344 PHYSIOLOGY CHAP. 



conclude that rabbits, too, can see perfectly after the purple has 

 been broken up by sunlight, before it can be regenerated. There 

 is no doubt that the accumulation of rhodopsin in the rods coincides 

 with an enormous increase in their sensibility to light ; but we 

 do not know if this really depends on an accumulation of the 

 purple, or on other concomitant chemical changes that take place 

 in the dark. The discovery of a highly photo-aesthetic sub- 

 stance in the retina nevertheless makes it very probable that 

 in all the elements of the retinal mosaic other photo-aesthetic 

 substances may exist which have not yet been detected, owing 

 to their being colourless, but which, by the alterations induced 

 in them by the luminous vibrations of the ether, may be cap- 

 able of exciting the peripheral and central neural apparatus of 

 vision. 



Of no less interest are the anatomical alterations, or better 

 the phenomena of movement, observed in the retina under the 

 influence of light and darkness. 



We have said that l>oll first described in the frog the move- 

 ment of the pigment granules of the retinal epithelium along 

 the filiform processes that penetrate between the rods and cones 

 (1876). Angelucci (1878), continuing Boll's work on the frog, 

 was altle better to observe this displacement of pigment, and found 

 that in darkness it travelled as far as the upper third of the 

 outer segments of the rods, while under the influence of light it 

 ascended to the membrana limitans externa. He further saw 

 that the movement of the pigment granules coincides with a 

 contraction of the protoplasm of the pigmented epithelial cells. 

 Under the action of light, both the outer unpigmented part and 

 the pigmented base of these latter diminish in height, while in 

 the dark both parts thicken considerably. Finally, he noted that 

 both the displacement of the pigment granules and the contraction 

 of the retinal epithelium increase progressively under mono- 

 chromatic light from red to violet, that is from the less to the 

 more refractive rays. 



Kiilme (1879) observed in the frog that the retinal pigment 

 (fuscin) disappears under the protracted action of light. This 

 fact was fully confirmed in fishes by Pergens (1896) and Chiarini 

 (1904), who further noted that after the fuscin had been entirely 

 used up by prolonged action of direct sunlight it was slowly 

 regenerated in the dark, and only attained its maximum after 

 15-20 hours (Fig. ] 68). 



Angelucci (1882) also found that the rods contracted under 

 the action of white and monochromatic light ; but he failed to 

 detect the striking fact that the inner segments of the cones 

 contract, which was discovered by van Genderen-Stort (1884). 

 This observation, which has been confirmed by all later workers, 

 completes the series of important morphological changes in the 



