Colour vision

by Sir William de Wiveleslie Abney

CHAPTER VIII.

Before quitting the measurement of luminosity, it may be as well to see whether the curves described are the same whatever the brilliancy of the spectrum may be. We can easily experiment with a very reduced brightness. Upon the screen we have an ordinarily bright spectrum. As the slit, through which the white light forming the spectrum comes, is narrowed, there is an evident change in the relative brightness of the different parts, though the energy of every ray must be proportionally reduced. The red is much more enfeebled than the green, and in brightness the green part of the spectrum looks much more intense than the yellow, which is ordinarily the brightest part. This we have assured ourselves of not only by casual observation, but also by direct measurement. Perhaps I can make this even more decisive to you. Two slits are now in the ordinarily bright spectrum, one in the red, and the other in a green which is near the E line of the solar spectrum. Instead of using one lens to form a single colour patch of mixed light, two parts of a lens, appropriately cut and of the same focal length as the large combining lens, are placed in front of the slits, one bit of lens before each. This artifice enables us to throw the patch of red on one white surface, and the patch of green light on another adjacent to it. By opening or closing one or other of the slits the brightness of the two patches of light are so arranged that there is no manner of doubt but that the red is the brighter of the two. The absolute energies of the rays forming each of the patches are proportionally reduced by closing the slit of the collimator, as before. At one stage both patches appear of about the same intensity. This might be taken for an error in judgment, but to make the change that takes place perfectly plain to you, the rotating sectors are introduced in front of the two slits, and the rays now pass through them. The apertures of the sectors are gradually closed, and we now come to such a reduction that the red is absolutely invisible; but the green still shines out. It is losing its colour somewhat, and appears of a bluish tint. The reason of this change of hue in the latter we shall shortly see. The sectors are withdrawn and the red re-appears, and is as bright as the green. The slit of the collimator is next opened, and there is no doubt that the red is much brighter than the green, as it was purposely made at the beginning of the experiment. The same class of experiment might have been repeated with the green and violet or the red and violet, and the same kind of results would have been obtained. The violet would have been the last to disappear when the green was so reduced in luminosity that it appeared in the ordinary brilliant spectrum to be equal to the violet ray selected. When the green was of the luminosity given by a slit equal in width to that of the violet, the violet would have disappeared first, owing to its feeble brightness to begin with. Now, if we measure a feebly illuminated spectrum we must adopt some special means to exclude all light, except that of the comparison light and the ray to be measured. This we can do by the box which is shown in the next diagram ([Fig. 24]).

Fig. 24.

At one end of a box, shown in plan, is an eye-piece, E. The other end has at its centre a white square of paper of 1½-inch scale. The monochromatic beam a, coming from the spectrum through the slit S and the reference beam b of white light, are reflected from glass mirrors M₁, M₂ to apertures in opposite sides of the box, and from close to these apertures by the right-angled prisms P₁ P₂, so as to fall on and cover S. Rods R₁, R₂ are inserted in the box in the paths of the beams, so that the opposite halos of S are illuminated. Diaphragms inside the box cut off any stray light, and rotating sectors placed at A and B regulate the intensity of the beams as required. The sector A is rotated with a previously determined-on aperture; the white light coming through B is altered till the luminosity of the two on the screen, as seen through E, are the same. Every part of the spectrum can be measured in this way; the result is shown in the diagram. [Fig. 25] (the measures will be found at [page 215] in the appendix). In this case the orange light at D where it fell on the screen was equal to 1/132 of an amyl-acetate light, which, in its turn, is closely ·8 of a standard candle. In the same figure the luminosity curve of the ordinary bright spectrum is given for reference, and it can be seen how the point of maximum luminosity is shifted into the green, lying almost over the E line of the solar spectrum. The maximum, of course, has been made 100 as before, for had it been drawn to the same scale as the other, the form of the curve would not have been demonstrated. There is a remarkable resemblance between it and the curve of luminosity of the monochromatic vision, and such a resemblance can scarcely be fortuitous. As a matter of fact, in this we seem to have come to the final curve for low luminosities, and is almost the same as that observed when the spectrum is reduced to such an extent that it is colourless throughout, a condition that it can assume, as we shall see very shortly. When the spectrum is rather more luminous, it gives a curve of luminosity which is similar to that of the ordinary spectrum when measured by a red-blind person. Here, then, we have an indication that a person with normal vision passes through a stage of red-blindness, as the intensity is diminished before he arrives at absolutely monochromatic vision.

Fig. 25.

This investigation is of practical as well as theoretical interest, as General Festing and myself quickly discovered when we first made it. The curious colour of a moonlight landscape is entirely accounted for by it. White light becomes greenish-blue as it diminishes in intensity, and the reds and yellows, being reduced or absent, are not reflected by surrounding objects. Hence, moonlight is cold, whilst the sunlight is warm owing to their presence.

When measuring these low luminosities, the various colours will in a great measure disappear. Part of the spectrum will be of that peculiar grey which was shown you in the experiment with the incandescent light (p. 34). By further experiment it is possible to arrive at an approximate determination of the point where all colour vanishes from the different parts of the spectrum. We use the same apparatus ([Fig. 24]) as before, the only difference being that each of the sectors is movable during rotation. The apertures of those through which the colour passes are reduced till all colour on the screen just disappears, the point being arrived at by a comparison with the white, which is itself also reduced. The apertures of the first sector alone need be noted, and from these readings the diagram ([Fig. 26]) is made (for measures, see [page 216]).

Fig. 26.