HISTORICAL REVIEW OF MODERN THEORIES 197 



and thought that one such was the so-called visual yellow. Haab^ 

 elaborated similar suggestions on the basis of Schultze's theory and 

 Hering's theory. 



Charpentier (1877 sqq.) distinguished " perception lumineuse brute " 

 or " vision diffuse " from " vision nette," the former with vague localisa- 

 tion, the latter with sharply defined. The " photaesthesic elements " 

 initiate impulses which mutually modify each other as in the inter- 

 ference phenomena of light and thus the two apparatus, each capable 

 of initiating only colourless perceptions alone, become capable of giving 

 rise to colour-perceptions. He did not attempt to define the exact 

 locality of the apparatus ; it is supposed to be partly peripheral or 

 retinal, partly central or in the nervous system. He conjectured that 

 diffuse vision is initiated by the rods and visual purple, but only in 

 achromatic scotopic vision, since the visual purple can itself be viewed 

 entoptically^. 



Parinaud^, in ignorance of Schultze's theory, elaborated much the 

 same views with more extensive material. He attributed the disease 

 known as night blindness -(often badly termed hemeralopia) to defects 

 in the rods and visual purple'*, and first demonstrated that central 

 vision is intact in this complaint. He asserted the failure of adapta- 

 tion at the fovea in 1884, having previously in 1881 pointed out that 

 the photochromatic interval is absent here. These facts, confirmed 

 by the night blindness of fowls and pigeons owing to absence of visual 

 purple, led him to the conclusion that the cones, which are alone present 

 in the fovea, are incapable of dark-adaptation, whereas achromatic 

 scotopic vision is a purely extrafoveal phenomenon and is the function 

 of the rods and visual purple. This is the " theory of two retinas." 

 Colour-perception according to Parinaud is a purely cerebral function. 



Similar views were expressed by Liesegang^ and by Parinaud's 

 pupil Weiss^. Berry'^ suggested that the pigment epithelium is the 

 seat of colourless light sensations. 



Konig^ also regarded the visual purple as the excitant of the rods 

 (KUhne) and with low intensities of light stimuli the basis of achromatic 

 scotopic vision. He considered the visual yellow, the first product of 



^ Correspondenzbl. f. Schweizer Aerzte, ix. 1879. 



^ C. r. soc. de biol. 1890 ; C. r. acad. des sci. 1891. 



3 C. r. acad. des sci. 1881, 1884, 1885; Ann. d'ocul. lxxxv. 113, 1881; cxii. 228, 

 1894 ; La Vision, Paris, 1898. 



* Arch. gin. de med. 1881. s photogr. Arch. 1891. 



6 Rev. gin. des sci. 1895. ' Ophth. Rev. 1890. 



8 Sitz. d. Akad. d. Wiss. Berlin, 1894. 



