We are now in a position to carry the investigations as to luminosity
a little further. When we look at small patches of light, we view the
colour through the yellow spot in the eye. If, when we have matched the
luminosity in the ordinary manner, we turn our eyes some 10° away from
the patches, we shall find that except at one place in the green the
equality in brightness no longer exists. By a little practice we can
make matches of luminosity when the eyes are thus diverted. This will
give us a different curve of luminosity, as the yellow spot absorption
is absent, and the difference in the heights of the ordinates between
the two curves will give us that absorption. Fig. 20 shows this very
well; and it will be noticed that the eye is appreciably not so
sensitive to the red and yellow at 10° from the axis as it is on its
central area. If we measure the areas of these two curves we get the
relative values of the light energy which is active on the two parts
of the eye, and these we found to be as 167 to 156. The heights at
which to put the maxima of the two curves were found from various
considerations, and the correctness of the deductions was verified by
directly comparing the intensities of two patches of white light some
10° apart, which, when looked at direct, were of equal intensities.
When one was compared with the other, the eye receiving one image
centrally and the other outside the yellow spot, the difference in
values was closely proportional to those of the above areas. The part
of this last curve showing a deficiency in red sensation is very
similar to that obtained from a person who is partially colour blind.
The absorption by the yellow spot derived from these measures is
graphically shown in the next figure (Fig. 21).
[Illustration: FIG. 20.]
[Illustration: FIG. 21.]
The question of the visual sensation at the “fovea centralis” (if it be
admitted that this is coincident with the visual axis of the eye, as is
usually accepted) may be very easily studied. When the luminosity of
the spectrum is examined at five or six feet distance, by throwing the
two patches on the whitened face of a small square of half-inch side,
we get a result differing from both of the above. The fovea appears
to be slightly more sensitive to red than the macula lutea, and is
generally less sensitive to the green rays (see Fig. 20). If a star,
or a distant light, be observed with the part of the retina, on which
the axis of the eye falls, as is the case in ordinary vision, and then
be observed with the eye slightly directed away, the difference in the
colours of the light is unmistakable. (The tables giving the measured
value of these curves will be found in the appendix, page 211.)
[Illustration: FIG. 22.]
Can we in any way find from these methods the colour sensation curves?
I think we can. Suppose we have a second instrument exactly like the
first placed side by side with it, we can then throw two patches of
colour on the two adjacent white surfaces, and we can mix with either,
or both of them, as much white light as we choose. From the second
instrument let us throw all the spectrum colours in succession on to
the one surface, and on to the other the three primary colours mixed in
such proportions as to match them accurately. This plan is, I venture
to think, a better way of obtaining the value of colours in terms of
standard colours than that adopted by Maxwell. This method gives the
values directly, and not by calculation from matches with white. Let
us place one slit near each of the extreme ends of the spectrum; that
in the red near the red lithium line, and another a little beyond G
in the violet of the spectrum, whilst the third slit should be in the
exact position in the green, where the _green-blind sees grey_. Now it
might be a matter of dispute as to whether one was entitled to make
this last one of the positions for the slits, for we use it entirely
on the assumption that two of the colour sensations which we suppose
we possess are identical with those of the green-blind. This might be,
or might not be, the case; but I think it can be shown very easily
that the assumption we are making is more than probably exact. Having
the slits in these positions, we may endeavour to match the spectrum
orange. We mix the red and the green lights together, and find that the
best mixture is always paler than the orange, but by adding a small
quantity of white to the orange we at once form a match. In the same
way if we have a greenish-blue to match, we shall find that we can only
make the match when we add a little white to the simple colour. Now
let us shift the position of the slit in the green just a little--a
very little--towards the blue, and again try to match orange. Do what
we will we cannot find apertures to the slits which will give us the
colour, though it be diluted with white. It will be too blue or too
red, but never exactly orange. This tells us that there is too much
blue in the green we are using. Next, shift the slit a little towards
the red below our fixed position, and endeavour to match the blue.
We shall find that this, too, becomes impracticable. The blue is
either too green or too violet, telling us that our mixture contains
too much green. As the neutral point of the colour blind is the only
position for the green slit which enables us to make a good match to
both the orange and the blue, it follows that this must be the point
where these two colour sensations are so arranged as to be in the
proportions required to form white when green is added; that is, that
there is neither an excess of red nor an excess of violet. To come
back to our measures of colour. We can make up every spectrum colour
with these three colours, and finally divide the luminosity curve into
the _colour luminosity_. In all these matches the violet luminosity is
very small indeed compared with the red or green. A match with white
is now made by a mixture of all these colours, and you will see, from
the images of the slits on the screen, that the _luminosity_ of the
violet is almost a negligible quantity compared with the others. We
may, therefore, as a first approximation, divide up the luminosity
curve into two parts, one being the luminosity of the green in the
different colours and the other of the red. The green, however, is
made up of red, of violet, and of an excess of green sensation, which
in this case comes practically to a mixture of white with the green
sensation. How can we tell how much is green and how much is white?
Suppose I, as a normal-eyed person, compare the luminosity of the
colour coming through the red slit with that coming through the green
slit, and then get the green-blind to do the same, it is evident that
any excess in the luminosity as measured by myself over that measured
by the green-blind must be due to the green sensation, and we can also
see how much red and violet make up his white. We shall not be far
wrong, then, in apportioning the constituents of the white thus found
between the green and the red; the violet being, for the time being,
negligible. We must subtract the red sensation from the green _colour_
curve and add it to the red colour curve: the two curves will then be
very closely the curves of the _red and green sensations_. By causing
the green-blind to make mixtures of red and violet for all the colours
of their spectrum, we can arrive at what must be finally taken away or
given to these curves, though such addition or subtraction of violet
will be small when the luminosities are considered. The accompanying
figure (Fig. 22) gives an idea of the shape and general features of
these curves. It may be remarked that we can check the general accuracy
of the measures of the colour mixtures by calculating or measuring the
areas of the two colour curves, the red and the green. If accurate,
they should bear the same ratio that the _luminosities_ of the two
colours bear to each other (when mixed with a little violet, which
is practically negligible) to form white light. So far, then, we can
utilize the luminosity methods to calculate and to trace the sensation
curves for the normal eye. It will not escape your notice that the
maximum heights of these two component curves are nowhere near the
parts of the spectrum where the colour is the purest. Another check to
these curves may also be made by taking the difference in the ordinates
of the luminosity curves of the colour blind and the normal eyed. Too
much stress must not, however, for the moment, be laid on this, as
this method depends on the absolute correctness of the scale of the
ordinates in the curves. It must be recollected that to the former
white light is deprived of at least one constituent sensation which is
perceived by normal eyes. This, in all probability, renders the white
less luminous to them than those possessing normal vision, so that the
comparisons of luminosities are referred to different standards.
It may seem a very simple matter to ascertain the correct scale, but it
is not, except by the extinction method, which will be described later.
At one time General Festing and myself tried to obtain a comparison
by finding the limiting illumination at which a book could be read.
We got results, but for the purpose in question the values are not
conclusive. What we really were measuring was the _acuteness of vision_
in different coloured lights. As a good deal depends upon the optical
perfection of the eyes under examination, besides the illumination,
we must be on our guard, even if there were nothing else against the
method, against taking any such measures as being conclusive.