io 4 8 VISION. 



mention here the experiment of Kiihne 1 on the vision of frogs whose visual 

 purple had been completely bleached. He found that they could evidently 

 see, were able to catch flies in bright sunlight, and behaved very differently 

 from blind frogs. He found that the great majority of normal frogs put 

 themselves under green in preference to blue glass, when differences of 

 temperature and intensity were excluded. Bleached frogs also preferred 

 green glass, while blind frogs were found indiscriminately under each. 



Other changes in rods. In addition to the changes in colour, the 

 outer limbs of the rods have been observed to change in size under the 

 action of light. In frogs kept in the dark, there are distinct spaces 

 between the rods ; the distances, according to v. Hornbostel, 2 varying 

 from -5 to '8 /*, ; in frogs exposed to the light, the rods are separated 

 only by narrow lines. The swelling, however, is only perceptible after 

 nearly one hour's exposure. No increase in diameter occurs in red light. 



Angelucci 3 and Gradenigo 4 describe contraction of the inner limbs 

 of the rods under the action of light, so that they become shorter and 

 thicker. In the dove, under the other hand, Stort 5 described the inner 

 limb as longer and thinner in the retina exposed to light. 



The pigment epithelium. The changes produced in the pigment 

 epithelium by the action of light were first studied by Kiihne, Boll, 

 and Angelucci. In the eye of a frog which has been kept in the 

 dark, the pigment layer separates easily from the rod and cone layer, 

 and the brown pigment is found heaped up in the body of the 

 epithelial cell and round the outer extremities of the rods, not reaching 

 beyond the outer third of the length of the rod. In an eye which has 

 been illuminated, the epithelium and rods are separated with difficulty ; 

 the body of the epithelial cell contains much less pigment, so that the 

 nucleus is clearly seen ; and the pigment is much more abundant between 

 the outer limbs of the rods, and extends also between the inner limbs as far 

 as the membrana limitans externa. In birds and mammals the pigment 

 layer naturally separates more easily than in the frog, but in them the 

 pigment shows the same change of distribution under the influence of 

 light and dark. In the frog, about ten minutes of illumination are 

 needed to bring the pigment to the membrana limitans externa, while 

 one and a half to two hours are necessary for return to the dark 

 position ; these times correspond very closely with those necessary for 

 bleaching and regeneration of visual purple respectively in the same 

 animal, and point to the close connection of the two processes. The 

 pigment, however, differs from visual purple in its behaviour to coloured 

 light, the movement forwards taking place actively in red light. Ac- 

 cording to Angelucci, 6 the violet end of the spectrum is that which acts 

 with the greatest intensity, and there is, according to him, a gradual 

 increase in the movement forwards with decreasing wave-length of the 

 light used. According to A. E. Fick, 7 the forward movement of the 

 pigment, once set up by a short exposure to light, goes on in the dark. 

 He found that frogs from the dark exposed to the light for one minute, 

 and then kept in the dark for twenty minutes, showed forward position 



1 Untersuch. a. d. physiol. List. d. Univ. Heidelberg, Bd. i. S. 119. 



2 Ibid., Bd. i. S. 409. 



3 Untersuch. z. Naturl. d. Mensch. u. d. Thiere, 1892, Bd. xiv. S. 237. 



4 Quoted from Angelucci. 



5 Arch. f. Ophth., 1888, Bd. xxxiii. Abth. 3, S. 275. 



6 Untersuch. z. Naturl. d. Mensch. u. d. Thierc, 1892, Bd. xiv. S. 252. 



7 Arch.f. Ophth., 1891, Bd. xxxvii. Abth. 2, S. 1. 



