The Electrical Response of the Eye to Stimulation hy Light 397 



rapid descent with which the first substance reacts to a strong light 

 stimulation. 



In the case of darkening of moderate duration, as for example at d^ 

 and d^, there therefore appears a curve which begins with a rapid ascent, 

 continues in the middle with a slow descent, and concludes with « rapid 

 descent (see diag., fig. 9, a). 



The rapid ascent at the beginning and the rapid descent at the end are 

 the reactions of the first substance ; the slow descent in the middle is 

 produced by the action of the second substance. 



As the duration of the darkening is shortened the middle pait of the 

 curve diminishes more and more until, with very short darkening (a flash 

 of darkness), it entirely disappears. In these circumstances, therefore, the 

 action of the second substance is totally cut out and the curve shows the 

 pure reaction of the first substance. These phenomena are fully depicted 

 in fig. 7. The effects of darkening at d^ and d^^ are reproduced diagram- 



J 



]- "1 T" ~T_r "ir"d ■' 



Fig. 9. — Diagram of the reaction of a light eye to darkness. The period of darkening 

 is progressively shortened from a to e. The initial upward and terminal down- 

 ward deflections are due to the first substance, and the intermediate slow descent 

 lo the second substance. In e, where the darkening is veiy short (a flash of 

 darkness), the reaction of the fir^t substance is seen pure. /, ti^ht v d, darkness. 



matically by fig. 9, b, at d^ by fig. 9, c, and the effects of the very short 

 periods of darkening at d^, cLj, and dg, where the reaction of the first 

 substance appears practically pure, are shown by fig. 9, e. 



The absolute amount of the darkening reaction is in d^ very great 

 The deflection being 31 mm. in height, should represent a potential 

 difference of 650 microvolts, if the galvanometer had been able, with the 

 sensitiveness here employed, to follow the current variations exactly, but 

 the actual potential difference is, for reasons explained above, much greater, 

 and must be estimated at more than 1200 microvolts. 



The absolute amounts of the lighting reactions are not so great, but 

 the potential differences here developed may nevertheless be termed 

 considerable. At l^^ and l^ downward directed deflections occur of 17 

 and 18 mm. which, if the galvanometer were rapid enough, would represent 

 potential differences of 357 and 378 microvolts, but must in reality be 

 estimated at more than 600 microvolts. 



In cases where the darkening is of short duration and the darkening re- 

 action is greater than the lighting reaction, curves are recorded as at d^ and d^, 

 where the string do*^s not return to its original position (see diag., fig. 10, a). 



