It will be seen, then, that in measuring colour or light several
circumstances have to be taken into account. These are not simple, and
require differentiating one from another before the results of colour
measures can be finally laid down as correct, or as being held to be
applicable to all cases.
We must naturally ask, what is colour? The answer I should like to pass
over entirely. It can only be described as a sensation, just as we
should describe touch as a sensation. It has, however, one advantage
over most sensations, in that it is a sensation which can be submitted
to empyric measurement. The question whether certain phenomena, such
as the colours produced by simultaneous contrast, are subjective or
real, does not require answering for the purpose that we have in view,
but the results recorded may probably help to throw light on it.
Colour is an impression caused by the stimulation in the eye of some
apparatus, that lies near the outer wall of the retina, the effect of
the stimulation being conveyed by the optic nerve to the brain. If this
apparatus be complicated by being made up of distinct parts, each of
which transmits its own kind of impression to the brain, it is not only
quite possible, but more than probable, that when one part is absent or
injured the particular impression for which it is responsible will be
lacking, and that the sum of the impressions due to the remainder will
be unlike that perceived when they are all working together.
In every investigation, whether it be in physical or in any other
branch of science, it is better to work up from the simple to the
more complicated; and acting on this plan, it is better to commence
experimenting with simple rather than with complex colours, though
they may apparently produce precisely the same sensations. I shall,
with this in view, devote most of the remaining part of this chapter
to some necessary experiments with simple colours. The simple colours
are those of the spectrum, and are the result of motion in the ether,
which pervades all space. The motion is in the form of undulations or
waves, and each colour is due to a series of these waves, which have a
definite length. Thus, 6562 ten-millionths of a millimetre produces
to most of us a red colour in the spectrum (see Plate I.), occupying
the position indicated by a black line known as the C line in the solar
spectrum.
A table of wave-lengths of certain lines in the solar spectrum is given
below:--
TABLE OF WAVE-LENGTHS IN TEN-MILLIONTHS OF A MILLIMETRE.
B, deep red 6866
Lithium, cherry red 6705
C, red 6562
D, orange 5892
E, green 5269
b, green 5183
F, bluish green 4861
Lithium, blue 4603
G, violet 4307
H, extreme violet 3968
The rays in the different parts of the spectrum being due to these
simple vibratory motions, cannot be decomposed further. And it makes
no matter whether we _see_ them as different colours or not, they will
always issue at the same angle from the same prism (if the prism be
used to form the spectrum), when it is turned to the same angle to the
incident light. Milestones are useful along a road to tell us where
we are in reference to some central place, and these black lines in
the spectrum serve the same end. But they have the advantage over the
milestone, for whilst the last will tell us how far we are from, say,
York or London, the former tell us our distance from a zero point.
We thus have a scale of light of different wave-lengths laid down for
us, which we can apply to the study of the sensations stimulated in
the eye, and so have the means of instituting a comparison between
the colour vision of different eyes. A mixed or composite colour is
in a different category, however, to the simple colour, as you will
see directly. It is one which may be formed by any number of rays of
different wave-lengths falling on the eye. What these rays are we can
only tell by analysing the light and referring them to the spectrum.
The instrument before you is one which I have used before in this
theatre; but as the major part of my experiments have been carried
out with it, in case those who are present may not be acquainted with
it, it will be necessary to describe it very briefly. The general
arrangement of the apparatus is given in the accompanying diagram, Fig.
4.
[Illustration: FIG. 4.]
R R are rays coming from the source of light, be it sun light or the
electric light, and an image of the one or the other is formed by a
lens L₁ on the slit S₁ of the collimator C. The parallel rays produced
by the lens L₂ are partially refracted and partially reflected. The
former pass through the prisms P₁, P₂, and are focussed to form a
spectrum at D by a lens L₃. D is a movable screen in which is an
aperture S₂, the width of which can be varied as desired. The rays are
again collected by a lens L₄, and form a white image of the surface
of the last prism on the screen E. If the light passing through S₂ is
alone used, the image at E is formed of practically monochromatic
light. Part of the rays falling on P₁ are, as just said, reflected, but
as it and the refracted part are portions of the light passing through
the slit S₁, they both must vary proportionally. If then we use the
reflected portion as a comparison light to the spectrum colours, the
relative intensities of the two, though they may vary intrinsically,
will remain the same. The rays reflected from P₁ fall on G, a silver
or glass mirror, and, by means of another lens L₅, also can be caused
to form a white patch on the screen E, alongside the patch of colour.
At M, or anywhere in the path of the beams, an electro-motor driving a
sector with apertures which can be opened or closed whilst rotating, is
placed, and the illumination of either beam can be altered at will. To
obtain a large spectrum on the screen E, all that is necessary is to
interpose a lens of fairly short focus in front of L₄, when a spectrum
of great purity and brightness can be formed.
If it be required to measure the width of the slits S₂ (which we shall
see further on is often necessary), a small lens of short focal length
placed behind L₄ and near the slit will cast a magnified image on E,
and by means of a scale placed there, the widths of each slit, if there
are more than one, can be read off on the scale by bringing them
successively into the same colour.
[Illustration: FIG. 5.]
Originally the comparison light was a candle, and it answered its
purpose fairly well, and for obtaining absolute measures is convenient
at the present time. Fig. 5 will show its arrangement, but as both the
candle and the electric light may vary independently of each other, it
will be seen that for merely the comparison of the different spectrum
colours, the previous arrangement is the better. In both cases the two
beams--the direct and the comparison--may be made to cast shadows
by placing a rod in their path, the shadow cast by one light is then
illuminated by the other light. By moving the rod towards or from the
screen the shadows can be brought side by side.
With this instrument it is easy to demonstrate that a mixed colour
may be mistaken for a simple colour of the spectrum. In a glass cell
with parallel sides is a solution of potassium bichromate, which,
to myself and probably most of you, has a beautiful orange colour.
The spectrum of white light is now on the screen, and if this orange
liquid is placed in the path of the white light before it reaches
the prisms, all the violet, blue, and most of the green is cut off,
leaving some green-yellow, orange and red only on the screen. That
these form the orange colour of the bichromate is readily shown by
removing the auxiliary lens. The spectrum, which has its focus at D, is
now recombined into a patch of light, which is at once seen to be the
colour of the solution.
[Illustration: FIG. 6.]
The colour of the bichromate is therefore a complex or mixed colour
according to our definition, for it is made up of a large number of
simple colours. What I desire to show, however, is that this complex
colour can be mistaken by the eye for a simple colour. First, let us
interpose the cell with the bichromate in the path of the _reflected_
beam, and throw the patch of light formed by it on a white surface A
(Fig. 6), alongside the patch of light B formed by the spectrum. Next
let us pass a single aperture (Fig. 7), which can be opened and closed
by a screw arrangement, through the spectrum. By careful movement we at
length come to an orange ray, which is spread out by the apparatus to
form a patch on B, that to the majority (and the word majority is used
with intention) of people exactly matches the colour of the bichromate.
Thus we have a proof that, as far as the eye is concerned, the simple
and the complex colours are identical. This illustration of the want
of power of the eye to analyse colour might be repeated as often as
we like. We may pass coloured wools, for instance, through the length
of the spectrum and show that they have the property of appearing
bright in, and therefore of reflecting, some colours and of almost
disappearing in others--a sure indication that these colours are mixed
colours as they are made up of the rays which are reflected. Yet when
viewed in white light they can in many cases be matched with simple
colours in the way we matched the colour of the bichromate solution.
This tells us that there is something which requires investigating as
to the constitution of the perceiving apparatus, and points to the
probability that it is less complicated than it would be were it able
to differentiate, without the aid of the spectrum, between simple
and complex colours. If the eye had a separate apparatus--and when I
say apparatus I use the word for want of a better--for taking up the
impression of every simple colour, it might well be assumed that a
differentiation must take place.
There is one class of colours, it must be remembered, which can never
be mistaken for simple colours. I refer to the purples--mixtures of red
and blue--for there are no spectrum colours which unmixed can possibly
match them. All other colours, as no doubt will soon be apparent, can
be referred to some one spectrum colour, either in its pure state or
else mixed with some variable quantity of white light. We are all
familiar with the fact that there are three primary colours, and we
are naturally led to consider these in the light of the experiments
just made. As good a definition as any other of a primary colour is
that it is a colour which cannot be formed by the mixture of any two or
more colours. The original investigators in colour phenomena were the
artists, and they found that neither red, nor yellow, nor blue could be
formed by any mixture of pigments on their palette, but that all other
colours could be made by a mixture of two or more of these three. Hence
to these three were given the name of primary colours. When, however,
the physicist began to work with the simple colours of the spectrum, it
was speedily found that, at all events, the yellow was not a primary
colour, as it could be formed by a mixture of green and red, whilst a
green could not be formed by a mixture of any other two colours. This
we can prove with our apparatus.
[Illustration: FIG. 7.]
Three apertures, all of which can be opened or closed as required (see
Fig. 7), are placed in the spectrum, one in the red, one in the green,
and one in the violet. The last we shall not require at present, so it
is entirely closed; but we vary the width of the other two. We find
that with a little red added to a bright green, a yellow green is
produced; with more red added we have yellow; with still more red, an
orange. The relative brightness of the two colours mixed together can
be shown by removing the lens which recombines the spectrum to form the
patch of light. Each colour issues through its slit and forms its own
patch on a white screen which, for the purpose, we make rather larger
than usual. The two patches overlap in the middle (Fig. 8), and the
pure colours are seen one on each side of the mixed colours.
[Illustration: FIG. 8.]
Now, placing one slit in the yellow and another in the blue of the
spectrum, we find that whatever width of slit we take, no green is
produced, but that, in fact, a yellowish or a bluish white results,
and that when the two slits are properly adjusted, a pure white is
produced. Evidently since none of the intermediate spectrum colours
between the blue and the yellow can be made by their mixture, certainly
green cannot. Hence, with pure colours a green and not a yellow is one
of the primaries.
Further investigation on these lines has placed the violet of the
spectrum as a primary rather than the blue, but this is still a matter
of debate. Suffice it to say that a red and a green in the spectrum are
really two of the primary colours, and most probably the violet the
third. Experiment shows that there is no other primary colour in the
strict sense of the word. We thus arrive at the fact that, except the
primary colours themselves, every colour in nature may be made by a
mixture of two or three of these primaries.
Just a word of explanation as to why, with pigments, the primary
colours appear to be red, yellow, and blue, and not red, green, and
blue. The colour of a pigment, it must be recollected, is a complex
one. If we analyse a yellow--a yellow glass will be just as good an
example as anything else--we find it is made up of green, yellow,
orange, and red. A blue is made up of blue and green. If a yellow is
placed behind a blue glass, and we look at a white surface through
them, the only light that can get through the glass is the green. If
the light, coming through each glass _separately_, falls on the same
spot on a white surface, it will be either colourless or bluish white,
or yellowish white, whichever colour preponderates. As the light
reflected from mixed pigments is made up principally by the light
coming through the different particles, first coming through one and
then through another, and only partially by mixed lights, it will be
gathered why the primary colour, when deduced from experiments with
pigments, was yellow, and not green.
With the spectrum colours there is this fact to remember, that
though all intermediate colours between the pairs of primaries can be
formed by their mixture, yet in some cases the resulting colours are
_slightly_ diluted with white, and that they thus appear less saturated
than the spectrum colours themselves. The reason for this we shall be
able to account for when we consider the colour sensations themselves.
When making matches to simple or other colours by the method of
mixtures, we have to be careful of the conditions under which we
experiment. This can be shown by a very simple experiment. I will make
a match on B with the white light, which is thrown on the surface A
(Fig. 6), by mixing the red, green, and violet that pass through the
three adjustable apertures or slits already described. The apertures
are altered till the match appears to myself perfect. From an appeal
made to those of the audience who are at least 25 feet away from the
patches of light, as to the correctness of the match, I gather that
the match is to them imperfect. The mixed colours appear to them to
give a pinkish white. The reason of this defect in the match is due
to the fact that, as the lecturer is viewing the two square patches
of 2 in. side from a distance of 2 ft. 6 in., their images on his
retina extend beyond the boundary of the yellow spot, whilst the
audience receives the whole of the image on that portion of the retina
which is completely covered by it. To the lecturer only part of the
blue and green is absorbed by the yellow spot, and the part of the
retina outside it on which the image falls receives and records the
full intensity of these colours. To the audience the full amount of
absorption takes place, with the result that the patch of mixed colours
must appear too red when it is correct to the lecturer. In this case
habit makes the eye take an average of the different intensities which
must exist at the various parts of the image. We can, however, cause
a perfect agreement between all parties if the experimenter views the
surfaces in a mirror placed some 12 feet away and then makes the match,
for he is viewing the patches from what is practically a distance of
24 feet. If after making the match without the aid of the mirror the
lecturer’s eyes are directed a little to one side of the illuminated
surfaces, a match will no longer exist; the mixed colour, which is
to the audience pinkish, will now appear a bluish green to him. The
reason for this alteration in hue is that the whole of the images falls
outside the yellow spot.
It will now be quite apparent that we must discount any assertion in
regard to colour matches, unless we are told the distance of the eye
from the surface on which the match is made, together with the size
of that surface. This yellow spot is often provokingly tiresome in the
study of colour mixtures, and one might almost be justified in doubting
whether any _absolutely_ exact matches can ever be vouched for, owing
to the important region of the retina which it occupies.
The fatigue of the retina to colour after it has been presented to
the eye for any length of time is a difficulty, but in a less degree.
That the retina does experience fatigue can be shown by a very simple
experiment. The lecture theatre is now illuminated by the incandescent
light, and if we throw an image of the bright carbon points of the
electric arc light on the screen and steadily fix the eyes on the
image of the white-hot crater for some (say) twenty seconds, and
then we suddenly withdraw it, a _dark_ image of the points will be
seen on the partially lighted screen, and will appear to travel with
the eyes as they move away from the fixed point. This phenomenon is
due to the fact that the perceiving apparatus for white light gets
fatigued on the parts of the retina on which the bright image of the
white carbon points thrown on the screen fell, and that when the
source of brightness was removed, the less intense illumination of
the screen failed to stimulate the vision apparatus at those parts
to the same extent that they were stimulated over the rest of the
field. We can vary the experiment by placing a red glass in front
of the electric light, and, following the same course as before, we
shall see a greenish-blue image of the carbon points upon the screen.
In this case the retinal apparatus which has not been stimulated by
the red sensation will be capable of the maximum stimulation by the
feeble white light, whilst that part which has suffered fatigue will
not respond so freely to the red contained in the white light. If we
abstract a certain amount of red from the spectrum, its recombination
will give a white tinged with greenish blue, which is a counterpart of
the colour we feel when the eyes have been fatigued by the red light.