PROPERTIES OF THE RETINA. 331 



conduction paths for the cones seem to be more direct than in 

 the case of the rods. These latter elements, moreover, possess the 

 visual purple, which is lacking in the cones. Lastly, in the eye of 

 the totally color blind, in the dark-adapted eye in dim lights, in the 

 color-blind peripheral area of the normal eye, and in the eyes of 

 most distinctly night-seeing animals, such as the mole and the owl, 

 vision seems to be effected solely by the rods. These facts find 

 their simplest explanation perhaps in the view advocated by Pari- 

 naud, Franklin, von Kries,* and others, according to which the 

 perception of color is a function of the cones alone, while the rods 

 are sensitive only to light and darkness, and by virtue of their power 

 of adaptation in the dark through the regeneration of their visual 

 purple they form also the special apparatus for vision in dim 

 lights (night vision). Color blindness, therefore, whether total or 

 partial, may be regarded as an affection or lack of normal develop- 

 ment of the cones. On the other hand, those interesting cases in 

 which the vision, while good in daylight, is faulty or lacking in dim 

 lights (night blindness, hemeralopia) may be referred to a defective 

 functional activity of the rods, probably from lack of formation of 

 visual purple. 



Theories of Color Vision. A number of theories have been 

 proposed to explain the facts of color vision. None of them has 

 been entirely successful in the sense that the explanations it affords 

 have been submitted to satisfactory experimental verification. The 

 immediate stimuli that give rise to the visual impulses are assumed 

 to be of a chemical nature, and it seems probable that in this 

 case as in that of many other problems of physiology, we must 

 await the development of a more complete knowledge of the 

 chemical processes involved. The theories proposed at present, 

 while all tested by experimental inquiries, are in a large measure 

 hypotheses constructed to fit more or less completely the facts that 

 are known. Three of these theories may be described briefly as 

 examples of the modes of reasoning employed: 



/. The Young-Helmholtz Theory. This theory, proposed essen- 

 tially by Thomas Young (1807) and afterward modified and ex- 

 panded by Helmholtz,t rests upon the assumption that there are 

 three fundamental color sensations, red, green, and violet and 

 corresponding with these there are three photochemical substances 

 in the retina. By the decomposition of each of these substances cor- 

 responding nerve fibers are stimulated and impulses are conducted 

 to a special system of nerve cells in the visual center of the cerebrum. 



*Von Kries, "Zeitschrift f. Psychologie u. Physiol, d. Sinnesorgane," 

 9, 81, 1895. 



f Helmholtz, "Handbuch der physiologischen Optik," second edition, 

 1896, 1, 344. 



