416 The Electrical Response of the Eye to Stimulation by Light 



comparable with the electric reaction of a nerve or a muscle, not with the 

 mechanical reaction of the latter, for the muscle in contracting is able to 

 develop a quantity of energy whrch far exceeds that of the stimulus. 



VI, Summary of Conclusions. 



1. The photo-electric reaction of an isolated eye is specially adapted to 

 the study of the effect of stimuli of very different intensities. In our ex- 

 periments light stimuli have been used whose energy varies from 305 x 10~'^ 

 to 3-95 X 10~^ g. cal. per sec. 



Short illumination with the strongest light has not been found to damage 

 the eye, while the weakest light was capable of producing a fairly consider- 

 able photo-electric reaction. 



2. The form under which the photo-electric reaction manifests itself 

 under different conditions gives ground for the supposition that there occur 

 in the eye three separate processes, and each of these may be dependent 

 upon a separate substance. We speak of three substances for the sake of 

 convenience. 



3. The first substance reacts more rapidly than the other two. On 

 lighting it develops a negati\'e, on darkening a positive potential difference. 

 Its action comes strongly into prominence in a light eye and appears almost 

 unmixed on sudden darkening of short duration (a flash of darkness). 



The second substance reacts less rapidly than the first, and in an opposite 

 sense. On lighting it develops a positive, on darkening a negative 

 potential difference. Its action appears almost unmixed in a dark eye 

 which is illuminated for a short time with weak light. 



The third substance reacts in the same sense as the second, but much 

 more slowly. Its action fails in a completely light eye, and also in a dark 

 eye which is illuminated very weakly for a short time. 



4. The latent period of the photo-electric reaction is in a high degree 

 dependent upon the intensity of the stimulus. With strong stimuli it is of 

 the order of 001 sec, while with very weak stimuli it may be lengthened 

 to more than 2 sec. These values are in agreement with the latent periods 

 of light perception in the human eye. 



5. For each of the three substances the rule holds good that with 

 moderate and strong light the energy of the stimulus increases much more 

 quickly than the energy of the reaction. 



6. Although the eye is far more sensitive than the most sensitive 

 artificial bolometer, the energy of the reaction remains below that of the 

 stimulus even in the most favourable circumstances. The photo-electric 

 reaction of the eye is in this respect comparable with the action current of 

 a muscle or nerve. 



O 



