400 Einthoven and Jolly 



duration of the lighting times. In the four figures the successive amounts 

 are 31-2, 66-8, 110 and 116 microvolts The first and second members of 

 the series — figs. 11 and 12 — exhibit the reaction of the second substance 

 practically unmixed. In the third member of the series — fig. 13 — a 

 complication begins to be visible. There is formed another summit at 

 C which, as we shall later explain, must be considered as the efffect of 

 the action of the third substance. 



In the last member of the series — fig. 14 — the complication has con- 

 siderably increased. Not only does summit C of the third substance 

 appear more clearly, but the action of the first substance also becomes 

 apparent; thus at A^ we perceive the darkening effect of this substance. 



The lighting effect of the first substance is the negative deflection A. 

 This does not yet appear in fig. 14, which need not surprise us, since the 

 first substance acts specially strongly in a light eye, and therefore from 

 the nature of the case the darkening reaction must appear sooner than 

 the lighting reaction. Nevertheless the lighting eflfect makes itself 

 appreciable to some extent during the record of the curve ; for if we 

 regard the duration of lighting and the proportional heights of the 

 summits of the four figures in the series (figs. 11, 12, 13, and 14), it is 

 evident that the summit height in fig. 14 is only little greater than that 

 in fig. 13. The increase is only 6 on 110 microvolts, while the duration 

 of lighting is increased from 1"9 to 3 sec. It is the lighting effect of 

 the first substance which here hinders the development of a higher 

 summit B. 



The Third Substance. 



The third substance reacts in the same direction as the second, but 

 more slowly. On lighting it displaces the image of the string slowly 

 upwards, and on darkening still more slowly downwards. So much slower 

 is the third substance than the other two that its effect in a recorded curve 

 appears, as a rule, almost entirely isolated, and can thus be easily followed. 



The summit of the wave which is evoked by the action of the third 

 substance is denoted by the letter C in figs. 3, 4, and 5. In the case of 

 the momentary but very strong illumination of fig. 4, this summit occurs 

 16 sec. after the beginning of lighting. In the case of the less strong 

 illumination of figs. 3 and 5, it occurs 20 sec. after such beginning. 



The effect of the third substance falls out under two conditions : (1) in 

 a dark eye exposed tx) very faint light for a short time, and (2) in a 

 completely light eye. The first condition is realised in figs. 11 and 12, 

 while on the contrary, in the figs. 13 and 14 of the same series, where the 

 energy of the light stimulation exceeds certain limits, the eflfect of the 

 third substance appears again. 



The second condition, a completely light ej^e, is practically realised in 

 fig 7. After the short lighting at l^.^ there is seen in the curve no slow 

 deflection which could be the analogue of the deflection C in figs. 3, 4, and 5. 



