410 PROCEEDINGS OF THE AMERICAN ACADEMY 



tissues connecting tlie disks, and another different curve representing 

 neural fatigue ; so that there would be two waves of excitation along 

 the cone-spectrura, each followed at different intervals by slower, damp- 

 ing waves of fatigue : and it seems quite possible that the combined 

 effects of both must be estimated in explaining all species of after- 

 images. Many circumstances, which will readily occur to those who 

 have followed us thus far, would lead us to believe that persistence of 

 impressions would be found due more to tlie terminal apparatus than 

 to the nerve action. The functions of the rods still remain unex- 

 plauied. It has been suggested that, as white light is analogous to 

 noise, the functions of the rods correspond to what was, until lately, sup- 

 posed to be that of the vibratory hairs or the otoliths of the labyrinth. 

 The very fact that these disks are smaller and more numerous and more 

 uniform in size, while they contain apparently less nervous elements, 

 suggests indeed that their function is to emphasize position more mi- 

 nutely than is done by the cones.- That they do not perceive color is 

 macje to some extent probable by the fact that retinal purple, which 

 seems to exist only in them, is not specifically modified by color as 

 such ; while, finally, if there is any reason to believe, as has been sug- 

 gested, that cones are modified or developed rods, then the surprising 

 theory of Hugo Magnus, which is so strangely countenanced by j)hilo- 

 logical facts, — viz., that the color-sense has been developed out of sim- 

 ple perception of light and shade within the historic period, and in the 

 S{)ectral order, beginning with red, and with some corresponding loss 

 in the accuracy of form-perception, — has at least one physiological 

 fact in its favor. 



Many other possible but unverified analogies with the ear are 

 suggested: e. g., may there be found any such correspondence between 

 the length or number of cones and the brilliancy of colors developed 

 among lower animals — and especially birds — by sexual selection as 

 has been observed between the length of the ductus cochlearis and 

 the development of songs and love-calls? Is there any special reason 

 wliy, as the lower notes affect only the vibratory fibres most remote 

 from the entrance of the labyrinth, so the red rays pass the other sen- 

 sitive disks, to be perceived at the further end of the cone? 



It is high time to remember that our theory can be called, at most, 

 only probable, until the course of the rays through the retina can be 

 more accurately traced. First, let us suppose light to undergo a spe- 

 cial and final refraction, in the substance of the retina itself, before it 

 is perceived. This is rendered highly probable by the great refractive 

 power of the cone substance and of the retinal purple ; by the fat 



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