Quantum Theory of Vision. 293 



in time and space. It is probable that some quanta are 

 expended in conferring purely thermal movements on 

 electrons, and along with the destructive effects of quanta 

 upon the sensitiser the regeneration of this must all the 

 time be progressing within the nerve. A confused now 

 of stimuli, too impure and too crowded for analysis, is the 

 outstanding character of the sensory contribution of the rod. 

 It corresponds to noise in the case of audition. Nevertheless, 

 the conservation and integration within the rod of the 

 stimuli arisino- from the interaction of light and sensitiser 

 render it a most sensitive exponent of luminosity reaching 

 the retina. 



Its sensitivity is, indeed, extraordinary. Henri and des 

 Bancels have shown that the retina is sensitive to an amount 

 of light energy of the value of 5 x 10 -1 " erg. Now the 

 quantum for green light is 4 x 10~ 12 erg. We may assume, 

 therefore, that one quantum is sufficient to excite vision 

 (Bayliss, General Physiology, p. 512). That is to say, the 

 liberation of a single electron bv o-reen or blue light will 

 excite visual sensation. 



Again, consider the following relatively commonplace case. 

 A standard candle removed to a .distance of 3000 metres 

 projects on each square centimetre luminous energy of the 

 amount 4xl0~ 7 erg per sec. This luminosity will evoke 

 vision ; and such feeble radiants are known to be best appre- 

 ciated when the image falls on parts of the retina rich in rods. 

 How many quanta are involved in this excitation of vision ? 

 The pupil admits, say, the luminosity reaching one-half a 

 square centimetre or 2xl0~ 7 erg per second. This corre- 

 sponds to about 7 x 10 4 quanta per second ; such quanta as 

 would be associated with yellow light. The number of rods 

 which receive this energy is considerable. The size of the 

 image is indefinite, but it will not be a. point image. Suppose 

 it covers one-tenth of a square millimetre, we can roughly 

 estimate the number of rods involved. The total area of the 

 retina is about 1000 square millimetres, and the total number 

 of rods has been estimated as 130 millions, and again as 

 half this number. We shall take the number to be 100 

 million. There will be about 10,000 rods illuminated. The 

 quanta are in fact distributed over this number of rods : 

 that is, 7 quanta enter each rod per second. We may 

 translate this into 7 electrons liberated in each rod per 

 second. It is evident that these small individual stimuli 

 must be so far conserved as to make their way to the optic 

 nerve. If they did not do so but died out within the nerve, 

 there would be no vision. We have it then, however 



