302 Prof. J. Joly on a 



accumulations of electrons occurring till a point is reached 1 

 when these revert to the parent molecules. But in what 

 manner in the case of the nerve could such effects be stored 

 and saved from immediate degradation ? A possible explan- 

 ation suggests itself. The cone during light stimulus 

 contracts ; subsequently it again elongates. When con- 

 tracting we may suppose the cone moves towards parts of 

 the sensitiser still unacted upon. [And this is probably the 

 primary object of the movement.] The effect will be to 

 remove the cone from the field of positive ionization and so 

 free the internal, fixed electrons from electrostatic attraction. 

 But when the retina is again darkened the cone moves back 

 into the exhausted sensitiser ; a region rich in free positive 

 ions. Hence as it elongates an electrostatic positive field 

 accumulates till there is break-down and discharge during 

 which the fundamental sensations are successively evoked. 

 If the successive colour cycles attending the movement of 

 the cones overlap there will ensue the irregular sequence 

 often perceived. On the other hand, if the cones move under 

 the influence of a common stimulus and advance, not 

 uniformly, but with pauses of quiescence, then the 

 repetitional colour cycles find complete explanation ; each 

 cycle corresponding to the latent " image " breaking down 

 over one short segment of the cone. That the movement of 

 the cones is general and not due to a stimulus local to each 

 cone is, I suggest, shown by the fact that in the case of the 

 frog, where the cone-movement is very marked, " light on 

 one eye causes reaction on both as also light on the skin so 

 long as the brain is intact'- (Engelmann, 'Nahmacher.' See 

 Parsons, loc. cit. p. 12). 



It is consistent with the view that after-images are of the 

 nature of the latent photographic image that they may persist 

 for very considerable time intervals. 



The negative after-image is probably explained by fatigue. 

 It appears a little after the positive image, and when the 

 retina is re-exposed to feeble illumination. What was 

 brio-ht now appears as dark, and the colours change to 

 the complementary hues. 



A discontinous motion of the cones will naturally arise if 

 the nervous actions involved are reflex in character ; as 

 the observations on the frog very surely indicate. In this 

 case electronic stimulus of the cone initiates its retraction 

 and the cessation of the stimulus initiates its extension. 

 Hence when, on the extension of the nerve, the luminous 

 after-image begins to be formed the electronic movements 

 act as the afferent stimulus and extension ceases or con- 

 traction may ensue. Only when the after-image dies out is 



