474 ELECTRO-PHYSIOLOGY CHAP. 



for the same stimulus, but they are qualitatively different. In 

 " light " frogs such, i.e., as have been exposed for hours to the 

 full effect of daylight the positive fore-swing of the negative 

 variation concomitant with the impact of light is entirely 

 wanting, or appears as a trace only. The same reaction, 

 according to Kiihne and Steiner, is exhibited by the retinae of 

 winter-frogs even when confined for days in darkness in a warm 

 room. 



If the current of rest in an unbleached " dark " retina is of 

 low E.M.F., and the negative variation in illumination only 

 moderately developed, there is usually a reversal of the current 

 at excitation, immediately after the positive fore-swing, which 

 lasts throughout the period of light (Fig. 283). In other cases 



Fio. 283. Fir;. 284. 



(with a strong current of rest), there is often no genuine 

 negative variation, merely a more or less considerable decrement 

 of the preliminary positive fore-swing (Fig. 284). The current 

 of rest often begins to sink rapidly from the moment of 

 preparation. It may not merely sink to zero, but be reversed. 

 The photo-electric variations are only thereby affected in so far 

 that the individual phases collectively exhibit the opposite signs, 

 while entry, order, course, and magnitude undergo no alteration. 

 The three variations accordingly present the indices + 

 instead of the normal + + . 



The fact that the three phases of the retinal action current, 

 due to transitory illumination, appear in sensitive preparations 

 even when, as with the electric spark, the impact of light is 

 momentary, shows that the medium negative phase must not be 

 regarded merely as the consequence of pcri/xtnait illumination, 



