by the Action of Luminous Points and Lines. 171 



retina produces phasnomena of a very interesting kind. If we 

 look at the sun through a small aperture at a great distance 

 from the eye ; or, what is the same thing, if we look at the di- 

 minutive image of the sun formed by a convex lens or a con- 

 cave mirror, or, seen in a convex surface, the light which falls 

 upon the retina does not form a sharp and definite image of 

 the luminous point, but it sends out in all directions an in- 

 finity of radiations covering in some cases almost the whole 

 retina. These radiations are extremely bright, and are ac- 

 companied by mottled colours of great variety and beauty. 

 The bright point of light propagates around it circular undu- 

 lations, which are broken and coloured by interference, and 

 which, being in constant motion from the centre of the retina 

 in all directions, occasion the radiations which have been 

 mentioned. 



4. If we now look at the radiant image just described, 

 through a narrow aperture, a very singular effect is produced. 

 A vortex of circular rays appears on each side of the radiant 

 point, and the rays have a rapid whirling motion. The line 

 joining the centres of the two vortices is ahvays perpendicular 

 to the narrow aperture. This remarkable configuration of 

 the rays is evidently produced by the union of a system of 

 parallel undulations with a system of circular ones, the inter- 

 sections of the parallel fringes and the diverging radiations 

 forming the circular rays, as in the case of ordinary caustics. 



The preceding phaiuomena, whatever be their true cause, 

 clearly prove that light incident upon the retina exerts an 

 action on parts of it upon which it does not directly fall, and 

 that the same action renders other parts of the retina insensi- 

 ble to the light which actually falls upon these parts. 



This remarkable effect is still more distinctly shown in 

 tlie interesting experiment first described by Dr. Smith of 

 Fochabers*. Upon looking with both eyes at a narrow slip 

 of white paper, in such a manner as to see it double, he of 

 course saw two slips equally white; but upon.bringing a can- 

 dle near one eye, the right one, for example, the image seen 

 by the right eye became g?-ce?iis/i, while that seen by the left 

 eye was reddish white. 



Dr. Smith remarks that the two colours are complemen- 

 tary, and form white light when the two images overlap each 

 other. As the left eye was entirely protected from the light 

 of the candle, and yet gave an image apparently complemen- 

 tary in its colour to that given by the right eye, it was diffi- 

 cult to resist the conclusion, that an influence was propagated 

 iioiii the rig/d to the le/l eye through the medium of the optic 



* Edinburgh Joiunjil of Science, vol. v, p. 52. 

 Z2 



