( 700 ) 



nuiinier lluxt a current passiui; fVoin tlic coniea (lin)ii«^li Ihe iiistru- 

 inciit (o llic )H)!s(eri()r siii-racc of llie ove dotlects llic iinage of the 

 string- in aii u{)\var<l diroctioii. Aii action cnrrcnt in this dii-ection 

 may be tenned positive, and iji the reverse direction jiegative. 



On momentary ilhiminatioii of tiie eye there is observed a small 

 preliminary negative deflection which is immediately followed by 

 an upward movement of the striiip'. Aftei' a somewhat acute peak 

 the cm've sinks, at first i"a[)idly then more gradually, but while 

 still distant from the zero line it mounts again. This latter ascent 

 begins a couple of seconds aftei- the l)eginning of the illumination, 

 and the second summit, which is reached much later, often consi- 

 derably exceeds the peak in height. Finally the curve graduall}' 

 regains the zero line. 



If the illumination be continued for some time, a new elevation 

 occurs at the moment of darkening whose height is greater the longer 

 the illumination has endured. 



The complicated form of these curves and the striking fact that 

 a deflection in the same direction takes place both on illumination 

 and on darkening suggest that there are in the eye two or more 

 different processes occurring partly simultaneously partly successively 

 whose fusion determines the form of the electric reaction. 



Further investigation confirms this suggestion, and if recourse is 

 had to very weak or very strong light it seems even to be possible 

 to bring about a separation of the supposed processes. The pheno- 

 mena are explained in the simplest manner by the assumption that 

 tiie processes are three in number, whether they are together depen- 

 dent upon the same substance or each upon a separate one. For the 

 sake of conxenience we shall «peak of three substances and as we 

 do not intend in the meantime to attempt to define them anatomically 

 in the eye, we prefer to try to describe their characteristics and to 

 mentio]] the conditions, under which their effects appear as pure 

 as possible. 



The first substance. 



The substance which we have termed "the first" reacts more 

 (piickly than the other two. On lighting it displaces the image of 

 the string downwards, on darkening upwards. Its effect can with 

 diffjculty be obtained pure but nevertheless it 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. 



In the nature of the case the darlvening stimulation can be taken 



1) An eye which is dark adapted may be called a daik eye. Both terms are 

 unuios'ous to "Lichtfrosch" and "Dunkelfrosch" which are commonly used. 



