The Senses of Animals. 279 



lateral edges of the spectrum would be the limits of the 

 instability of E. If G. only were developed, we should see 

 only a band of monochromatic g. light. Its centre would 

 not coincide with that for E., but would lie in a region of 

 smaller wave-length. Here would be the maximum in- 

 stability for G. On either side the green would fade away. 

 Its lateral edges would mark the limits of the instability 

 of G. But though their centres would not coincide, the 

 E. band and the G. band would to a large extent overlap. 

 Similarly with the band for V. It, too, would have its 

 centre of maximum instability and its lateral edges of 

 lessening instability. Its centre would lie in a region of 

 yet smaller wave-length than that for G. And the v. band 

 would overlap the green and the red. 



Normally, all three bands are developed, and their 

 blended overlapping gives the colours of the rainbow. For 

 this reason the monochromatic bands r., g., and v. are un- 

 known to us in experience. All the colour-tints we know 

 are blended tints. What we call full-red light causes 

 strong disruptive change in E., but decomposes slightly 

 G., and probably also, but in much less degree, V. 



Whether E., G., and V. are all three present in each 

 cone, or whether they are each developed in separate 

 cones, we do not know for certain. Nor are we certain 

 that there are separate nerve-fibres for the transmission of 

 stimuli due to E., G., and V. 



When we look steadily at a red object we cause the 

 disruption of E. ; and since it takes some time for the 

 reformation and reconstitution of this explosive substance, 

 on turning the eye to a grey surface, G. and V. are alone, 

 or in preponderating proportions, caused to undergo dis- 

 ruption. Hence the phenomena of complementary after- 

 images. It is not merely a matter of the tiring of certain 

 nerve-fibres, but a using-up of the explosive material in 

 certain of the cones. 



What is called colour-blindness is probably due to one 

 of several abnormal conditions. It is possible that in some 

 cases E., G., or V. may be entirely absent. More fre- 



