Primary Colours--Molecular Swings--Colour Sensations--Sensations
absent in the Colour-blind.
For some purposes it is advantageous to show experiments before
indicating the deductions from them which may lead to a theory. Those
described in Chapter IX. will enable us to treat the theory of colour
perception from a standpoint of some advantage. How is it that the
combination of three colours suffices to form white, or to match any
colours we wish, be they spectrum colours to which a little white is
added, or the colours of pigments? The most plausible theory that can be
advanced is that it is only necessary for the eye to be furnished with a
three-colour-perceiving apparatus to give the impression of every
colour, and yet this would be somewhat difficult to believe had we not
had the experiments narrated in that chapter before us. We should have
almost expected some machinery in the eye to exist, which would answer
to the rhythmic swing of the rays of every wave-length which together
make up white light. But now we have to stand face to face with the
results of experiment, and we find that at the most only three colours
are necessary to make up white light, and that from these three spectrum
colours we can form any others, with the limitation already mentioned,
when some simple colours are in question.
We must here digress for a moment, and notice the fact that from our
experiments we have derived the three primary colours as they are
called, viz. red, violet, and green; the definition of a primary colour
being that it cannot be formed by the mixture of any other colours. We
have ascertained that yellow and blue make white. It is therefore
evident that blue, yellow, and red cannot be primary colours, since two
of them form white; and we have moreover shown that yellow can be made
from green and red; hence it might be fair to assume that the three
primary colours are red, green, and blue. But blue, when mixed with a
very small percentage of white light, can be made by green and violet.
Hence, in the white light formed by the two colours yellow and blue, we
have the first made by green and red, and the second by green and
violet; hence the three colours which really make the white light are
red, green, and violet. The approximate positions of these three colours
in the spectrum are those already indicated; though, as we shall
presently see, it is highly improbable that any person whose eyes are
what are called normal, has ever experienced the fundamental green
sensation.
The fact that red, yellow, and blue cannot be primary colours has been
mentioned, as even now it is sometimes taught that they are so. As long
as the theory of colour principally lay with artists there was
reasonable ground for their assumption, since they worked with impure
colours, viz. those of pigments; and as we shall see later on the truth
of the assumption agreed with such experiments as they would make. When,
however, the question was taken up by the physicist with more exact
methods of experimenting, and with pure colours, the falsity of the old
triad was soon capable of proof.
To return from our digression: how it is that three mixed colours can
give the sensation of white light is at first sight hard to understand;
but a reference to the action of light on a photographic salt helps us
in some degree. In the case of a sensitive salt, such as the
bromo-iodide of silver, we find that a chemical decomposition is caused
by the violet end of the spectrum, and is only feebly affected by any
other part, though with prolonged exposure even the red will cause it.
The annexed figure (Fig. 33) gives the idea of the relative action of
different parts of this violet portion.
Fig. 33.--Curve of Sensitiveness of Silver Bromo-iodide.
The height of the curve shows the relative effects produced. Now this
curve is not symmetrical, but has a maximum effect nearer to the violet
end of the spectrum than to the red. The atomic composition of the
silver bromo-iodide is probably two atoms of silver and one of bromine
and one of iodine oscillating together, and we can conceive of some one
atom, the period of whose swings in its molecule is isochronous with
some wave-length of light. Further, we can conceive that, like a
pendulum whose vibrations are increased in magnitude by well-timed
blows, the swing of the atom is also increased, and that eventually it
gets beyond the sphere of the attraction of its parent molecule, leaves
it, and is attracted to some neighbouring molecule of different
constitution, and that thus a chemical change is induced. This we can
conceive, but how can other waves, which are not isochronous with the
rhythmic swing of the atoms, alter the composition of the molecule? If
we have an impulse given to a pendulum exactly timed with the period of
oscillation, there is no doubt that the swing is increased. If we have
one nearly in accord, it will be found that though the swings are not
increased in amplitude so greatly as when there is perfect accord, yet
an increased swing is given, and as exact accord is removed further and
further, so the increase in the swing of the pendulum gets smaller and
smaller. In somewhat the same manner it is possible that many series of
waves, differing in wave-length, and therefore in periods of
oscillation, may be capable of increasing the amplitude of a swing, and
with the photographic salt this probably occurs, with the result which
we see in the above figure. Suppose in the eye we have three such
sensitive pendulums which are capable of responding to the beats of
waves of light, it requires no great imagination to see that one such
pendulum will respond not only to that wave of light which is
isochronous with it, but also with waves shorter and longer than that
particular wave. The same pendulum indeed may respond to the whole of
the visible spectrum, but when far off from the maximum the response
would be very small indeed. We may therefore assume that though each
pendulum may have its maximum increase of oscillation at one part of the
spectrum, yet at other parts not only it alone answers to the beating of
the waves, but that the other pendulums are also affected by the same,
and thus the whole spectrum is recognized by the swings more or less
long, of either one, two, or of all three.
To Thomas Young is usually attributed the three-colour theory, though it
seems to have been promulgated in an incomplete state some time before;
Clark-Maxwell and Helmholtz revived it in later years, and it is usually
known as the Young-Helmholtz theory. It should be remarked that the
three fundamental colour sensations are not of necessity the same
sensations as are given by the three primary colours, as we shall see
further on. The following figure (Fig. 34) is taken from Helmholtz's
physiological optics, as diagrammatic of the three sensations.
Fig. 34.--Curves of Colour Sensations.
To this diagram there is an objection, in one respect, viz. that it
gives the same luminosity-value to the blue of the spectrum as it does
to the red and green. It has been seen that if we call the luminosity of
the yellow 100, that of the blue is about 5. The objection does not hold
if it is remembered that the three maxima of impressions are taken as
equal. If the ordinates were increased, so that the maxima were of the
same height as that of the photographic curve, the resemblance between
them and this last would be very marked. It will be noticed that each of
the three colour sensations is not only excited by a limited portion of
the spectrum, but by all of it, the height of the curves being a measure
of their response.
Now assuming that this is the case, since a certain degree of
stimulation given simultaneously to the three sensations causes an
integral sensation of white light, it follows that the colour perceived
in every part of the spectrum is due to the excess of stimulation of
either one or two of the fundamental sensations, together with the
sensation of white light. If this diagram were correct, at no point in
the spectrum is one fundamental sensation excited alone, but we believe
that the diagram obtained by K[oe]nig (Fig. 35), from colour equations
(which will be explained in our next chapter), is more exact, and that
it is probable that in the extreme violet and extreme red of the
spectrum the only sensations which are stimulated are the violet and red
respectively. Our measures in the red and violet of the spectrum make it
appear that each of the two sensations can be perceived unaccompanied by
any others, and the fact that the red colour blind person perceives a
shortened spectrum in the red end, is a further proof of this deduction,
so far as the red is concerned.
The colour which the fundamental green sensation excites in the normal
eye has probably never been seen, nor can be seen. This is due to the
fact that all three sensations overlap in the green; that is, that the
pendulum which answers to the green colour in the spectrum also affects,
but with much less energy, the other two pendulums, which respond to
the red and violet sensations.
The word pendulum has been used advisedly, for it may equally as well
apply to a molecular aggregation as to one which is visible and
measurable. Without entering into the physiological structure of the
eye, we may say that it has usually been assumed that the pendulums are
the ends of nerves which vibrate with the waves of light; but this seems
rather doubtful. Gross matter, such as these ends are, compared with the
molecules of which they are built up, cannot, as a rule, vibrate with
waves of light, and there seems to be no reason why there should be an
exception in the case of the eye. It seems much more probable that a
chemical decomposition takes place in some substance attached to them,
and where such decomposition takes place electricity of some kind must
be produced. In other sensations of the body the nerves act as telegraph
wires, carrying messages to the brain, and it is not improbable that the
nerves of the eye are employed in somewhat the same manner. Professor
Dewar has shown that when light acts on an extirpated eye, a current of
electricity does traverse the nerves, and of such an amount that it can
be shown to a large audience. This experiment is not, however,
conclusive, as the effect may be mistaken for the cause. This idea,
however, is only hypothetical, as is indeed the hypothesis of the
mechanical action of light on the gross matter of which the rods and
cones attached to the retina are composed.
We have in a previous chapter stated that there are some eyes in which
the sensation of some colour is altogether absent, and in others in
which it is more or less deficient. Thus some eyes appear to be lacking
wholly in the sensation of red, others of green, and some very few of
violet; and there have been cases known in which two sensations, the red
and violet, have been totally absent. In the first case, where the
sensation of red is entirely absent, what is known to the normal-eyed as
white can be matched with a mixture of blue and green, and there is a
place in the spectrum that is recognized as white. Similarly white can
be matched by a green blind person with a mixture of red and blue.
To those who may be curious to see the colour which red and green blind
persons would call white, a very simple means is at hand to demonstrate
it. Using the colour patch apparatus with the three slits inserted in
the slide, and in the positions we have indicated in the violet, green,
and red, and forming white light for ourselves on the screen, if we
cover up the red slit entirely we shall have a patch of sea-green
colour, which a red blind person would call white; and if we cover the
green slit, uncovering of course the red, we shall have a brilliant
purple, which to a green blind person would be white. They both would
call white what the normal-eyed person sees as white, for the simple
reason that either the red or the green mixed with the remaining colours
would be unperceived. The examination of colour-blind people is of prime
importance for testing any theory of colour vision. For instance, if it
were asserted that the fundamental sensations did not overlap as shown
in the diagram above, then it would follow that at some place in the
spectrum there would be a dark point. If they do overlap, it must follow
that both for the red and for the green colour blind person there must
be some place in the spectrum where what is white light to them is
produced.
Colour-blind people were tested with the colour apparatus. The reflected
beam and the colour patch were made to cast shadows as before, and the
rotating sectors placed in the path of the former. A slide with one slit
was passed across the spectrum, and the position noted where it was said
that the two shadows were illuminated with white light; to the
normal-eyed person one shadow of course appeared illuminated with the
sea-green colour, or bluish green, according as the observer was red or
green colour blind. The ray in the spectrum which to the red colour
blind is white, has a wave-length of about 4900, and that for the green
colour blind a wave-length of 5020, which corresponds to the position in
which we usually place the green slit when a normal-eyed person is
making colour matches.
It may be further remarked, that if the maxima of all the three colour
sensations are taken, as in the diagram, as of equal value, that the
place in the spectrum where the white light is perceived by the
colour-blind is where the two sensations are of equal strength, that is,
where the two curves cut one another, and are of equal height. By
obtaining the proportions of the different colours with colour-blind
persons which make up what to them is white light, the curves for the
two sensations can be worked out in the form of simple equations.
The experiments carried out with colour-blind people are of the most
interesting character, and a good deal remains to be done with the data
already obtained from them.
To the popular mind a colour-blind person is usually thought a strange
creature, and it is a matter of wonderment, if not of amusement, that
they cannot distinguish between the red of cherries and the leaves of
the cherry tree. The physicist, studying the theory of colour, views the
matter quite differently, and he looks upon an intelligent observer of
this class as a boon. It may be remarked that both the red-blind and the
green-blind persons would be unable to distinguish between the cherries
and the leaves. The red-blind person would see the cherries as green, as
also the leaves; whilst the green-blind person would see both as red.
Without regarding form it is probable that the red-blind would see the
leaves as a bright green, whilst the green-blind would see them as
darker red than the cherries. Failure to distinguish between the two is
more likely to occur with the green of leaves, and the red of such
fruits as cherries, since the former contains a marked proportion of red
in it, and the latter a small proportion of green.
One highly-educated gentleman was led to know his deficiency in colour
sense, by hearing a companion on a tour going into raptures over a
sunset. He saw but little difference between it and that to be seen at
midday. Testing his vision it appeared that he was totally blind to the
sensation of green, and that white and purple would consequently be
mistaken by him for one another. The crimson on the clouds, illuminated
by the setting sun, would appear to him as only slightly different to
the white clouds which he would see at midday; in fact he would be
always seeing what to us would be a sunset. For this gentleman to mix
spectrum colours to match others would evidently be no guide to
normal-eyed persons.
We believe that amongst us in our daily life we have many persons who
are blind to some colour, but who are not aware of it, or if they are
aware of it, hide their defect as far as possible. That some are
ignorant of it to a late period of their life we know.
We have said that there are cases in which persons are only defective in
colour perceptions, and not wanting in them altogether. The former are
more common than the latter, and to the experimenter are by no means so
interesting. They are only alluded to here to indicate that there are
degrees in the defectiveness of eyes to colour. One point which must be
remembered here is that all colour production for registration by the
mixture of three colours is delusive, unless the eye of the operator is
tested for its colour sense.