THE CONTRAST OF COLORS. 3 



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 par- 

 tial 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 in- 

 tervals 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. In- 

 stead, then, of gray, let us take j-ellow 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 



YELLOW 



\ ORANOE \ 



Fig. 4.— Yellow Groxtnd with 

 Green Slip. 



Fig. 5.— Yellow GRotrND wth Oeange- 

 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 vio- 

 let 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 



