396 



Einthoven and Jolly 



On further shortening the period of darkness the proportion becomes 

 continuously changed in favour of the first substance, so that on lighting 

 at l^, l^, li, l^, and l^^ the negative wav.e of the first substance is about as 

 great as the positive wave of the second (see diag., fig. 8, b), and finally, 



Fig. 7.— The reaction of the first substance. Light eye. Absc. 1 mm. =02 sec. Ordin. 1 ram. = 

 21 microvolts. White light. Intensity of illumination = Iw. Z, light ; rf, darkness. 



after very short periods of darkening, the positive wave of the second 

 substance at l^^, l^, l^, Ig, and l^^ becomes smaller than the negative of the 

 first (see diag., fig. 8, c), especially at l^, l^, and l^^ the lighting effect of the 

 first substance appears almost pure. 



To understand full}- the significance of fig. 7, we must further pay 

 special attention to tlie darkening deflections. As already mentioned, the 



Fig. 8. —Diagram of the reaction to strong light of a light eye which has 

 previously been darkened for a short time. The preceding period of 

 darkness has been longest in a, shorter in b, and shortest in c, and the 

 upward deflection, due to the action of the second substance, becomes 

 progressively less, while the down^eard deflection due to the first sub- 

 stance becomes greater. I, light ; d, darkness. 



first substance reacts in a completely light eye on stimulation by darkness 

 with the development of a huge positive potential difference, which, 

 however, cannot have a long duration, since by the darkening the light eye 

 begins to change into a dark eye. The descent of the image of the string 

 caused thereby, a descent which is produced by the action of the second 

 substance, is relatively slow. It can easily be distinguished from the very 



