VISUAL PURPLE. 585 



spending to the intensity of the light. It passes from red and orange 

 to yellow. Red light bleaches the visual purple slowly; the ultra-red 

 light does not bleach it at all. A solution of visual purple shows no special 

 absorption bands, but only a general absorption which extends from the 

 red side, beginning at D and extending to the G line. The strongest 

 absorption is found at E. 



KOETTGEN and ABELSDORF l have shown that there are, in accordance with 

 KUHNE 's views, two varieties of visual purple, the one occurring in mammals, 

 birds, and amphibians, and the other, which is more violet-red, in fishes. The 

 first has its maximum absorption in the green and the other in the yellowish 

 green. 



Visual purple when heated to 52-53 C. is destroyed after several 

 hours, and almost instantly when heated to 76 C. It is also destroyed 

 by alkalies, acids, alcohol, ether, and chloroform. On the contrary, 

 it resists the action of ammonia or alum solution. 



As the visual purple is easily destroyed by light, it must therefore 

 also be regenerated during life. KUHNE has also found that the retina 

 of the eye of the frog becomes bleached when exposed for a long time 

 to strong sunlight, and that its color gradually returns when the animal 

 is placed in the dark. This regeneration of the visual purple is a function 

 of the living cells in the layer of the pigment epithelium of the retina. 

 This may be inferred from the fact that a detached piece of the retina 

 which has been bleached by light may have its visual purple restored 

 if it is carefully laid on the choroid having layers of the pigment-epithe- 

 lium attached.- The regeneration has, it seems, nothing to do with the 

 dark pigment, the melanin or fuscin, in the epithelium cells, A partial 

 regeneration seems, according to KUHNE, to be possible in the retina 

 which has been completely removed. On account of this property of 

 the visual purple of being bleached by light during life we may, as KUHNE 

 has shown, under special conditions and by observing special precautions, 

 obtain after death, by the action of intense light or more continuous 

 light, the picture of bright objects, such as windows and the like so- 

 called optograms. 



The physiological importance of visual purple is unknown. It follows 

 that the visual purple is not essential to sight, since it is absent in certain 

 animals and also in the cones. 



Visual purple must always be prepared exclusively in a sodium light. 

 It is extracted from the net membrane by means of a watery solution 

 of crystallized bile. The filtered solution is evaporated in vacuo or 

 dialyzed until the visual purple is separated. To prepare a visual-purple 

 solution perfectly free from haemoglobin the solution of visual purple 



1 Centralbl. f. Physiol., 9; also Maly's Jahresber , 25, 351. 



