1046 



VISION. 



bleaching, and the process may be repeated two or three times. Boll supposed 

 that the regenerating substance is derived from the coloured globules of the 

 epithelial cells, and described the globules after regeneration as more faintly 

 coloured. But no definite relation has been found by other observers between 

 the action of light and the condition of either the coloured globules or the 

 my fluid granules of the pigment epithelium. Regeneration takes place when 

 there is no pigment, as in the albino rabbit, and in those mammals (man, ox, 

 pig) in which there are no fat globules in the epithelium. 



The times necessary for bleaching and regeneration of visual purple have 

 been determined. In the, living frog ten minutes' exposure to strong sunlight 

 is necessary, while regeneration requires one to two hours ; in a warm-blooded 

 animal bleaching is far more rapid, according to Kiihne, sixty times more rapid 

 than in the frog. Solution of visual purple from the frog and mammal, how- 

 ever, bleach in about the same time. The difference in the two animals may 

 depend partly on the temperature, partly on the much greater length of the 

 rods in the frog, and consequent greater thickness of the purple layer. The 

 size of the pupil is also of influence. Kiihne found by experiments on the 

 rabbit with one pupil dilated by atropin, that the amount of bleaching differed 

 greatly in the two eyes. The effect of temperature on the time of bleaching 

 differed to some extent in solutions obtained from the frog and rabbit, and 

 Kiihne supposes that there are different visual purples as there are different 



Fia. 379. Kiihno. 



In order to investigate the absorption spectrum of visual purple, the retina 

 must bo treated with a solution of bile salts. Owing to the fact that bile has 



a powerful solvent action on 

 red blood corpuscles, the solu- 

 tion of visual purple so obtained 

 is mixed with haemoglobin, 

 especially in man and other 

 mammals. 1 Kiihne ' 2 has re- 

 cently described two methods 

 for separating the haemoglobin. 

 In one, bile salts and visual 

 purple are precipitated by an 

 excess of magnesium sulphate, 

 leaving the haemoglobin in solution ; in the other, a retina which has 

 been hardened in alum is left for some hours in a 10 per cent, solution of 

 sodium chloride, after which treatment with bile salts extracts visual purple 

 only. JSTo observations have yet been recorded on the spectrum of human visual 

 purple dissolved with these precautions. The spectra of visual purple and 

 visual yellow of the frog are shown in Fig. 379. 



Kiittgen and AbelsdorlP 5 have determined the absorption of visual purple 

 in a larg' number of animals. They found that in mammals, birds (owl), and 

 amphibia, the maximum absorption took place at 500 A, while in lishes the 

 maximum was at 540 A. The curves representing the absorption in the two 

 lasses are shown in Fig. ."SO. Kiihne's new method was not used, but the 

 differences seem to be too great to be explained by the presence of haemo- 

 globin in one and not in the other. Kb'ttgen and Abelsdorff failed to 

 obtain any evidence of the existence of visual yellow in their investigations. 



The influence of coloured light on visual purple is of great importance in 

 relation to its possible function in colour vision. The red of the spectrum 

 has least action, and, as already mentioned, in moderate intensity does 



1 In the frog a pure solution may be obtained, owing to the vessels in this animal being 

 limitiMl to the hyaloid ,Kuhm-\ 



* Ztschr.f. B\ol. t 1895, Bd. xxxii. S. 21. 



* Ztschr.f. Psychol. u. Physiol. & Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 161. 



