( 702 ) 



and the very slow second elevation which follows must be attributed 

 to the functioning of the third substance. 



llie third substance. 



The third substance reacts in the same direction as the second 

 substance but more slowly. On lighting it displaces the image of 

 the string slowly upwards and on darkening still more slowly down- 

 wards. So much slower is the third substance than the other two 

 that its effect in a recorded curxe appears as a rule almost entirely 

 isolated, and thus can be easily followed. 



The effect of the third substance falls out under two conditions 

 (J) In a fully light adapted eye and (2) in a dark eye submitted to 

 very faint light for a short time. 



Specially remarkable are the curves obtained if the duration of 

 the lighting of a dark eye is systematically changed, and we wish 

 to direct attention more particularly to the "off elfect" in such cases. 

 If the duration of tiie light is very short and the light is weak, then 

 as already mentioned the effects of the second substance appear 

 unmixed. The off effect here consists in the descent of the curve to 

 the zero line. 



If the duration of the light is taken a little longer, and the effect 

 of the other two substances begin to become perceptible, the off 

 effect is determined by the resultant of three forces : The first sub- 

 stance 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 elfect of the other substances. In the 

 case of longer lighting the off effect 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 can not 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 third substance itself, consisting in a slow descent of 

 the string, appears much later and fairly isolated. 



The general result is that we can observe in a series of curves, — 

 ol)tained from a dark eye where the light has been gradually 

 lejigthened in duration, — that the darkening effect, in the first 



