398 



Einthoven and Jolly 



In ng. 10, b, on the contrary, is diagrammatically reproduced a curve 

 with a long period of darkness as at d-^^, where the string descends below 

 its original position. 



The Second Substance. 



The second substance reacts less quickly than the first. On lighting 

 it moves the string with moderate velocity upwards, and on darkening 

 slowly downwards ; thus on applying stimuli of the same kind, it develops 

 potential differences which are opposed to those of the first substance. Its 

 effect appears almost unmixed in a dark eye which is illuminated for a 

 short time by weak light. 



If when illuminating with light of very low intensity, the darkening 

 follows rapidly upon the lighting, in a similar way as in a momentary 

 illumination, there is recorded a curve of simple form with a steeper 



Fig. 10. — Diagram of the reaction of a light eye to darkness. In o, where 

 the darkness is of short duration and the darkening reaction is greater 

 than the lighting, the curve does not return to its original position. 

 In 6, where the darkening is .of long duration, the curve descends below 

 its original position. I, light ; d, darkness. 



ascending part which is evoked by the lighting and a less steep descending 

 part evoked by the darkening. The top of the curve lies, within certain 

 limits, higher the more the energy of the illumination is increased either 

 by using greater intensity or longer duration of the light. These limits 

 are determined by the functioning of the other two substances, which, 

 when their effects become perceptible, influence the form of the curve and 

 considerably complicate it. 



The series of four photographs which are taken from one and the same 

 eye and are reproduced in figs. 11, 12, 13, and 14, may serve as examples 

 of illumination of this description. Here 1 mm. abscissa is equal to 0*2 

 sec, and 1 mm. ordinate equals 4 microvolts. The eye is illuminated 

 each time by green light, which, by making use of diaphragms, is reduced 

 to • one ten millionth of its original intensity, and which we thus name 

 10 '^ Ig. The duration alone of the illumination is varied. From fig. 11, 

 where this duration amounts to 048 sec, it increases gradually : in fig. 12 

 it is 112 sec ; in fig. 13, 19 sec. ; and in fig. 14, 3 sec 



The heights of the summits increase regularly with the increasing 



