14 



COLOUR VISION 



40 



3-0 



2-0 



10 



00 



OS 



^ 



Fig. 1. • • — the bleaching values of spectral lights for frog's visual purple, 



and X — X — X the human achromatic scotopic luminosity curve. Abscissae, wave- 

 lengths of the prismatic spectrum of the Nernst light ; ordinates, an arbitrary 

 scale. (Trendelenburg.) 



curve will be referred to later (pp. 53, 55). Actual readings are as 

 follows : 



Visual purple is regenerated after bleaching in the living and " surviv- 

 ing " retina, but only on contact with the pigment epithelium. Regene- 

 ration commences in the frog after 29 minutes and is complete in one 

 to two hours. The process is more rapid in the rabbit, commencing in 

 seven minutes and being complete in 33 to 38 minutes. (It may be 

 noted here for convenience that increase of sensibility in dark adaptation 

 in man after good light adaptation commences in 7 to 8 minutes and 

 reaches nearly the maximum in 40 to 45 minutes (Piper^).) 



It has already been stated that the visual purple occurs in nearly 

 if not all animals possessing rods from petromyzon to man. It is present 

 in albinotic animals, e.g. rabbit, and has been found in the 9- and 7- 

 months' human foetus (Klihne, Fuchs and Welponer'^). Kiihne said that 

 it was absent in the bat {Rliinoloj^lius hijjposiderus), but this statement 

 has been disproved by Trendelenburg. Kiihne found no visual purple 



^ Ztsch. f. Psychol, m. Physiol, d. Siimesorg. xxxi. 161, 1903> 

 2 Wiener med. Woch. 221, 1877. 



