand, suppose that rays
of light, differing in their wave-length, may affect the chemical
changes of the visual substance in different ways, some promoting
constructive changes (changes of assimilation), others promoting
destructive changes (changes of dissimilation); and on the other hand,
that the different changes in the visual substance may give rise to
different sensations.
“We may, for instance, suppose that there exists in the retina a
visual substance of such a kind that when rays of light of certain
wave-lengths--the longer ones, for instance, of the red side of
the spectrum--fall upon it, dissimilative changes are induced or
encouraged, while assimilative changes are similarly promoted by the
incidence of rays of other wave-lengths, the shorter ones of the blue
side. But it must be remembered that in dealing with sensations it
is difficult to determine what part of the apparatus causes them; we
may accordingly extend the above view to the whole visual apparatus,
central as well as peripheral, and suppose that when rays of a certain
wave-length fall upon the retina, they in some way or other, in some
part or other of the visual apparatus, induce or promote dissimilative
changes, and so give rise to sensations of a certain kind, while
rays of another wave-length similarly induce or promote assimilative
changes, and so give rise to a sensation of a different kind.
“The hypothesis of Hering applies this view to the six fundamental
sensations spoken of above, and supposes that each of the three pairs
is the outcome of a particular set of dissimilative and assimilative
changes. It supposes the existence of what we may call a red-green
visual substance of such a nature that so long as dissimilative and
assimilative changes are in equilibrium, we experience no sensation;
but when dissimilative changes are increased, we experience a sensation
of (fundamental) red, and when assimilative changes are increased, we
experience a sensation of (fundamental) green.
“A similar yellow-blue visual substance is supposed to furnish,
through dissimilative changes a yellow, through assimilative changes
a blue sensation; and a white-black visual substance similarly
provides for a dissimilative sensation of white and an assimilative
sensation of black. The two members of each pair are therefore not
only complementary but also antagonistic. Further, these substances
are supposed to be of such a kind that while the white-black substance
is influenced in the same way, though in different degrees, by rays
along the whole range of the spectrum, the two other substances
are differently influenced by rays of different wave-length. Thus,
in the part of the spectrum which we call red, rays promote great
dissimilative changes of the red-green substance with comparatively
slight effect on the yellow-blue substance; hence our sensation of red.
[Illustration: FIG. 17.]
“In that part of the spectrum which we call yellow, the rays effect
great dissimilative changes of the yellow-blue substance; but their
action on the red-green substance does not lead to an excess of
either dissimilation or assimilation, this substance being neutral to
them; hence our sensation of yellow. The green rays, again, promote
assimilation of the red-green substance, leaving the assimilation of
the yellow-blue substance equal to its dissimilation; and similarly
blue rays cause assimilation of the yellow-blue substance, and leave
the red-green substance neutral. Finally, at the extreme blue end
of the spectrum, the rays once more provoke dissimilation of the
red-green substance, and by adding red to blue give violet. When
orange rays fall on the retina, there is an excess of dissimilation of
both the red-green and the yellow-blue substance; when greenish-blue
rays are perceived, there is an excess of assimilation of both these
substances; and other intermediate hues correspond to various degrees
of dissimilation or assimilation of the several visual substances.
When all the rays together fall upon the retina, the red-green and
yellow-blue substances remain in equilibrium, but the white-black
substance undergoes great changes of dissimilation, and we say the
light is white.”
It has been said by the same writer that this theory is tri-chromic.
For my own part I do not see that it is so in the sense in which that
word is used in the theory of Young. It may be a tetra-chromic, for
as far as _colour_ is concerned the black-white sensation must be
excluded; but it appears to me that it cannot be strictly brought under
the head of tri-chromic.