292 Prof. J. Joly on a 



free path in water for electrons supposed to carry the 

 quantum for yellow light to be 5*5 x 10~~ 7 cm., and the 

 maximum free path to be 154xl0~ 7 cm. These are, of 

 course, only approximations and must be regarded as only 

 admissible in the absence of more secure results. The 

 quantities arrived at by Patterson and by A^incent for the 

 mean free path would assign to it a value from five to ten 

 times the above, even if we make no assumption as to the 

 influence of density. A correction on the score of initial 

 velocity is not called for, as the initial velocity affecting 

 Patterson's deductions is about 7'6 x 10 6 cm. : a value not 

 greatly different from that acquired by electrons carrying 

 quanta associated with visible light. 



Bespecting these figures, we must of course bear in mind 

 that the " collisions w refer in general to a deviation of 

 path, not an arrest of motion and loss of energy. 



(5) The velocity just referred to as acquired by the 

 electron when a quantum of energy is imparted to it by 

 visible light is very great — of the order 10 7 cm. per sec. 

 The course of the electron before its kinetic energy is given 

 up in work of ionization, in thermal agitation, or otherwise, 

 is a brief one : probably less than the billionth of one second. 



(6) The rods contain the sensitiser in the form of 

 rhodopsin. I assume that this substance emits electrons 

 in the same manner as other light-absorbing and optically 

 unstable substances *. The electrons set free from the 

 sensitiser by the rays absorbed expend their kinetic energy 

 in stimulating the nerve, and, perhaps, establish an electronic 

 current into the ganglion cell with which the nerve makes 

 connexion. I assume that the electronic emission in the rod 

 constitutes an intimate and generally copious source of 

 stimulus — the light being very completely absorbed. The 

 conditions are therefore very favourable to the appreciation 

 of feeble illumination. 



In this association of the photosensitive substance with 

 the nerve, the quantitative value of the stimulus is developed 

 at the expense of its qualitative value. Colour will not be 

 interpreted to the brain, or only defectively. The liberated 

 electrons are in no degree selectively presented to the nerve. 

 All that escape from the rhodopsin molecules contribute to 

 the total stimulus. In the case of some the energy of the 

 quantum is all but spent ; others reach the nerve possessed 

 of the maximum kinetic energy- The stimuli may overlap 



* Kiilme dwells on the remarkable instability of rhodopsin towards 

 light and its great stability towards chemical reagents (' Photo- 

 chemistry of the Retina and on the Visual Purple,' 1878). 



