12 COLOUR VISION 



of the pigment in both (Engelmann). Light on the skin produces the 

 effect in frogs ; so too electrical stimulation of the optic nerve, probably 

 through the centrifugal nerve fibres described by Ramon y Cajal 1 . 

 The photo tropic reaction of the pigment epithelium has not yet been 

 proved to occur in mammals 2 . 



In the frog's eye in the state of rest (darkness) the cones are extended ; 

 on exposure to light they become contracted. This reaction occurs in 

 all animals that have been examined, including man (van Genderen 

 Stort 3 ) ; but Garten (in Graefe-Saemisch, loc. cit.) found the reaction 

 doubtful in monkeys. The reaction is slow, taking two or more minutes 

 even with intense light. The violet end of the spectrum acts most 

 strongly (Engelmann), but the reaction to the red end is greater than 

 that of the pigment. Light on one eye causes reaction in both, as also 

 light on the skin, so long as the brain is intact (Engelmann, Nahmacher 4 ). 



Chemical Changes. Light on the retina causes it to become acid 

 (Angelucci, Lodato 5 , Dittler 6 ), and its staining reactions are said to alter. 



The most important chemical change, however, has to do with the 

 visual purple or rhodopsin. This remarkable substance was discovered 

 in the rods of the frog's retina by H. Miiller in 1851. Boll in 1876 

 discovered that it was bleached by exposure to light. Kiihne in 1878 

 first studied it exhaustively. It occurs in all animals which possess 

 rods, and is present in the rods only (Kiihne). Hence it is absent from 

 the fovea. This statement is denied by Edridge-Green and Devereux 

 Marshall 7 for monkeys, but their observations have not been confirmed. 

 Hering, however, points out that visual purple may possibly not be 

 wholly absent from the cones, and the peculiar rod-like character of 

 the human foveal cones makes it not improbable that they contain 

 some of the substance. The question merits further investigation. 

 Kiihne investigated the chemical characteristics of rhodopsin, the most 

 noteworthy facts being its solubility in bile acids and their salts, and its 

 resistance to strong oxidising and reducing agents. It is not the cause 

 of the fluorescence of the retina. It can only be seen ophthalmoscopically 

 in fishes which possess a white tapetum (Abelsdorff 8 ). Tait 9 and Boll 



1 Die Retina der Wirbeltiere, Wiesbaden, 1894. 



2 Hess, Vergleichende Physiol. d. Gesiclitsinnes, Jena, 1912. 



3 Arch.f. Ophth. xxxra. 3, 229, 1887. 4 Arch. f. d. ges. Physiol. LIII. 375, 1893. 



5 Arch, di Oil. ix. 267, 1902. 



6 Arch. f. d. ges. Physiol. cxx. 44, 1907. 



7 Trans. Ophth. Soc. xxii. 300, 1902. 



8 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. xiv. 77, 1897 ; Sitz. d. Berliner Akad. 1895. 



9 Proc. R. S. of E din. vn. 605, 1869. 



