vii EETINAL EXCITATION 343 



He found that a retina, previously dissected and bleached by 

 light, can recover its red hue if it is again brought into contact 

 with pigmented epithelium in the dark. This influence of the 

 epithelium cannot, however, depend on the pigment, because the 

 regeneration of visual purple takes place also in the retina of 

 albino rabbits, in which the epithelium contains no pigment. 



Eesearches subsequent to the work of Boll and Klihne have 

 added little to our knowledge of the rhodopsin (erythropsin) or 

 visual purple. It is now proved to exist in all classes of animals 

 in which the retina is provided with rods except pigeons and 

 fowls and to be absent in retinae that contain only cones. 

 Schenck and Zuckerkandl found rhodopsiu in the retina of an 

 executed criminal ; Fuchs and Yelponer found it also in the 

 retina of the human foetus of 7-9 months. 



Previous to the discovery of visual purple, Helmholtz (1855) 

 and Setschenow (1877) had noted that the retina observed under 

 ultra-violet light is fluorescent. Klihne pointed out that this 

 fluorescence cannot be due to the rhodopsin, 

 because retinae previously exposed to bright 

 light and thus entirely colourless and trans- 

 parent are more fluorescent than those 

 previously kept in the dark and charged 

 with purple. Obviously the fluorescence 

 must depend on the colourless derivatives of 

 the rhodopsin. Nagel and Himstedt (1902) 

 found that colourless solutions of visual FI. i67.-optoram or photo- 

 purple in sodium glycocholate are more Sne n reti " a obtained by 

 fluorescent than simple solutions of bile 



salts. On the other hand, the retina of pigeons is also fluores- 

 cent, particularly after exposure for a few minutes to daylight, 

 although it is entirely destitute of purple. 



The pigmented epithelial layer of the outer surface of the * 

 retina prevents ophthalmoscopic observation of the visual purple 

 in man and in most living animals (Becker, Coccius). In fishes, 

 on the contrary, which have a white tapetum behind the layer of 

 rods, e.g. Abramis brama, it can be seen and its discoloration 

 watched by means of the ophthalmoscope (Abelsdorff). 



Immediately after .the discovery of the visual purple not a 

 few hoped with Boll that it might represent an important factor 

 in the construction of a complete physiological theory of vision. 

 But we are now forced to admit that vision is entirely independent 

 of the retinal purple. There is none in the cones, and therefore 

 in the fovea where vision is most acute ; in pigeons and fowls 

 the rods are destitute of purple ; snakes have only unpigmented 

 cones. Kiihne made investigations to determine if in the rabbit's 

 retina, in which only rods are present, the purple is the substance 

 which on its decomposition excites vision ; but he was forced to 



