The Electrical Response of the Eye to Stimulation by Light 407 



latency of only 00001 sec, so we may assume that the light stimu- 

 lation begins or ceases simultaneously with the interruption of the 

 signal line. 



The breadth of the image of the slit in front of the sensitive plate in 

 our photographic apparatus is 005 mm., and as the majority of our 

 photogi-aphs were made on a sensitive plate moving at the rate of 5 mm, 

 per second, time differences of 00 1 sec. may be observed. 



So it is just possible to deduce from figs. 6, 7, 15, 19, 20, 21, and 22 that 

 the latent period of the negative deflection A, as it occurs on very strong 

 illumination, may be as short as 0'02 sec. or even 001 sec. 



In order to be able to measure this duration exactly, we would require 

 to give greater velocity to our sensitive plate. In the meantime we must 

 content ourselves with the round figures given above. 



The duration of the latent periods of the different deflections of the 

 photo-electric reaction is in a high degree dependent upon the intensity of 

 the stimulation.^ Thus we see that the latent period of the negative 

 deflection A in fig. 5, where the light stimulation amounts to 10~* Ig, is 

 increased to about 1 sec. 



In other photographs not reproduced here we have even been able to 

 measure a latent period of this deflection amounting to 0-14 sec, but much 

 larger periods are difficult to obtain, since, on employing weaker light, the 

 negative deflection diminishes, and soon fails entirely. 



Our observations, which are in agreement with those of Brticke and 

 Garten, extend over a wider range. These investigators have made exact 

 measurements, but their intensity of stimulation has not been greatly varied. 

 They find, as we have mentioned, for tjie shortest latent period of the 

 negative deflection A 0'078 sec. and for the longest 0*099 sec. 



Fuchs ^ gives much shorter latent periods than we do, but the method 

 employed by Fuchs is, as shown by Gotch,^ open to serious criticism. 



The latent period of the darkening deflection A, shows greater variations 

 than that of the negative deflection A. With very strong illuminations the 

 latent period of A^ is also very short, 004 sec. and less, even diminishing 

 to 001 sec. With weaker illuminations it becomes longer, and since we 

 can still obtain a summit A^ with very weak illuminations, there occur also 

 very long latent periods of this summit. 



In fig. 5 the latent period of A^ is about 02 sec, with the intensity of 

 illumination of 10"* Ig, In fig. 14 the latent period of A^ has the value 

 of 08 sec, with an intensity of illumination of 10"^ Ig; while in fig. 

 24, where an intensity of illumination of 10~* Ig is employed, the latent 

 period attains the enormous value of 22 sec. 



The latent period of the summit B, the reaction of the second substance, 

 cannot be measured when illuminating with strong light, since the beginning 

 of the wave is here masked by the reaction of the first substance. Where 



1 This relation has already been mentioned by Briicke and Garten, loc. cit. 

 * Loc. cit. ^ Loc. cit. 



