Quantum Theory of Vision. 303 



the extension of the cone continued. But now a fresh part 

 of the " latent image " becomes involved and again there is 

 arrestment : and so on. 



(19) The momentary electrical response which is noticed 

 in the retina when light is cut off and which is the game in 

 direction as the light response (Bayliss, loc. cit. p. 522) is not 

 difficult to explain on the present theory. It is due to the 

 break-down partially or completely of the latent " image " 

 in the cones ; that is to the stimulus which arises, when the- 

 anchored electrons are attracted back to the positive field 

 surrounding the cone ; the electrostatic effects of inflowing 

 electrons attending light stimulus having ceased to affect 

 them. 



(20) Simultaneous contrast effects are, according to my 

 own observations, largely due to imperfect fixation. There- 

 is probably a psychological factor also involved. The tissue 

 paper increases the effects of adjacent colour-patches because 

 it renders fixation inaccurate. 



(21) The Purkinje Effect has been explained by Von 

 Kries on the assumption that the cones are sensitive to colour, 

 possess a maximum sensitiveness in the yellow, and are 

 responsible for vision at high luminous intensities ; the 

 rods being responsible for vision at low intensities. Into 

 this matter the dark adaptation of the retina enters, for with 

 it scotopic values rise. There is nothing here inconsistent 

 with the Unitary Theory of vision. 



(22) The dependence of the colour-sensitivity of the cone 

 upon its surface area is well shown by the increasing colour- 

 blindness of the retina towards the periphery ; the active 

 area of the cone diminishing as retinal colour-blindness 

 increases. 



The study of colour vision is hampered by many diffi- 

 culties, chief among which is the elusive and variable nature 

 of the effects under observation. On this account we find 

 disagreement among high authorities as to many phenomena 

 of vision. I shall not here pursue the matter into further 

 details. 



The foregoing theory is founded on the conception of the 

 quantum. The nerve is supposed to discriminate between 

 the quanta of three or more representative spectral centres. 

 And should not the quantum be regarded as a vera causa, 

 when we find that a single one of them acting on the retina] 

 nerve suffices to stimulate the sense of vision ? What alter- 

 natives have we to a quantum theory at the present time ? 

 One thing seems certain. No interpretation of colour in 

 terms of the frequency seems possible, whether primary or 



