Quantum Theory of Vision. 295 



Some do not escape from the sensitiser. Others fail to travel 

 beyond the delicate covering of neurokeratin which is 

 believed to invest the nerve. The fastest electrons carry 

 into the nerve almost the full quantum of energy which is 

 characteristic of the frequency giving rise to them. These 

 are the most effective in exciting a nerve stimulus. If the 

 intensity of the light is considerable there are many such. 

 If feeble, there are only a few ; but the speed, trajectory, 

 and energy of these electrons remain characteristic of the 

 frequency. Such stimuli are too few and too brief to 

 confuse one with another by overlapping. They are ap- 

 preciated at their cerebral destination as would be successive 

 notes heard in music. 



The penetration of the faster electrons into the nerve must 

 be considerable. A maximum free path of 154xl0~ 7 cm. 

 may be assumed. The radius of the cone is about 5 x 10 ~ 5 

 cm. It follows that electrons moving in a radial direction 

 may traverse one third of the radius before they become 

 deflected. One half the total cross sectional area of the nerve 

 is traversed by these direct movements. Deviated electrons 

 may be supposed to reach the centre or travel beyond it. 



As to the nature of the stimuli arising from electronic 

 bombardment and as to the manner and form in which the 

 energy of the electron is transmitted outwards from the 

 retina to the optic nerve, we have much to learn. The trans- 

 mission is probably electric in character according to many 

 physiologists. It seems, however, to be probable that the 

 velocity of the absorbed electron is not that which is 

 quantitatively appreciated by the nerve, the time interval 

 involved is far too small. But we may assume that the 

 disturbance set up by the shock is not so short-lived. It 

 travels relatively slowly from its point (or rather line) of 

 origin. Probably what the nerve appreciates is the energy 

 value of the individual stimuli, and this depends on the 

 quantum of energy associated with the electron ; which 

 in turn is determined by the frequency of: the light, i. e. 

 by its " colour.'' That each individual electronic stimulus 

 must possess a certain sensory value appears from the figures 

 cited above respecting threshold vision by the rods. 



It is necessary to consider a little more fully the specialized 

 nature of the cone functions. 



In the central foveal area, covering about 0*16 square 

 millimetres, there appear to be some 2xl0 4 oones *. 

 Suppose such an amount of light as would certainly excite 



* Rather more according- to Goldiiig-Bird's drawing, if this is 

 intended to depict these organs numerically. 



