ioo8 THE SENSES 



proverbial ' blindness ' of that animal, such a hasty deduction 

 would be at once corrected by the fact that birds with as sharp 

 vision as the pigeon are equally devoid of visual purple, while in 

 other nocturnal animals, like the owl, it is plentifully found. The 

 most probable hypothesis of the function of the visual purple is 

 indeed that which attributes to it the property, in virtue of its 

 capacity for regeneration in the dark, of adapting the eye for night 

 or twilight vision in other words, of increasing the sensitiveness 

 of the retina for faint light, especially of the shorter wave-lengths. 

 If this is the case, it is precisely in nocturnal animals that we should 

 expect to find it in large amount; and recently visual purple has 

 been obtained from more than one species of bat (Trendelenburg). 

 The fact that central vision (p. 1018) in which the rodless fovea is 

 concerned is but little, if at all, susceptible of dark-adaptation, 

 while peripheral vision shows a marked capacity of adaptation, 

 agrees well with this hypothesis. We shall see later that there is 

 some evidence that it is the mere perception of luminous impressions 

 as such and of their intensity, without any distinction of quality 

 or colour, with which the rods have to do. They are, then, on the 

 hypothesis under discussion, elements concerned in achromatic 

 sensations under conditions of feeble illumination (twilight vision). 

 The cones are supposed on this theory to be more highly developed 

 elements than the rods, their function being connected, especially 

 with the perception of colour, but also with the perception of 

 achromatic sensations under daylight conditions. 



The pigmented retinal epithelium is undoubtedly sensitive to light, 

 and has important relations to the formation of the visual purple. 

 When the eye is exposed to light, black pigment migrates along the 

 processes of the epithelial cells between the rods, even as far as the 

 external limiting membrane. In the dark the pigment moves back 

 again, and gathers around the outer portions of the rods, where the 

 visual purple is being regenerated. The precise meaning of the changes 

 in the pigmented cells is obscure. 



The pigmented epithelium is known to be concerned in the regenera- 

 tion of the visual purple. When a frog is curarized, oedema occurs 

 between the retina and the choroid, so that the former membrane is 

 separated from the hexagonal epithelium. If the frog is now exposed 

 to sunlight till the visual purple is bleached, and the retina then taken 

 out and placed in the dark, no regeneration of the purple takes place. 

 When the same experiment is repeated on a non-curarized frog, the 

 visual purple is restored in the dark, and may be seen under the micro- 

 scope in the rods. The only difference in the two experiments is that 

 in the latter the pigmented epithelium adheres to the retina, and it 

 must therefore have a hand in the regeneration of the pigment. Even 

 the visual purple of a retina from which the epithelium has been de- 

 tached will, after being bleached, be restored if the retina is simply laid 

 again on the epithelial surface. And it does not seem to be the black 

 pigment of the hexagonal cells which is the agent in this restoration, 

 for it takes place in the pigment-free retinae of albino rabbits or rats. 

 Even a retina isolated from the pigmented epithelium, and then 

 bleached, may, to a certain extent, develop new visual purple in the 



