( 701 ) 



very strong in a light eve, and accordingly an eye which has been 

 ilhiminated strongly develops on darkening a hnge positive potential 

 difference. The npward deflection so evoked can ho\ve\er not he of' 

 long duration, because by the darkening the light eye is beginning 

 to be changed into a dark eye and therefore the effect of onr first 

 substance is no longer so clearly indicated. 



Although in the light eye the conditions are less favourable for 

 the lighting than for the darkening stimulus it is nevertheless possible 

 to apply the former in either of two ways. In the first place we 

 may suddenly increase the intensity of the light that is radiating on 

 the eye, and secondly we may darken the light eye for a short 

 period, so that it has not yet become a dark eye and then suddenly 

 illuminate it. 



The second method gives better results than the first and we 

 possess numei'ous curves where after a short darkening of a light 

 eye a strong light stimulus was applied. The "on effect"') is a stee{) 

 downward deflection and attains the considerable amouiit of 120 to 

 130 microvolts. It is true that it is followed immediately by an 

 upstroke, the latter however is but small in comparison with the 

 strong upstroke which under similar conditions is evoked in a 

 dark eye. 



The second suhsUince. 



The second substance reacts less quickly than the first. (Jn 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 wliich are opposed to those of the first 

 substance. Its effect appears almost nnmixed in a dark eye which 

 is illuminated for a short time by weak light. 



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

 follows rapidly upon the lighting, in a similar way as in a momen- 

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

 steeper anacrotic part which is evoked by the lighting and a less 

 steep katacrotic part evoked by the dai'kening. The top of the curve 

 lies, luithin certain limits higher the more the energy of the illumi- 

 nation is increased either by using greater intensity or longer dura- 

 tion 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. If a 

 strong momentary illumination bo applied there appears a short 

 negative preliminary deflection by the function of the first substance 



\) A convenient expression introduced by Gotch. 



