VISUAL PURPLE AND VISUAL YELLOW 677 



Visual Purple and Visual Yellow and Accommodation of the Eye for 

 Different Degrees of Illumination. The outer segments of the rods of 

 the retina sometimes present a peculiar red or purple color which disap- 

 pears after ten or twelve seconds of exposure to light. This was first 

 observed by Boll (1876) in the retinae of frogs that had been kept for a 

 certain time in the dark. From his preliminary researches Boll con- 

 cluded that this coloration of the retina exists only during life and per- 

 sists for but a few moments after death ; that it is constantly destroyed 

 during life by the action of light and reappears in the dark ; and finally 

 that it plays an important part in the act of vision. Kiihne and others 

 have since confirmed and extended the original observations of Boll; 

 and the visual purple (rhodopsin) has been noted in the mammalia and 

 in man. It has been extracted from the retinae of frogs and dissolved 

 in a five-per-cent solution of crystallized ox-gall, still presenting in solu- 

 tion its remarkable sensitiveness to light. Finally it has been found 

 possible to fix images of simple objects, such as strips of black paper 

 pasted on a plate of ground glass, upon the retina, by a process very like 

 photography. 



The visual purple is produced by the cells of the pigmentary layer 

 of the retina and from them is absorbed by the outer segments of the 

 rods. It is not present in any part of the cones and does not exist, 

 therefore, in the area of distinct vision, at the fovea centralis. The 

 rapid disappearance of the color under the influence of actinic rays of 

 light renders it necessary to examine the retina under a non-actinic 

 (monochromatic) sodium-flame. When thus examined and gradually 

 exposed to actinic rays, the color quickly fades into a yellow and finally 

 disappears, being restored, however, in the dark. If the choroid and the 

 pigmentary layer of the retina are removed, the rods are bleached, and 

 the color is restored in the dark when the choroid is replaced. In the 

 eye of the frog, kept in the dark, the hair-like processes that extend 

 from the pigmentary layer of the retina downward between the rods 

 and cones are retracted, and the pigment is then contained chiefly in 

 the cells themselves. After prolonged exposure of the retina to light, 

 these processes, loaded with pigment, extend between the cones as far 

 as the limitary membrane. 



The fact that visual purple is not found in the fovea centralis is 

 opposed to the theory that its existence is directly essential to distinct 

 vision ; nevertheless, certain phenomena observed in passing from a 

 bright light to comparative obscurity, and the reverse, show that the 

 purple has, at least, an important indirect action. In passing from the 

 dark to a bright light, the eye is dazzled and distinct vision is difficult. 

 It may be assumed that this is due to unusual general sensitiveness of 



