Quantum Theory of Vision. 2$T 



nerve. The sensory stimulus emanating from the rods is 

 compounded of many sources of stimuli, i. e., such as may 

 originate in electrons possessing every velocity and kinetic 

 energy up to the maximum proper to the frequency of the- 

 activating light, such as may arise in electronic movements 

 associated with the regeneration of- the sensitiser and in, 

 thermal electronic agitation excited by the quanta taking- 

 part in these operations. The stimulus emanating from the 

 cones, on the other hand, is purified of all stimuli save those 

 arising from the kinetic energy of electrons, which are 

 activated by the energy of absorbed quanta. The electrons 

 are, in fact, selectively presented to the nerve : all other 

 sources of stimuli take place outside the cone and are cut off 

 from it by the filament which invests it. In the cone the 

 more intense stimuli tap out to the brain the sensation of 

 colour which we associate with the intensity of the quanta 

 involved. To this succession of characteristic nerve impulses 

 there is added an underlying accompaniment : the white 

 or luminous sensation made up of all the feebler electrons 

 which impart to the nerve but a fraction of that which is- 

 characteristic of the frequency of the light entering from 

 the world without. The cone is the more highly specialized, 

 organ of the two and is probably a more recent product of 

 evolution. 



(8) The higher luminosity of the colour threshold arises 

 out of the conditions affecting the stimulation of the cones ; 

 the sensitiser being external and hence a part only of the 

 evoked electrons producing visual sensation. In the rods all 

 absorbed radiations are expended on exciting the sensation of 

 luminosity ; the electrons being liberated within the rod. 

 There will be a colourless interval attending foveal stimula- 

 tion of low intensity for the reason that' the characteristic 

 electrons constitute only a fraction of the stimulus and it 

 requires a certain density of such electrons before colour 

 vision is experienced. In other words, the achromatic effect 

 of very low light intensities or of very brief exposures is due 

 to commencing stimulation by non-characteristic electrons. 



(9) One consequence of the different disposition of the 

 sensitiser respecting the rod and the cone is that the 

 characteristic quanta stimulating the cone cannot possess the 

 full energy value proper to the originating frequency. That 

 is to say, it reaches the nerve with a quantity of energy such 

 as would be associated with a frequency v\ less than v. This 

 is because the electron in the case of the cone must part with 

 some of its energy in penetrating the outer sheath o( the cone, 

 We may expect from this that the entire luminosity curve of 



