we have presented to us, mainly, a mixture of the sensations red and violet, giving as a final result rose-red or purplish-red. The green nerves, of course, are not so fatigued that they do not act at all when the gray light is presented to them, but the only effect that their partial action has, is to render the rose-colored image somewhat pale or whitish in appearance. The fatigue of the optic nerve mentioned here does not differ essentially from that which it undergoes constantly even under the conditions of ordinary use, where the waste is constantly made good by the blood circulating in the retina, and by the little intervals of rest constantly occurring. In our experiment we have merely confined the fatigue to one set of nerves, instead of distributing it equally among the three sets.
The above experiments and explanation will enable us easily to comprehend the more complicated case, where, instead of placing our little green square on gray, we lay it on a sheet of colored paper. Instead, then, of gray, let us take yellow paper, placing the green square on it as before (see Fig. 4). On suddenly withdrawing the green square, we find it replaced by an orange-colored ghost (Fig. 5), which
|Fig. 4.—Yellow Ground with Green Slip.||Fig. 5.—Yellow Ground with Orange-colored Image.|
we account for thus: As before, the green nerves are fatigued, the red and violet nerves remaining fresh; when the square is removed, yellow light is presented to the retina, and this yellow light, as explained in Chapter IX., tends to act on the red and green nerves equally, but the green nerves in the present case do not respond with full activity, hence the action is more confined to the red nerves, and, as explained in Chapter X., the resultant tint is necessarily orange, that is to say, we have a strong red sensation mingled with a weak green sensation, and the result is the sensation called orange. In this experiment the violet nerves do not come into play to any great extent. If the green square is placed on a blue ground the image becomes violet, for the reason that the blue light which is presented to the fatigued retina acts, as explained in Chapter IX., on the green and violet nerves; but the green nerves being already fatigued, the action is mostly confined to the violet nerves, and hence the corresponding sensation. In this case the red nerves hardly come into play at all.
It follows, from the above examples and reasoning that the final effect is, that we obtain as an after-image what amounts to a mixture