300 Prof. J. Joly on a 



Colour vision curves constructed from the examination of 

 abnormal sight show the curves as overlapping. As I have 

 already pointed out, the entry of spent electrons into the 

 normal nerve — i. e. electrons possessing less kinetic energy 

 than is proper to the wave-length illuminating the retina — 

 introduces indeterminate stimuli which result in luminous or 

 white sensation ; for such, received in the sensory colour- 

 centre, could not be differentiated from the sensation arising 

 when white light falls on the retina, and quanta exciting all 

 three colour sensations stimulate the nerve. Such spent 

 electrons affect the sensations of the colour-blind also,, 

 according to his limitations. Thev can o-ive no new sensation 

 to one possessed of monochromatic vision. The subject is 

 full of obscurities and difficulties, and I shall not enter upon 

 it. I see nothing in the present theory to accentuate or add 

 to those difficulties. It is to be hoped it may contribute to 

 clearing them up. 



(16) The spectral limits of the colour- sensation curves find 

 a simple physical explanation in the failing absorption of 

 rhod opsin for these wave-lengths. This matter is, of course, 

 Hound up with the limitations imposed by the absorption of 

 quanta (associated with the higher and lower frequencies) by 

 the media through which the light has to pass before it 

 reaches the retina. 



(17) The light-sensitive substance, rhodopsin, is probably 

 extremely complex. It is said to be related to Anthocyanins 

 which are rich in chromogens and are held accountable for a 

 wide range of colour in flowering plants. Evidence for the 

 richness in chromogens of rhodopsin is, I think, to be found 

 in its sustained absorption of visible spectral rays. " Fatigue " 

 may evidently be referred to the exhaustion of such chro- 

 mogens as have been deprived of: an electron. This appears 

 to be the natural explanation. 



(18) When a bright object is looked at for some time and 

 the eyes then closed, " after-images " are seen. These are 

 both positive and negative. 



The phenomena of positive after-images suggest that 

 something of the nature of the latent photographic image is 

 formed in the nerve-substance. Indeed, a priori, one would 

 expect this tp happen, the conditions having much in common 

 in the two cases. On this view electrons entering the nerve 

 would in some cases remain attached to atoms within its 

 substance and an electrostatic field would prevail between 

 them and positive ions in contact with the cone. This system 

 must break down ultimately — probably is continually 

 breaking down and being rebuilt. The electrons attached 



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