The Evolution of Optics. 281 



the retina, nor are they supported by any known facts as to 

 the nature of light. The presence of three kinds of retinal 

 elements or of three chemical substances can not be demon- 

 strated. Neither can any division of light-vibrations cor- 

 responding to three primary colors, nor, in fact, to any color, 

 be found in the spectrum. 



When the spectral band falls on the retina, the material 

 with which the retina has to deal is simply a series of ether- 

 vibrations of varying periods, and in this series we find those 

 waves which we see as one color differ as widely from each 

 other as they do from the vibrations in the next adjacent 

 color band. The difficulty seems to be that, having started 

 with the assumption that the power to differentiate color 

 resided in the retina, a retinal structure has been assumed 

 to correspond to the theory ; but why this work has been 

 assigned to three sets of nerve fibers or chemical substances 

 is not clear. I see no escape for those who hold this theory 

 but the assumption of a retinal substance attuned to each 

 vibration in the visible spectrum, for, as we have before said, 

 no breaks occur corresponding to the color bands with 

 which we are acquainted in experience. 



There has been much good work done of late in illumi- 

 nating this dark field of color-perception, and the theory 

 that seems most acceptable to our evolutionary ideas is that 

 which transfers the seat of color-differentiation from the 

 retina to the brain. All the retina has to do is to receive 

 the rays of light which fall upon it, and which create, ac- 

 cording to their wave-length, a characteristic molecular dis- 

 turbance, thus making the function of the eye a refined 

 temperature-sense, and the translation of such impressions 

 into what we know as color a purely psychical phenomenon. 



That we do see definite bands of color, and that our ordi- 

 nary division of the spectrum must have a basis of fact 

 somewhere, seems self-evident. The explanation of this 

 phenomenon has been very ably given by my friend Dr. 

 Gould, and I would recommend his instructive monograph 

 on the human color-sense to all who are interested in this 

 subject. He starts with the fundamental idea that our 

 color-sense must be the organism's response in reaction un- 

 der stimulus. It has arisen as a response to light-stimulus ; 

 not to the stimulus of pure white light, but to the light that 

 we see every day in nature. 



" Sunlight, we are told, is composed of the following 

 parts: 



