342 PHYSIOLOGY CHAP. 



the one hand and the cells of the pigment epithelium on the 

 other. As in the organs muscles, electrical and luminous organs 

 -provided with centrifugal nerves, so, too, in the sens.-- >\ --.in- 

 provided with centripetal nerves definite material alterations of a 

 physical, chemical, and anatomical character correspond to the 

 physiological states of rest and activity." 



Kiihne (1877-79) successfully continued and developed Boll's 

 researches on visual purple, but unjustifiably attempted to claim 

 the credit of its discovery. He proved that it was not an inter- 

 ference colour, but a trui- pigment, as Boll had also stated; that it 

 is soluble in a solution of bile acids or their salts ; and that, con- 

 sequently, the changes which the visual purple undergoes in 

 white or coloured liubt come under the category of photochemical 

 phenomena. 



According to Kiihne, visual purple resists the death <>f the 

 tissue, putrefaction, and desiccation, so lonu as it is protected from 



J&Sf ?s 



n 



Ki. . L66. Toshow mosaic of rods in frog's retina. (Boll.) 

 A. attt-i IHD!' in daikii.'-s ; B, aftei Longer exposure in violet li{ 



the action of light. It also resists a number of oxidising and 

 reducing chemical reagents; but it is destroyed by most aci<ls 

 and caustic alkalies, by alcohol, ether, chloroform, etc. 



Visual purple can be fixed by 4 per cent solution of alum, and 

 preserved for a certain time from the decomposing action of 

 light, although it becomes more or less altered. By this means 

 Kiihne succeeded in obtaining optoyrams or photographic images 

 on the retina of rabbits or frogs, similar to those on photographic 

 plates. For this purpose the animal, which had previously been 

 atropiuised and kept in the dark, was placed for 1-i minutes in 

 front of a window, and then killed ; the retina was dissected out in 

 yellow sodium light, and the image fixed by the alum solution. 

 Even after several days the image of the window, etc. could be 

 plainly seen. As shown in Fig. 167, the illuminated part of the 

 window was white, and the remainder pink. 



Kiihne further confirmed and demonstrated the fact already 

 pointed out by Boll that the origin of retinal purple in the 

 outer part of the rods is associated with the pigment epithelium. 



