i:*4 



Dr. A. D. \Vullrr. 



arc Mental currents, provided they ! not due to excessive physio- 

 logical differences. 



The immediate effect of light is to arouse current in the half-shaded 

 leaf, directed from the illuminated to the shaded half (is., in the 

 galvanometer from shaded to illuminated; i.e., from resting to active 

 tissue, as in muscle and nerve). 



AYith illumination of moderate duration, i.e., not exceeding a few 

 minutes, this first effect lasts as long as its cause, rising towards a 

 maximum. With longer illumination, a maximum is reached from 

 which the effect begins to decline. The current drops to or beyond 

 zero, giving place to the reversed current, which is the regular after- 

 effect of illumination. 



At the end of an illumination of moderate duration, the current 

 rapidly subsides and gives place to a reversed current directed in the 

 leaf towards the previously illuminated half. 



This effect and after-effect of illumination are similar in appearance 

 to the effect and after-effect in nerve produced by tetanisation, extend- 

 ing, however, over longer periods of time (figs. 2 and 2l>). 



MH f/iii fmle. The electromotive force of the response has a value that 

 usually ranges from 0'005 to 0'020 volt. 



The leaf resistance (interpolar distance = 5 cm., and breadth = 

 about 1 cm.) is generally between 500,000 and 1,000,000 ohms. 



The current deflection with these values is between 5 and 40 cm. of 

 scale, with a possible accidental effect of 1 cm.* 



L. R. 



FIG. 5. Interval of Time between Illumination L and Response R, of a rigorous 

 Leaf of Iris. (1733.) 



* The sensitiveness at which this galvanometer was used was such that 10~ 9 A 

 1-eni. scale. With the recording galvanometer, 1 cm. of ordinate = 3'10~ 8 A. 



