The Electrical Response of the Eye to Stimulation by Light 395 



This enormous off-efFect is thus about sixteen times higher than that 

 shown in fig. 5. It is, as we have already remarked, of but short duration. 

 By the darkening the light eye is beginning to be changed into a dark eye. 

 The image of the string is seen to descend at first rapidly, then more 

 slowly. 



Although in the light eye the conditions are less favourable for the 

 lighting than for the darkening stimulus, it is nevertheless possible to 

 apply the former in either of two ways. In the first place, we may 

 suddenly increase the intensity of the light that is radiating on the eye, 

 and secondly, we may darken the light eye for a short period, so that it has 

 not yet become a dark eye., and then suddenly illuminate it. 



The second method gives better results than the first, and we possess 

 numerous curves where, after a short darkening of a light eye. a strong 

 light stimulus was applied. An example of this is seen at the end of fig. 6. 

 The potential diflferenee of 200 microvolts which was introduced at a is 

 cut out at c, and the strong white light I„ is suddenly allowed to radiate 

 upon the eye at e. The reaction A of the first substance attains here as a 

 downward directed deflection a value of —90 microvolts. The positive 

 deflection B following thereon belongs, as we shall explain later, to the 

 action of the second substance. It has nothing to do with the reaction of 

 the first substance, and bcQomes smaller the more the reaction of the first 

 substance appears unmixed. The positive deflection E (lighting reaction 

 of the second substance) must be reckoned as beginning at the lowest point 

 of the negative deflection A. The height of B must thus be measured 

 from this lowest point to the peak. It amounts at e in fig. 6 to 370 

 microvolts, and is therefore more than four times greater than the lighting 

 reaction A of the first substance. 



But it is not difficult to increase the lighting effect of the first substance 

 and at the same time to diminish that of the second substance, which is, in 

 other words, to produce the lighting eff'ect of the first substance more 

 purely. For that purpose one requires to darken the light eye during a 

 shorter time so that it preserves better the attributes of a light eye. In 

 an eye that is darkened during a very short time the lighting eflfect of 

 the first substance can even surpass that of the second. The negative 

 deflection A becomes then larger than the immediately following positive 

 wave B. Fig. 7 may illustrate this. 



Here 1 mm. abscissa is equal to 02 sec, 1 mm. ordinate to 21 

 microvolts. The eye is illuminated with white light of the full strength 

 I,. The periods of lighting are, as in all our other figuree, denoted by I, 

 those of darkening by d. 



If ?i2 01 fig. 7 be compai-ed with l^ of fig 6, it is clear that the reaction 

 of the first substance at l^^ has become greater, that of the second substance 

 has become smaller than at I.,. The reaction on lighting at /.g ^^ followed 

 upon a shorter period of darkness. We may remark, however, that at ly. 

 B is still twice- as great as A (see diag., fig. 8, a). 



