700 SPECIAL SENSES. 



Nerve of my Eight Eye, whilst I kept my Left shut. Then I plac'd myself 

 over against the First paper, and drew back by little and little, keeping my 

 Eight Eye fixt and very steddy upon the same ; and being about 10. foot 

 distant, the second paper totally disappear'd." 



In this experiment the i,;ays of light from the paper which has disap- 

 peared from view are received upon the punctum caecum, at the point of 

 entrance of the optic nerve. If the observer withdraw himself still farther, 

 the second circle will reappear, as the rays are removed from the punctum 

 caecum. With the ophthalmoscope, the point of penetration of the optic 

 nerve may readily be seen in the living eye. If the image of a flame be 

 directed upon this point, the sensation of light is either not perceived or it 

 is very faint and indefinite, and it is then probably due to diffusion to other 

 portions of the retina. 



The relative sensibility of different portions of the retina has been meas- 

 ured by Volkmann and has been found to be, in an inverse ratio, equal to 

 .about the square of the distance from the axis of most perfect vision. This 

 observer calculated the distance between the sensitive elements of the retina 

 at which he supposed that two parallel lines would appear as one. In the 

 axis of vision, the distance was 0-00029 inch (7*366 /x), and at a deviation in- 

 ward of 8, it was 0-03186 inch (809-244 /*), a diminution of acuteness of 

 more than a hundred times. 



Visual Purple and Visual Yellow, and Accommodation of the Eye for 

 Different Degrees of Illumination. The outer segments of the rods of the 

 retina sometimes present a peculiar red or purple color, which disappears 

 after ten or twelve seconds of exposure to light. This was first observed by 

 Boll (1876) in the retinae of frogs that had been kept for a certain time in 

 the dark. From his preliminary researches, Boll concluded that this colora- 

 tion of the retina exists only during life and persists but a few moments after 

 death ; that it is constantly destroyed during life by the action of light and 

 reappears in the dark ; and finally that it plays an important part in the act 

 of vision. Kuhne and others have since confirmed and extended the original 

 observations of Boll ; and the visual purple (rhodopsine) has been noted in 

 the mammalia and in man. It has been extracted from the retinae of frogs 

 and dissolved in a five-per-cent. solution of crystallized ox-gall, still present- 

 ing in solution its remarkable sensitiveness to light (Ayres). Finally it has 

 been found possible to fix images of simple objects, such as strips of black 

 paper pasted upon a plate of ground glass, upon the retina, by a process very 

 like that of photography. 



The visual purple is produced by the cells of the pigmentary layer of the 

 retina and from them is absorbed by the outer segments of the rods. It is 

 not present in any part of the cones and does not exist, therefore, in the area 

 of distinct vision, at the fovea centralis. The rapid disappearance of the 

 oolor under the influence of actinic rays of light renders it necessary to ex- 

 amine the retina under a non-actinic (monochromatic) sodium-flame (Ayres). 

 When thus examined and gradually exposed to actinic rays, the color quickly 

 iades into a yellow and finally disappears, being restored, however, in the dark. 



