90 PROCEEDINGS OF THE AMERICAN ACADEMY. 



are likely to be increasingly so, since, as I have noted in a previous 

 paper (loc. cit.), selective radiation is necessary to high luminous effi- 

 ciency. One has to deal with the yellow of the flaming arc, the yel- 

 lowish green of the Welsbach, the blue green of the mercury tube, and 

 the violet of the enclosed arc, all of which may have to be compared 

 with the deep orange of the Hefner lamp. 



Practically the question of suitable color resolves itself into two parts, 

 — first, the effect of color on the proper functioning of the visual appa- 

 ratus, and second, its relation to our observation of colored objects. I 

 shall not take up here the theories of color vision, save to note that 

 many of their difficulties may now be charged to the existence of at least 

 two kinds of independent visual elements, the rods and cones, differently 

 distributed in the retina, and possessing two radically different types of 

 visual sensitiveness. That the cones are highly evolved rods has been 

 shown beyond much doubt by Cajal, and is in evidence in the simple rod 

 structure found in the parietal eyes of some fishes and lizards and in 

 lower organisms generally. Whether, as Mrs. Franklin ^° surmised, there 

 are definite intermediate phases of sensitiveness between the achromatic 

 vision of the rods and the full chromatic vision of the cones is an 

 important topic for research. 



May I venture to suggest that there are some reasons for thinking 

 that there may even be a difference in kind between a simple photo- 

 chemical rod stimulation and the strongly selective stimulation of the 

 highly specialized cones 1 Selective activity does not necessarily con- 

 note chemical instability. They may coexist, as in some organic dye- 

 stuffs, or may be entirely independent, as in the fluorescence of heavy 

 paraffin oils. The presence of strong pigmentation at the rods and its 

 absence at the cones, coupled with the absence of visual purple in some 

 nocturnal creatures whose eyes are presumably specialized for very weak 

 light, suggests that the evolution of the retinal elements may have pro- 

 ceeded along more than one line. In fact, the Young-Helmholtz and 

 Hering doctrines may find in a heterogeneous retina a certain amount 

 of common ground. Be this as it may, mankind certainly has super- 

 imposed a very sensitive but achromatic rod vision, and a much less 

 sensitive but chromatic cone vision, the latter being mainly central 

 and the former mainly peripheral. The passage from predominant rod 

 vision to predominant cone' vision is shown in the sharp flexure of 

 the curves in Figure 1. The exact point at which the color sensitive 

 cones begin to get into action undoubtedly varies greatly in different 

 eyes, and in the same eye in different conditions of adaptation. As the 



" Mind, N. S., 2, 473 et seq. 



