Colour Mixtures--Yellow Spot in the Eye--Comparison of Different
Lights--Simple Colours by mixing Simple Colours--Yellow and Blue form
White.
The colour of an object in nature, without exception we might almost
say, is due, not to one simple spectrum colour, or even to a mixture of
two or three of them, but to the whole of white light, from which bands
of colour are more or less abstracted, the absorption taking place over
a considerable portion or portions of the spectrum. Notwithstanding this
we shall now experimentally show that every colour can be formed by the
simple admixture of not more than three simple colours, if they be
rightly chosen, and from this we shall make a deduction regarding vision
itself. We are in a position to obtain three simple colours by means of
a slide containing three slits. Now for our purpose we require that the
three slits can be placed in any part of the spectrum, and that they
can be narrowed or widened at pleasure. Instead of a card the writer
uses a metal slide, as shown in Fig. 30.
Fig. 30.--Slide with slits to be used in the Spectrum.
It will be seen that the three slits can be closed or opened from the
centre by a parallel motion. They also slide in a couple of grooves, so
that they can be moved along the frame into any position. The position
they occupy is indicated by a scale engraved on the front of the slide.
Behind the grooves in which the slits move are another pair of grooves,
into which small pieces of card CCCC can slide, and thus close the
apertures between the slits. By this arrangement all rays except those
coming through the slits themselves are cut off. The metal frame fits on
to an outer wooden frame, which slides in the grooves used with the card
in the apparatus as already described. It is convenient always to keep
the scale on the back of this wooden slide in the same position as
regards the shadow of the needle-point used for registering the
position, and to move the slits along their grooves when a change in
position is required. Using these three slits three different colours
can be thrown on the same square patch on the screen.
A very crucial experiment is to see if we can make white light by the
admixture of three colours, for if this can be done it almost follows
that any colour can be formed. We must use the colour patch apparatus,
and begin with placing one slit in the violet near the line G, another
between E and F, and a third between B and C of the solar spectrum, and
fill up the gaps between them with cards as shown in the figure. For our
present purpose it is better to make the colour patch and the white
patch touch each other, not using the rod, as by this means we avoid
fringes of colour. We shall find that the aperture of the slits can be
so altered that we can produce a perfect match with the white reflected
light. By placing the rotating sectors in front of the reflected beam we
can reduce its intensity, so that the two patches are equally bright. By
a tapering wedge we can measure the width of the slits, and thus get the
proportions of these three different colours which must be used to give
the white. This is a sample of the method that we employ when we match
any other colour. Suppose, for instance, it be wished to measure the
colour of a solution of bichromate of potash; it is placed in the path
of the reflected light, and we have an orange strip of light which we
have to match. In this case it will be found that the slit in the blue
has to be closed entirely, and only the green and red slits opened. The
intensities of the two lights are equalized by the rotating sectors as
before. So again with a solution of permanganate of potash. In this
instance no green light will be required (or if any of it but a trifle),
and the colour of the permanganate will be formed by the rays coming
through the blue and red slits.
This plan is a very useful one for measuring all kinds of transparent
colours in terms of three rays. The method of finding the intensity of
any ray of the spectrum transmitted by any such medium has already been
explained. The latter has one advantage over the former, in that the
measurements by it are exact, whatever source of light be used to form
the spectrum. By the method now described this is not the case. For
instance, the colour of permanganate of potash may be matched in the
electric light with the red and blue slits. If the limelight were
substituted for the electric light, it would be found that the slits
would require other apertures, not proportional to those already formed,
to match the colour of this substance.
Fig. 31.--Screen on which to match Gamboge.
If we wish to register the tint of any pigment, we have to slightly
alter our mode of procedure. Suppose, for instance, we wish to register
the colour of gamboge. In such a case we paint a small bit of card (Fig.
31) with the pigment, and divide the white space on which the colour
patches are thrown into two parts, and cover one-half with the pigmented
card, leaving the other half white. The reflected beam illuminates the
pigment, and the spectrum patch the white. The widths of the three slits
are then altered till the two tints agree, and the brightness matched by
means of the rotating sectors.
There are certain sad and æsthetic colours which it might be considered
cannot be matched by a mixture of three colours. A brown colour, or "eau
de nil," might appear to come out of the range of matching. These
colours, however, can be matched in precisely the same manner as the
brighter colours are matched. Thus a brown pigment will be found to
require red and a little green, and a trifle of blue; and the only
difference between it and a brighter shade of the same colour, is that
more total light has to be cut off from it to give the sombreness. A sad
colour only means a pigment or dye which reflects but little light, and
if that be so it can naturally be matched by using but very small
quantities of the compounding colours.
There is one curious phenomenon to which attention may be called in this
matching, which is worthy of remark. The match will be found to differ
according as the patches are compared from a distance of a couple of
feet, or from a considerable distance. More green will be required in
the latter case than in the former. If matched at a distance of about
six feet, and the eyes be then turned so that the edge of the patch
falls on their centres, it will be noticed that the colour mixture
appears of a green hue. This last experiment indicates that the retina
is not equally sensitive for all colours throughout its area.
Physiologists tell us that what is known as the yellow spot occupies a
central position in the retina, and that it absorbs a part of the
spectrum lying in the green. Now when the eyes are close to the patch,
its image occupies a considerable part of the retina, and the colour is
compounded as it were of the colour as seen on the yellow spot, and of
that beyond it, for the yellow spot will take in an image of from six to
eight degrees in angular measurement. When viewed at a distance we have
the image of the patch falling almost entirely on the yellow spot, and
hence a greater quantity of green is required, as it has to make up the
deficiency caused by the absorption. When the eyes are turned a little
on one side the image falls on the outside of the yellow spot, and the
patch illuminated by the mixed light appears green, compared with the
patch illuminated with the white reflected beam.
It is thus evident that when colour matches have to be made, the
distance of the eye from the screen should always be stated, as also the
dimensions of the patches viewed. It may be fairly asked why, if the
half patch illuminated by the mixed colours appears greener when the eye
is turned, the other should not equally do so. This is a very fair
question to ask. It must be remembered that one strip is illuminated
with white light, in which every coloured ray of light is compounded,
whilst in the other only three rays are blended. The green ray chosen
happens to be taken from that part of the spectrum which is absorbed by
the yellow spot; but all of the green rays of the spectrum are not so
much absorbed, hence in ordinary white light, in which all the green
rays are present, only a small percentage of the total green in the
spectrum is absorbed, compared with that absorbed from the single green
ray with which the match is made. No doubt both patches are really
greener when the eye receives the impression of their images outside the
yellow spot, but one is much greener than the other, and it is thus
_comparatively_ green. It is possible to make a match with some colours
with a blue-green in which the phenomenon described does not appear; but
in cases where a match has to be made with colours in which but little
blue is required, it would be impossible to make it, owing to the blue
existent in such a green-blue ray.
We will now return to our compounding of three colours to make white.
Why have we chosen the positions of the slits which we did in the
spectrum for its formation? Would not other positions answer as well?
Let us give our answer by experiment. Let us move the slit which is now
in the green towards the red; we shall find that as we do so--and
keeping the blue slit of the same width--that we shall have to close the
red slit, and alter the aperture of the green slit itself. If we reason
on this point we shall be forced to the conclusion that the green slit
lets through more red light of some description, as less red from the
red slit is required to make the match. If we move the green slit almost
into the yellowish green, we shall find that the red slit has to be
entirely closed, and that white light is formed of the two colours,
yellowish green and violet. This shows us that the yellowish green
colour here used is formed by a mixture of the red and green rays which
passed through the two slits in their original positions. If we replace
the slits in these positions and close the violet slit, we are at once
able to verify it.
If we again form white light with the slits in their original positions,
and move the green slit towards the blue, we shall find that, keeping
the red slit at a constant aperture, the blue slit will have to be
closed, and the green slit altered in width. The necessity of lessening
the aperture of the blue slit shows that there is a certain amount of
blue light coming through the green slit. At one point, when the slit
has travelled into the blue-green, the blue slit may be entirely closed,
and white light be formed of this and the red, showing that the
blue-green colour is composed of the same proportions of blue and green
which passed through the blue and green slits in their original
position. The positions chosen were arrived at by the writer from
experiments made in this manner, moving first one slit and then the
others, and the position of the green slit was confirmed by a
consideration of the neutral point which exists in a green colour-blind
person's spectrum.
The method of mixing three colours together gives us a means of
imitating all kinds of white light, as it does of coloured light. At
page 110 we have already given a diagram of the relative amounts of
spectrum colours in sunlight, skylight and gaslight. If we by any means
throw a patch of the light which we wish to match on the patch formed by
the colour patch apparatus, and interpose the rod, we can measure the
apertures of the three slits, and thus arrive at the relative
proportions of each colour present. In an experiment carried out,
sunlight, the electric arc-light, and gaslight were compared in this
manner. The following are the results, the red being near the C line,
the green near the E line, and the violet near the G line of the solar
spectrum.
+--------+-----------+----------+-----------+-----------+
| | Sunlight. | Electric | Gaslight. | Skylight. |
|--------+-----------+----------+-----------+-----------+
| Red | 100 | 100 | 100 | 100 |
| Green | 193 | 203 | 95 | 256 |
| Violet | 228 | 250 | 27 | 760 |
+--------+-----------+----------+-----------+-----------+
Now from the above it might seem that as three simple spectrum colours
will give us the colour of any pigment, that therefore two colours ought
to give us the same colour as any intermediate simple colours in the
spectrum which lie between them; for instance, that the simple
blue-green ought to be obtained by mixing spectral green and spectral
violet together. This can be ascertained with a single colour patch
apparatus, by cutting a slit in the card that fills up the aperture
between the two adjustable slits, and deflecting the beam transmitted
through it by a right-angled prism, and back on to the screen through
another similar prism, as described in chapter VIII. It is more
convenient, however, to use a duplicate apparatus precisely similar to
the first, with the exception that no collimator is required, placing
them side by side, and mirrors making the reflected beam from the first
traverse the second set of prisms. There will be a reflected beam from
the second apparatus, which can be utilized in the same way as was that
from the first apparatus, and the two spectra will vary together in
brightness, as will also the new reflected beam, since they all are
formed by the light coming through one slit. A patch of the colour
intermediate between the two is thrown on the screen from the second
apparatus, and the second patch from the first apparatus overlaps it. A
rod placed in the usual manner throws two shadows, which are illuminated
by the two different beams. If blue-green be a colour it is wished to
match, it will be found that no matter in what part of the violet and
green the slits are placed, no match can be effected. But if some very
small quantity of red light be mixed with simple blue-green, that then a
colour identical in every respect as regards the eye can be obtained
from the violet and green of the first apparatus. It must be remembered
that a mixture of red, green and violet form white, and that they are
mixed in definite proportions. No matter how feeble in intensity the
white may be, the same proportions will still obtain. In the above
experiment, as the blue-green must contain violet and green, the small
quantity of red must combine with the proper proportion of violet and
green, and will form white light, so that the match is obtained by the
residues of the violet and green mixed with the small quantity of white
light, of which the red is the indicator.
We can test the truth of this argument in a very simple way. If we add
to the colour with which the match has to be made a small quantity of
white light from the reflected beam, cutting off more or less by the
rotating sectors, we can get the exact hue of the impure blue-green made
by the mixture of the colours coming through the two slits; and further
we shall find that the amount of white added corresponds with the amount
of red which would be required when the components of the white light in
the terms of the three colours are taken into account. For spectrum
colours between the violet and the green it may therefore safely be said
that no match can be effected by the mixture of violet and green light;
but that it always gives the intermediate colour diluted with white
light. For colours between the green and the red of the spectrum, a very
close, if indeed not an exact match, can be made with the red and green
slits, without the addition of white.
If we take from the second apparatus light from above the position of
the violet slit in the first apparatus, that is, nearer the limit of
visibility, it will be found that a match is made, for at all events a
very considerable way with the violet slit alone, by merely reducing the
aperture, thus showing that the colour is the same, only less intense.
In the same way it will be seen that the rays coming from any point
between the lower limit of the spectrum to a little below the C line are
identical in colour.
As we have arrived at the fact that in colour mixtures of violet and
green, white light is to be found in the colour produced, it follows
that either the violet or the green, or both, must themselves contain
some small proportion of white. It might perhaps be said that violet is
really a mixture of red and blue, and hence the white in the mixture
with the green; but if in the first apparatus we place one slit in the
purest blue we can find, and the other in the red, and throw a violet
patch on the screen from the second apparatus, we shall be unable to
form the same hue of violet by any means; it will always be diluted with
white. Now as the very blue we are using, if matched as above by green
and violet, requires white light to be added to it, and as to match the
violet with the same blue and red, white light has also to be added to
it, it follows that the violet must be freer from white light at all
events than the blue.
There is one other experiment that must be mentioned before leaving for
a time this part of our subject, viz. the formation of white by a
mixture of yellow and blue. If one of the slits be placed in the yellow
of the spectrum, a position will be found in the blue where, if a second
slit be placed, and the apertures are adjusted, an absolute match with
the reflected white of the apparatus can be secured. This experiment
will be referred to later on, when considering the question of primary
colours.
The above experiments have a great bearing on the theory of colour
vision, and should be considered very carefully in connection with the
shortened spectrum which we have shown exists when red colour-blind
people are observing its luminosity.
There is one point to be recollected in relation to the mixtures of the
three or two different colours which make white light. If different
coloured pigments be illuminated by the "made" white light, they will
not appear of the same hues, as a rule, as when viewed by ordinary white
light. They will vary not only in colour, but in brightness. This might
be expected when the spectral light which they reflect is taken into
account.