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]).
Fig. 20.
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].)
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.