PROBLEM OF COLOR VISION—DANE. 615 
rod-like in form, and greatly exceed the rods in length. (Fig. 1, B.) 
Slender cones are also found in the thicixened area centralis which in 
many mammals replaces the human fovea. : 
In the ape, horse, pig, cow, sheep, and dog the rods and cones are 
similar to those of man. In rodents which avoid the light.the cones 
are “ very small and hard to detect since their inner segments scarcely 
differ from those of the rods, from which they may be distinguished 
by their much shorter outer segment. M. Schultze at first questioned 
the existence of cones in the mouse, guinea pig, mole, hedgehog, and 
bat. The cat undoubtedly has cones, but they are small, slender, and, 
except in the area, infrequent.” * Birds have a single or double fovea, 
like that of man. Cones are small but very numerous, and in their 
inner segments they often contain a drop of oily substance, either 
colorless or various shades of yellow, green, or red. Presumably 
these drops, which are absent from the rods and some of the cones, 
exert an important influence upon color perception. In owls the 
bright colored drops are lacking and the cones are said to be fewer. 
Some reptiles have fovexr; two kinds of visual cells are reported, 
neither of which resembles the mammalian rods. M. Schultze be- 
lieved that reptiles have only cones. In fishes and amphibia, both 
rods and cones occur; in some sharks, rays, and eels, however, the 
cones so resemble rods that they may be overlooked. Whether or not 
deep-sea fishes are without cones is apparently unknown. In the 
various groups of animals the rods and the cones each present modifi- 
cations of structure, with which as yet physiological observations 
have not been correlated. 
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 experiences a momentary blindness; after becom- 
ing accustomed to the darkness, a modified form of vision is re- 
gained. In this twilight vision the fovea is far less sensitive to hght 
than the more peripheral parts of the retina. Moreover all ob- 
jects 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 fluctua- 
@The quotation, and much of this account of the retina, is from Von Ebner’s 
résumé in Koelliker’s Handbuch der Gewebelehre, 1902, vol. 3, pp. 818-832. 
6 Von Kries presents this Duplizitiitstheorie in Nagel’s Handbuch der Physiol- 
ogie, 1904, vol. 3, pp. 168-195. 
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