The Electrical Response of the Eye to Stimulation by Light 377 



almost completely constant. The reaction on darkening has a latent 

 period of about 1287 sec. and consists of a renewed increa.se of electro- 

 motive force more gradual than the lighting effect and le.ss in amount. 

 Immediately thereafter the electromotive force sinks at first rapidly, then 

 slowly, to the original amount. Piper did not find in the case of the frog's 

 eye any gradual continual increase after the first increase and diminution. 

 In the course of his study of the vertebrate eye, however, he found such a 

 variation. The reaction of the eye of a cat, which had not been treated 

 with atropin, after a first deflection presenting the typical appearance of 

 the lighting cfiect, showed a second slow positive deflection. This was not 

 observed in eyes treated with atropin, and Piper attributes its presence to 

 electromotive changes accompanying contraction of the iris muscles. The 

 second deflection was considerably greater in amount than the initial 

 positive deflection which followed the on.set of light. 



We have seen that a positive deflection during the continuance of 

 illumination has been figured by several observers as an occasional, feature 

 of the reaction of the frog's eye, but it is to Ishihara^ tliat w.e owe the 

 recognition of this deflection as a typical constituent of the curve, inde- 

 pendent of the primary positive deflection on lighting. This worker followed 

 out the suggestion yielded by Waller's curves, and recognised clearly the 

 double nature of the positive deflection. According to his description the 

 positive deflection on lighting is at first rapid. It then continues more 

 slowly until it reaches a maximum, after which it gradually diminishes. 

 A close investigation shows that the ascending limb of the curve contains 

 a step or notch which occurs about the same time after lighting as the 

 maximum of the off-efl'ect after the moment of darkening. Ishihara 

 concludes that at lighting there is a rapid positive deflection similar to that 

 which occurs at darkening, although the former, owing to the slow move- 

 ment of the galvanometer used by him, was less clearly visible than the 

 latter, tending to be fused with tiie succeeding slower deflection which he 

 names the " Helligkeitsschwankung." On one of the curves fig\n-ed by 

 this obsoi'ver may be seen the positive deflection on lighting, which, after 

 a slight diminution, is followed by a slow rise above the value of the fir.st, 

 upon which th<i darkening reaction is superposed. 



BriJcke and Garten' have recently made valuable contributions to 

 our knowledge of the retinal currents by the employment of the capillary 

 electrometer and string galvanometer. By the aid of tlie latter instrument 

 they have demonstrated clearly the preliminary negative variation visible 

 to Gotch on examining his curves by a magnifying glass. This di'floctioti 

 occurs in the majority of cases, and its latency is 0078 sec. to 00!^9 see. 

 Its value is great(>st iti the freshest eyes. Tlie continuous eflect during 

 illumination of the isolMted eyeball consists of a slow rise independent of. 

 and two or three times greater than, the first positive variation. Its return 



' Pfliiger'.s Arch., Bil. 114, S. 5(i9, 190(5. 

 -' Ibid.JBd. 120, S. 290, 1907. 



