The Photo- Electric Theory of Vision. 347 



Obliquely Incident Light. — When the film is tilted rela- 

 tively to the direction of the incident rajs, the rings seen 

 on the film contract and move inwards. As in the case of 

 normal incidence, perfectly black rings may be seen on the 

 film, if it be viewed in monochromatic light very nearly in 

 the direction of the transmitted rays, and the appearance of 

 these rings alters with the obliquity of observation in much 

 the same way. It should be remarked, however, that in 

 the present case the effects observed vary not only with 

 the angle between the transmitted pencil and the direction 

 of observation, but also with the particular plane in which the 

 latter direction lies. The maximum permissible angle of 

 observation varies -with this plane. In the plane of incidence 

 it is tt/'I — o. and tt/2 -fa respectively on the two sides of the 

 transmitted pencil, where a is the angle of incidence. If a 

 is considerable, the rings continue to be visible when the film 

 is viewed nearly along the surface of the plate on one side of 

 the transmitted pencil, while on the other side the rings 

 move out and disappear from the film at a moderate obliquity, 

 so that in white light the film appears achromatic over a wide 

 range of angles of observation. 



Calcutta, India, 



18th September, 1920. 



XXXI. The Photo- Electric Theory of Vision. 

 By J. E.J. Poole, M. A* 



IN his address to the British Association in 1919, 

 Sir Oliver Lodge suggested that the light sensation 

 in the eye might be caused by the action of photo-electrons 

 excited by the incident light. He thought that, if expe- 

 riments were carried out on the various substances present 

 in the retina, some of them might be found to exhibit photo- 

 electric properties when exposed to visual light. The same 

 idea had occurred to Dr. Joly some years previously, and in 

 1915—16 some experiments were actually carried out in this 

 laboratory on the possible photo-electric properties of the 

 black 'pigment No evidence of the black pigment possessing 

 any photo-electric powers, when exposed to visual light, could 

 however be obtained, and as thus no direct experimental 

 evidence in favour of the theory could be given, Dr. Joly 

 refrained from publishing this view of the mechanism of 



* Communicated by Prof. J. Joly, F.R.S. 



