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THE A M ERIC A N NA T URA LIST [Vol. XLI 



believed that reptiles have only cones. Tn fishes and amphibia, 

 l)()th rods and cones occur; in some sharks, rays, and eels, however, 

 the cones so resemble rods that they may i)e overlooked. Whether 

 or not deep sea fishes are without cones is apparently unknown. 

 In the various (jroups of aninuils the rods and the cones each 

 present modifications of structure, with which as yet physiological 



Physiology. The physiology of color vision is the study of the 

 functions of the rod and the cone cells. In passing from a bright 

 to a very dim illumination one ex|)crienccs a momentary blindness; 

 after becoming accustomed lo tlic darkness, a modified form of 

 vision is regained. In this iirl/ii/hl risimi the fovea is far less 

 sensitive to light than the more peiiplit-rai parts of the retina. 

 Moreover all objects appear in shades of gray. The spectrum is 

 bright but colorless, and its brightest part has shifted from the 

 yellow portion toward the blue. Von Kries has explained these 

 facts by assuming that the cones are the agents of day vision, and 

 the rods of twilight vision.' Cones, exclusively, occur in the fovea 

 where day vision is most acute; and rods predominate where 

 twilight vision is at its best. The fluctuations in the visual purple 

 of the rods show that they respond to the varying intensities of 

 dim light, and tliis purple is known to desintegrate most rapidly 



or not the bleached t'dds are active in day'^vi.ion has not been 



It is probable that all cones do not respond to color stimuli. 

 In the periplieral })orti()n of the retina there is a partially color- 

 one another; and the outermost portion of th(< retina is always 

 totally color blind. Since cones occur in tliese areas they also 

 must'be color blind. From these con.si.h-ration^ it is reasonably 



