Photo- Electric Theory of Vision. 349 



electrons would excite both the red and green sensations and 

 hence such a colour would be visually indistinguishable from 

 a mixture of: red and green. In fact, if we adopt this view 

 of the resolving power of the cones for photo-electrons of 

 different velocities, we could explain all the facts which agree 

 with the Young- Helmholtz theory of three primary colour 

 sensations. 



Before making any direct tests of the possible photo- 

 electric powers of the retina, some preliminary indirect 

 trials were made. It seems probable that the formation of 

 the latent image in a photographic plate is really based on a 

 photo-electric action due to the silver halides which are found 

 to be vigorously photo-electric. Sensitisers are also photo- 

 electric, their activity being dependent on the absorption of 

 light of some particular colour. It is, therefore, perhaps 

 permissible to infer that if either the black pigment or 

 rhodopsin were really photo-electric to visual light, they 

 would also be capable of acting as sensitisers for a photo- 

 graphic plate, if treated in a suitable manner. Experiments 

 on this point were accordingly made by exposing a plate 

 which was insensitive to the red end of the spectrum to 

 intense red light. One half of the plate was treated with 

 the preparation from the eye, which it was desired to test. 

 The untreated portion acted as a standard for com- 

 parison purposes, and by putting a slight scratch with 

 a penknife on the treated half it could always be sub- 

 sequently identified. An ordinary carbon glow-lamp was 

 used as a source of light. Before falling on the plate, the 

 light passed through a picric-acid screen, ordinary photo- 

 graphic ruby glass, and also a special red screen, as it was 

 found that the ruby glass allowed quite a large proportion 

 of the blue end of the spectrum to pass through. 



The first material to be dealt with was the black pigment. 

 Various methods of applying it to the plate were tried, 

 but in no case was definite positive evidence of any sen- 

 sitizing action obtained. In the first attempts the black 

 pigment was simply removed from a freshly dissected bull's 

 eye, and placed directly in contact with the sensitive film. 

 The plate was exposed through the glass, as owing to 

 the opacity of the black pigment no effect could be expected 

 if the black pigment coating was between the film and the 

 incident light. After exposure, the pigment was carefully 

 washed off and the plate given a thorough rinsing with 

 water to remove as far as possible any trace of grease. 

 As already stated however, no sign of any sensitising 

 action could be discovered on development, and in Pact 



