390 Einthoven and Jolly 



Fig. 3 may serve as an example. The curve shown in this figure miist^ 

 like all the curves here reproduced, be read from left to right, while the 

 connections of the eye with the galvanometer are made in such a way that 

 a current passing from the cornea through the instrument to the posterior 

 surface of the eye deflects the image of the string in an upward direction. 

 An action current in this direction may be termed positive, and in the 

 reverse direction negative. 



Fig. 3 gives the reaction following on a momentary illumination. In 

 the rectangular system of co-ordinates 1 mm. of an abscissa represents 0'5 

 sec, 1 mm. of an ordinate 100 microvolts. The illumination is by means 

 of green light, which is reduced to 001 of its original intensity, and which, 

 in accordance with the two tables given in the previous chapter, we may 

 name 10"^ Ig. The duration of the illumination is 01 sec. 



One observes, after a latent period, a small preliminary negative deflec- 

 tion A which is immediately followed by an upward movement of the 

 string. After the curve has reached a somewhat acute peak B, it sinks 

 first rapidly, then more gradually, but while still distant from the zero 

 line it mounts again. This latter ascent begins 25 to 3 sec, after the 

 beginning of the illumination, while much later the curve reaches its second 

 maximum C, which lies about 1 mm. higher than the first positive peak B. 

 Finally, the curve gi-adually regains the zero line. 



The potential differences which have given rise to these three summits 

 amount to — 



For the preliminary negative deflection A . . — 70 microvolts. 



For the first positive summit B . . . 1050 



For the second positive summit O . . .1150 



The form of the photo-electric reaction when evoked under similar 

 circumstances to fig. 3 is always essentially the same, but the absolute size 

 of the deflections as well as their proportional size may differ. When a 

 number of curves are compared one with another, which all agree in having 

 summit B of the same height, the deflection A as well as the slow succeeding 

 wave C may show very different heights. 



In the case of illuminations of short duration, the energy of the light is, 

 as may be expected, the measure of the stimulation.^ A stronger light 

 must shine a shorter time than a weaker in order to produce the same- 

 effect. 



In fig. 4 is seen a curve which is obtained by illuminating another eye 

 during 001 sec. with the full amount of Ig. In the system of co- 

 ordinates 1 mm. abscissa is again 0'5 sec, but the sensitiveness of the 

 galvanometer is now ten times greater, so that 1 mm. ordinate amounts to 

 10 microvolts. 



Notwithstanding that the energy of the light stimulation is greater, 



' Cf. de Haas, loc. cit. 



