The Electrical Response of the Eye to Stimulation by Light 375 



tion, but regard such a division as indefensible. In a later research ^ it was 

 observ^ed that, in some cases, instead of a negative variation following the 

 positive, the latter merely suffered diminution. This, since it occurred in 

 the freshest prepatations, was regarded as the normal reaction of the 

 retina. 



No current was obtained from the posterior half of the eyeball from 

 which the retina had been removed, the pigment layer being left behind. 

 The observers concluded from this that the pigment epithelium possesses 

 no electromotive power, and no ability to give rise to electrical changes on 

 stimulation by light. 



The latent period of the lighting effect M'as investigated by Fuchs^ by 

 means of the rheotome. He obtained much smaller figures than have 

 been observed by other workers. The method employed by Fuchs, 

 however, is not applicable to the changes of long duration which constitute 

 the reaction of a dark-adapted eye. 



Waller^ studied the phenomena presented by the frog's eyeball with 

 the help of photographic records of the galvanometer variations. He found 

 that the effects obtained in the posterior half of the eyeball or in the 

 isolated retina were precisely those manifested by the injured eyeball, and 

 therefore regarded the reaction of the intact eyeball as typical. Waller 

 found that the deflection of the galvanometer with illumination is positive 

 at the commencement and at the end, and is also positive during the 

 continuance of the illumination. He regarded the final deflection as a 

 subordinate feature of the main change, and described the reaction simply 

 as a positive (upward) current during illumination and no such current 

 during darkness. 



Some of the curves figured by Waller indicate that the positive 

 deflection evoked by lighting is not simple, but of a double nature. In his 

 figure 15 it is evident that the curve, after a steep ascent, is interrupted in 

 a step-like manner, after which it progresses bj'' a more gradual rise and fall 

 during the course of the illumination, and upon this slow deflection the 

 positive effect due to darkening is superposed. The interruption of the 

 rise is noted by Waller and referred to a negative restraint setting in at 

 that point 



The period of latency between onset of light and positive deflection 

 were found by this observer to be very long amounting in sOme cases to 

 several seconds. This delay he explains as due to a period of hesitation 

 dniing which two opposed currents are developed from the retina at nearly 

 equal rates, and finds that a short negative swing frequently precedes the 

 main positive effect. 



De Haas,* working in this laboratory, investigated in detail the strength 



' Pliysiol. Untersiich., Bd. iv., S. 64, 1881. 



2 Pflugei's Arch., BrI. 56, S. 408, 1894. . Ibid., Bd. 84, S. 425, 1901. 

 ■■' Pliil. Trans vol. 193, B, p. 123, 1900. 



* Liclitprikkels eii retiuastroomeii in lain qnantiUtief verband, Dissertation, Leiden, 

 1903 ; and Onderzoekin<;.'n Physiol. Lah., Leiden, 2nd .ser., vol. 6. 



