212 COLOUE VISION 



arguments, and he regards Hess's observations on birds^ as also con- 

 firmatory. 



Baiier^ has brought forward arguments to show that the role of the 

 visual purple is not limited to the condition of dark adaptation. Since 

 Klihne's researches it has been known that it is difficult to bleach the 

 visual purple completely so long as the retina is in contact with the 

 pigment epithelium. Bauer found that though frog's visual purple was 

 rapidly bleached when exposed to bright sunlight, the retina again 

 became red after several hours in spite of continuous exposure. He 

 concluded that increased destruction of the substance is associated under 

 physiological conditions of exposure to bright light by correspondingly 

 increased production, and that therefore the visual purple plays an 

 important part in photopic as well as in scotopic vision. 



The clear explanation which the duplicity theory affords of the 

 chief differences between human photopic and achromatic scotopic vision 

 and of the peculiarities of the intermediate stage, scotopia with only 

 moderate dark adaptation (chromatic scotopia), tempts one to carry 

 the theoretical considerations farther than is perhaps warranted. 

 Dark adaptation appears to be directly associated with the visual 

 purple. On teleological grounds extreme sensitiveness to light and 

 shade are most needed in the dusk, and while the loss of visual acuity 

 for form is a serious disadvantage the gain in concentration is a counter- 

 balancing merit. The rods are more intimately connected with each 

 other by nervous paths than the cones (Ramon y Cajal), so that relatively 

 widespread impulses become summated in the resultant psychological 

 impressions. An even less complete insulation of the rods may be 

 brought about by dark adaptation, possibly through the retraction of 

 pigment in the retinal pigment cells. On the other hand the photopic 

 position of the pigment tends to protect the more sensitive rods with 

 their contained visual purple and to isolate the less sensitive cones. 

 The role of the pigment cells in dark adaptation has already been the 

 subject of conjecture^, but it must be remembered that the wandering 

 of the pigment has not been conclusively proved to occur in warm- 

 blooded animals. 



1 Arch.f. Angenhlk. Lvn. 298, 317, 1907 ; lix. 143, 1908. 

 * Arch. f. d. ges. Physiol, cxli. 479, 1911. 



3 Exner, Sitz. d. Wiener Akad. xcvm. 3, 1889 ; Nagel, in v. Helmholtz, 3rd ed. n. 

 331. 



