402 



Einthoven and Jolly 



exhibit complete agreement in essentials. The analogous summits are 

 denoted in both figures by the same letters A, B, A- and C. 



Specially remarkable are the curves obtained if the duration of lighting 

 of a dark eye is systematically changed, and we wish to direct attention 

 more particularly to the ofF-effect in such cases. If the duration of the 

 lighting is very short and the light weak, then, as already mentioned, 

 the effects of the second substance appear umnixed. The off-effect here 

 consists in the descent of the curve to the zero line. 



Fig. 16. — Diagrammatic representation as three separate curves of the reactions 

 to light of the three substances. A, first ; B, second ; and C, third sab- 

 stance. /, light ; d, darkness. 



If the duration of the lighting is a little longer and the effects of the 

 other two substances begin to become perceptible, the off-effect is determined 

 by the resultant of three forces. Tlie first substance tends to displace the 

 image of the string upwards. It is at first acting weakly, but its strength 

 increases regularly during illumination so that it soon surmounts the 

 effect of the other substances. In the case of longer lighting the off-effect 



~T^ 



FiO. 17. — Diagram of the combined action of the three substances. 

 A is the lighting, and Aj the darkening reaction of the first sub- 

 stancf ; B is the action of the second, and C of the third substance. 



therefore is always an upward movement which increases with the 

 duration of the lighting. 



The second substance tends to depress the image of the string, acts 

 first with moderate strength, but decreases gradually during lighting. As 

 the second substance in particular is acting in a dark eye, the conditions 

 for its functioning grow during the illumination more unfavourable. A 

 strong darkening effect cannot be expected in a dark eye. 



The third substance is so slow that the darkening effects of the first 

 and second take place usually at a moment when the third substance is 

 still tending to displace the string upwards. The darkening effect of the 



