394 Einthoven and Jolly 



the simplest manner by the assumption that the processes are three in 

 number, whether they are together dependent upon the same substance or 

 each upon a separate one. For the sake of convenience we shall speak of 

 three substances, and as we do not intend in the meantime to attempt to 

 define them anatomically in the eye, we shall try to describe their charac- 

 teristics and to mention the conditions under which their effects appear as 

 pure as possible. 



The First Substance. 



The substance which we have termed the first reacts more quickly than 

 the other two. On lighting it displaces the image of the string downwards, 

 on darkening upwards. Its effect, which can be obtained pure only during 

 a short period, is very marked in a light-adapted eye — which for the sake 

 of brevity we may call a light eye^ — and the more so the stronger the 

 illumination has been. 



From the nature of the case the darkening stimulation can be employed 

 very strong in a light eye. and accordingly an eye which has been illumin- 

 ated strongly develops on darkening a huge positive potential difference. 

 The upward deflection so evoked cannot, however, be of long duration, 

 because by the darkening the light eye is beginning to be changed into a 

 dark eye, and therefore the effect of our first substance is no longer so clearly 

 indicated. 



An example of these phenomena is reproduced in fig. 6. Here 1 mm. 

 abscissa is 0-2 sec, 1 ram. ordinate is 20 microvolts. The eye has been 

 illuminated for a fairly long time with almost the strongest light at our 

 disposal ; that is to say, practically the full amount ^ of I„. At a a constant 

 potential difference of 200 microvolts is introduced into the circuit whereby 

 the image of the string is displaced 10 mm. downwards. At b the eye is 

 suddenly darkened. It is seen that after a latent period which, according 

 to a rough estimate, has a durati-^n of about 004 sec, a high positive 

 off-effect A^ occurs, whose top is elevated 29*5 mm. above the original 

 position of the string. If the indicating speed of the galvanometer left 

 nothing to be desired, this height should correspond to a potential difference 

 of 590 microvolts, but the galvanometer was in this case given too great 

 sensitiveness to be able to follow exactly the extremely rapid current 

 variation. The amount of the latter must be calculated from the form of 

 the curve.2 We have not made this calculation in detail but it is easy to 

 ascertain that the actual potential difference attained is considerably higher 

 than the amount above mentioned. We may estimate it at about 1000 

 microvolts (1 millivolt). 



' An eye which is dark-adapted may be teemed a dark eye. The terms are analogous to 

 " Lichtfrosch " and " Dunkelfrosch," which are commonly used. 



^ There was placed in the path of the light rays a very weak smoked glass, whereby 

 the intensity was a little lessened. 



^ For a method of calculation cf. " Weitere Mitteilungen liber das Saitengalvanometer," 

 Annalen der Physik., Bd. xxi., S. 483, 1906. 



