February 24, 192 1] 



NATURE 



827 



Letters to the Editor. 



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 opinions expressed by his correspondents. Neither 

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 the rvriters of, rejected manuscripts intended for 

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A Quantum Theory of Vision. 



In a paper appearing in the February issue of the 

 Philosophical Magazine 1 have described a theory of 

 vision which ascribes visual stimuli to the activity of 

 light quanta in liberating electrons from the visual 

 purple. Various phenomena associated with scotopic 

 and photopic vision are considered. Those coming 

 under the designation of simultaneous contrast did not 

 appear to me, at the time of writing, referable to 

 purely retinal actions. Since then it has occurred to 

 me that simultaneous contrast effects find explanation 

 in a very simple way on the same data as serve to 

 explain successive contrast, i.e. in the external loca- 

 tion of the sensitiser with reference to the cones and 

 in the motion of these organs attending light stimulus. 



Consider the case of a grey patch bordered by a 

 black area. This disposition secures unused sensi- 

 tiser around the retinal image of the central patch. 

 When those cones which are covered by the image 

 of this central patch retract, the unused sensitiser 

 flows in around them. The conditions are, therefore, 

 favourable to specially luminous sensation, and the 

 grey patch looks bright. In the other case, when the 

 ^rey patch is surrounded by a white area, the reverse 

 conditions prevail. The retreat of the cones involves 

 the inflow of used-up sensitiser around the cones 

 covered by the central image. The conditions are 

 favourable to lowered luminous sensation, and the 

 grey area looks dark. When we substitute for the 

 white border a coloured border, then the sensitiser 

 invading the central area of the retinal image is 

 "fatigued" for the particular tint of the border, and 

 hence the central grey looks tinted with the com- 

 plementary colour. The tissue-paper, which when 

 laid over the patches accentuates these effects, acts 

 probably in two ways. It renders good fixation im- 

 possible and, by the increased luminosity which it 

 brings in, it causes the cone-movements to become 

 more active. 



In my paper I have invoked the "latent image" 

 familiar to photographers. I think it explains more 

 even than I claimed for it. 



The latent image in the photographic plate may be 

 ascribed to electrons which, having travelled a cer- 

 tain distance from their point of origin, become loosely 

 attached to atoms. Afterwards they take part in the 

 chemical effects attending development, or, if ex- 

 posure is carried so far as to cause an accurnulation 

 of electrons to the point of instability under increas- 

 ing electrostatic forces, the latent image runs down 

 ■of itself. This is " solarisation " or "reversal." 

 There may be a succession of such reversals under 

 continued exposure. 



It is quite to be expected that something of this 

 sort will occur in the case of the cones, and possibly 

 of the rods also. After-images find explanation in this 

 way, and their theory becomes very complete when 

 the motion of the cones under light stimulus is taken 

 into account. As briefly referred to in rny 

 paper, the latent image serves also to explain 

 the "dark" electrical response of the retina — 

 a response which has the same sign as the "light" 

 response. To understand this we have to consider 



NO. 2678, VOL. 106] 



that the latent image in the nerve-substance has less 

 stability, very probably, than that which forms in the 

 photographic plate, owing to the nature of the 

 medium. It is probably kept in being by the con- 

 tinuous inflow of electrons from without the cone, and, 

 normally, is also continually breaking down. When 

 light is cut off, the whole accumulation runs down, 

 attracted back to the positively charged ions developed 

 around the cone. Hence there is a second stimulus, 

 and it will, of course, be of the same sign as that 

 attending the primary movement of the electrons. 

 The final discharge of the latent image may be rela- 

 tively slow, as the curves in some cases show. Look- 

 ing at such a curve as that which Piper obtained for 

 a pigeon's eye under brief periods of darkness alter- 

 nating with light intervals, it needs little imagination 

 to picture the happening of these events. 



Frohlich's results on the Cephalopod eye seem to 

 involve the same effects. In this case the latent 

 image builds up rapidly within the rods which are 

 exposed directly to the light and as rapidly runs down, 

 giving rise to rhythmic electrical responses from 20 to 

 100 per second. Just on account of this extreme in- 

 stability of the latent image there is no definite dark 

 response. In short, the dark response is a pheno- 

 menon connected entirely with the quasi-stability of 

 the latent image, and is probably favoured by the 

 location of the sensitiser external to the nerve. 



From all this I think we must conclude that the 

 stimulus is ascribable ultimately to the motion of the 

 electron, its amount depending upon the kinetic 

 energy, and this, in turn, upon the particular quantum 

 which activates the electron. The return of the elec- 

 tron in some cases, under the electrostatic attraction 

 at the point of origin, involves a fresh stimulus. This 

 is a phenomenon similar to that which Lenard invokes 

 in his explanation of phosphorescence. 



On this view colour is appreciated in terms of the 

 energy of the stimuli ; brightness in terms of the 

 concentration or density of the stimuli. Rhythmic 

 succession of stimuli is not required, and does not 

 exist. It will be understood that this theory does 

 not involve views respecting the origin of the quantum. 

 Thus, whether we believe that quanta originate at the 

 source of light or come into existence upon its absorp- 

 tion — as Sir Oliver Lodge has suggested — the basis 

 of the theory remains. The one thing essential is the 

 relation between the energy of the photo-electron and 

 the frequency of the light which gives rise to it. 



I conclude with a question : Are there any good 

 data available respecting the rate of motion of the 

 cones under light stimulus? It is generally stated 

 that it is slow even with strong lights (and faster for 

 violet than for red). Definite information on this 

 point seems worth seeking. For if the reaction 

 towards the light were rapid, we could regard it as 

 diminishing the effects of dispersion in the refractive 

 system. It is probable (in harmony with the present 

 theory) that the displacement would be greater for 

 violet than for red rays. This would tend to bring 

 cones illuminated by violet light nearer to the lens 

 than cones illuminated by red light. There would be 

 at least a partial correction for dispersion. 



J. JOLY. 

 Trinity College, Dublin, 

 January 30. 



The Constitution of Lithium. 



Positive rays of the alkali metals were first ob- 

 tained bv Gehrcke and Reichenheim by using as anode 

 a heated metal strip upon which a suitable salt was 



