CHAP. XI.] PERIPHERAL NERVOUS END -ORGANS. 461 



phanes slowly become decolourized even in the dark. They are 

 much more rapidly bleached in the light, though very much less 

 rapidly than the visual purple to be afterwards described. Under 

 the most favourable circumstances a solution of chlorophane exposed 

 to the direct rays of the sun will be bleached in a few hours ; a 

 solution of xanthophane under similar circumstances will resist for a 

 period three times longer, and a solution of rhodophane for a period 

 twenty times longer. The process of decolourization is stated by 

 Kuhne to be dependent upon the presence of oxygen and to be there- 

 fore probably due to oxidation changes. 



Special Chlorophane is of a greenish yellow colour; its 



characters of alcoholic and ethereal solutions possess this tint. They 

 Chlorophane. present two absorption bands; these (in the case of a 

 petroleum ether solution) are situated between F and G. 



Special Xanthophane, ' unlike Chlorophane, is but slightly 



characters of soluble in petroleum ether, but readily soluble in al- 

 Xanthophane. cohol, ether and carbon disulphide. The solutions ex- 

 hibit a strong absorption of the violet end of the spectrum and a 

 single absorption band, which in the case of the ethereal solution is 

 situated near F, and on its violet side. In the case of solutions in 

 bisulphide of carbon the absorption band is situated between b and F. 



Special This colouring matter is not at all soluble in petro- 



characters of leum ether or carbon disulphide. It is most readily 

 Rhodophane. soluble in oil of turpentine and in alcohol which has 

 been acidified with acetic acid ; these solutions become decolourized, 

 after some hours, even in the dark. Solutions in benzol may be kept 

 indefinitely. These solutions exhibit marked absorption of the violet 

 end of the spectrum and a single absorption band between b and F. 



Colouring matters associated with the rods, 

 (Visual Purple or Ehodopsin) 



Historical In the year 1851 Heinrich M tiller 1 pointed out that 



Notes. the rods of the retina of the frog when seen en masse 



often present a reddish colouration. In 1857 Leidig 2 referred to the 

 satiny-red colour possessed by the retina of the frog. Later Max 

 Schultze drew attention to the satiny-red colour of the rods of the 

 retina of the rat and owl. 



These observations did not however attract marked attention and 

 were lost sight of until the publication of a remarkable paper by 

 Boll, presented to the Berlin Academy towards the close of the year 

 1876 3 , in which that observer announced the startling fact that the 

 bacillary layer of the retina of all animals is during life not colourless, 

 but of a purple red colour. 



1 H. Mueller, Zeitschr. f. wiss. Zoologie, Vol. in. p. 234. 



2 Leidig, Lehrbuch d. Histologie, p. 238. 



3 Boll, "Zur Anatomie u. Pbysiologie der Retina." Monatsber. d. Berl. Acad., 

 12 Nov. 1876. 



