616 BRAIN AND NERVES. 



absent, and is also necessarily absent in the macula lutea. In a variety of bat 

 (Rhinolophus hipposideros), in hens, pigeons and new-born rabbits, no visual 

 purple has been found in the rods. 



A solution of visual purple in water which contains 2-5 per cent crys- 

 tallized bile, w r hich is the best solvent for it, is purple-red in color, quite 

 clear, and not fluorescent. On evaporating this solution in vacuo we 

 obtain a residue similar to ammonium carminate which contains violet 

 or black grains. If the above solution is dialyzed with water, the bile 

 diffuses and the visual purple separates as a violet mass. Under all 

 circumstances, even when still in the retina, the visual purple is quickly 

 bleached by direct sunlight, and with diffused light with a rapidity cor- 

 responding 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 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 regenera- 

 tion 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. 



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



