This extinction of colour is one which often occurs, but is seldom noticed. The figure tells us that the orange is about the last colour of the spectrum left, some of the others still appearing as greys. The next to retain its colour is the green, and the most rapid to lose them are the red and violet. It must not be supposed that the colours remain of the same hue up to the time that they vanish. Pure spectrum red (red sensation) remains the same up to the last, but the scarlet becomes orange, and the orange yellower, and the green bluer. This is what would be predicted from the Young theory if the order of extinction of sensation be red, green, violet. This we shall see is the case. At nightfall in the summer the order of disappearance of colour may often be seen; orange flowers may be plainly visible, yet a red geranium may appear black as night; the green grass will be grey when the colour of the yellow flowers may yet be just visible. An early morning start in the autumn before daybreak will give an ample opportunity of satisfying oneself as to the order in which colours gradually re-appear as daybreak approaches. Red flowers will be at the outset black, whilst other colours will be visible as grey. As more light comes from the sky the pale yellow and blue flowers will next be distinguished, though the grass may still be a nondescript grey. Then, as the light still increases, every colour will burst out, if not in their full brilliance, yet into their own undoubted hue.
CHAPTER IX.
Not only, however, may we lose a sense of colour, but we may also lose all sense of light by reducing the energy of the different rays. We have seen that colour goes unequally from the different parts of the spectrum. We may therefore prognosticate that the light itself may disappear more rapidly from some parts than from others. You will scarcely, however, I think, be prepared for the enormous difference which exists in the stages of disappearance of the grey of the reduced red and of that of the reduced green.
Fig. 27.
But how are we to measure this extinction of light at the different parts of the spectrum? This is a problem which I have attacked during the last few years by a variety of methods; but as is the case with almost every scientific problem, when the mode of attack is reduced to its simplest form, it yields the more readily to solution. If we have a box, like that figured in [Fig. 27], and combine it with our colour patch apparatus, the problem is solved. B B is a closed box 3 feet long and about 1 foot high and wide, having two similar apertures 1½ inch in diameter in the positions shown. The aperture at the side is covered on the inside by a piece of glass a, ground on both sides, and a tube T is inserted, in which diaphragms, D, of various apertures can be inserted at pleasure. The most convenient form of diaphragm is that supplied with photographic lenses—an iris diaphragm. E is a tube fitted at the end of the box through which the screen S is viewed. S is black except in the centre, where a white disc is fastened to it. A mirror, M, placed as shown, reflects the light scattered by the ground glass on to the screen S. The rotating sectors are placed where shown, and are in such a position that they can be readily adjusted by the observer. The patch of any desired colour of the spectrum is thrown on a, and an appropriate size of diaphragm used, so that when the sectors are not less than 5° to 10° open, the light totally disappears. We can now make observations throughout the whole spectrum, and knowing the value of the different apertures of the diaphragm and the angular opening of the rotating sectors, we can at once find the amount of reduction of the particular part of the spectrum that is being required in order to just extinguish all traces of light from the white disc at the end of the box. From these measures we can readily construct a curve or curves which will graphically show the reduction given to the different parts of the spectrum. [Fig. 28] gives the curve of extinction for ordinary normal colour vision. The spectrum was of such a brilliance that the intensity of the square patch of light formed on a of the orange light (D) was exactly that of an amyl-acetate lamp, placed at one foot distance from the receiving screen. Knowing this, the actual luminosity of all the other rays of the spectrum can be derived from the curve of luminosity (see [Fig. 20]). Extinguishing the various parts of the spectrum by this plan, it is found that the red rays cease to stimulate the retina sufficiently to give any appearance of light long before the green rays are extinguished. It is only the rays in the extreme violet of the spectrum, and which consequently possess very feeble luminosity, that make any approach towards requiring the same amount of reduction as the red rays.
Fig. 28.
There is the fact to remember in making these measures in the extreme red and the extreme violet, that the luminosities of the colours are so small that the illumination of the prism itself, by the white light falling on it, has to be taken into account, since it forms an appreciable portion of the patch of feeble colour. By placing a proper shade of blue or red glass in the front of the collimator slit this white light disappears or becomes negligible, and when the absorption of the coloured glass is known from measurement, we can get a very accurate measure of the extinction of these parts. Some people may propound the idea that the rotating sectors may in such kind of measurements give a false result. Now such a criticism is quite fair, and it is absolutely necessary that it should be answered. Well, to test the accuracy or the reverse of the assumption that such measures are correct, the following small piece of simple apparatus was devised. A and B ([Fig. 29]) are two mirrors placed at angles of 45° to the angle of incidence of the beam. The path the beam takes can be readily ascertained from the figure. This piece of apparatus was placed in position in front of the spectrum, and the reflected beams used to form the patches of colour. For convenience only a small pencil of light was allowed to issue from the prism, a diaphragm of some ½-inch in diameter being placed in front of it. This allows a spot of any desired colour to fall on the screen, the ground glass being removed. The slit through which the spectrum colours pass is moved along the spectrum, and a position is arrived at where the last glimmer of light disappears.