Colour vision

by Sir William de Wiveleslie Abney

Fig. 29.

The mirrors A and B may both be of plain glass blackened with smoke on one side, or one may be plain glass and one silvered, or they both may be silvered. This, with the power possessed of altering the aperture of the slit of collimator, puts us in possession of ample means of making our measures. We may also use the ground-glass arrangement and use different diaphragms, which puts a further power of variation in our hands. I may at once state that the resulting measurements fell on the curves, obtained by measurements made with the rotating sectors, a sufficient proof that the sectors may be used with confidence. There is still another method which avoids a resort to the sectors. A tapering wedge of black glass can be moved in front of the colour slit, and a different thickness of glass will be required to cause the extinction of each colour. Recently I have modified the extinction box, more particularly for the purpose of using it where the spectrum is to be formed of a feeble light, such as that of an incandescent lamp or a candle. If a really black wedge could be obtained, this would seem to be the best method, but no glass is really black. We have, therefore, to make a preliminary study of the wedge to ascertain accurately the absorption co-efficients for the different rays, a piece of work which requires a good deal of patience, but which, when done, is always at command.

In [Fig. 28] two branches of the curves are given at the blue end of the spectrum; one is shown as the extinction for the centre of the eye, and the other of the whole eye. Of course the former observations were made by looking direct at the spot. This may appear a very easy matter, but it is not really so simple as it sounds. It is curious how little control there is over the absolute direction of the eyes when the light has almost disappeared. The axes of the eyes are often directed to quite a different point. When the extinction for the whole eye is made, the readings are really much easier, as then the eye roams where it likes, and a final disappearance is noted. When the eye has once been invested with a roving commission, it is hard to control it. In making these observations it was therefore advisable to have data for the first branch of the curve, before commencing to observe for the later. The main cause of difference between the two branches of the curve is due to the absorption by the yellow spot.

It might be thought that with the curves ([Fig. 28]) before us, we have learnt all we can regarding the extinction of light, but is it so? Surely we ought to know something as to the reduction necessary for extinction of the different parts of the spectrum when they are all of equal luminosities and of ordinary brightness.

We arrive at this by simple calculation. Supposing we have two luminosities, one double the other, it does not require much thought to find out that you have to reduce the greater luminosity twice as much as the other in order for it to be just extinguished. In other words, if we multiply the extinction by the luminosity, we get what we want. Now, in the curves before us, we have taken the luminosity of the yellow light near D as one amyl-acetate lamp, and that has a height in the curve showing the spectrum luminosity very closely approaching 100. We may, therefore, multiply the extinctions of a ray by the value of its ordinate in the luminosity curve and divide the result by 100, and this will give us the extinction of each colour, supposing it had the luminosity of an amyl-acetate lamp. A portion of the curve so calculated is shown in the same diagram ([Fig. 28]) as a dotted line. It appears at the violet end as an approximately horizontal line, and then starts rapidly upwards, and would, if carried on to the same scale, reach far out of the diagram; but at the extreme red it would be found to bend and again become horizontal. I would have you notice that the same is true not only for the extinction observed with the centre of the eye through the yellow spot, but also for the whole eye. Such straight, horizontal parts of the curve must mean something.

Fig. 30.

In the diagram ([Fig. 16]) of colour sensations we see that in each of these two regions there is but one sensation excited, viz. the violet and the red. Now, if these sensation curves mean anything, the reduction necessary to produce the extinction of the same sensation when equally stimulated should prove to be the same, for there is no reason to the contrary, but exactly the reverse. Primâ facie, then, taking the Young theory as correct, we may suppose that these horizontal parts are due to the extinction of one sensation. Let us treat it as such, and go back to the original extinction curve shown in the continuous lines. The parts of the curve which lie over the fairly horizontal dotted line, at all events, should be the extinction curve of the same sensation, but more or less stimulated or excited. As before explained, if we have double the stimulation at one part of the spectrum to that we have at another, the reduction of the greater luminosity to give extinction will be double that of the lesser. If, then, we take the reciprocals of the extinction, it ought to give us a curve which is of the form of some colour sensation; and when we arrive at the maximum, we may for convenience make that ordinate 100, and reduce the other ordinates proportionally. This has been done in [Fig. 30] in the curves C and D. For the sake of a name my colleague and myself have named such curves “persistency curves.” Perhaps some other name might be more fitting; but still a poor name is better than none at all.

When the persistency curve was scrutinized to see what might be taken as its full signification, I must confess that the result astonished us somewhat, though we ought not to have been surprised. The persistency curve C, when applied (in a Euclidean sense) to the curve of luminosity recorded for the men who had monochromatic vision, almost exactly coincided with it. In other words, by far the largest part of the extinction was due to the extinction of the sensation which in the monochromatic vision was alone excited. If this be not the case, there is something in colour vision which no theory which I am acquainted with can account for. Then, again, the persistency curve agrees with the curve of luminosity when the intensity of the spectrum is very feeble, which is another coincidence of a remarkable character which some theory should explain. [[Fig. 30] gives, besides the persistency curves, the luminosity curves of the normal eye, of monochromatic vision, and of the violet-blind; and an exaggerated curve of the difference between the normal luminosity curve and that of the violet-blind, and others which I think will be found useful for general reference.]