138 A. E. Bostwick—The Influence of Light 
Method I.—The plate was connected as one arm of a Wheat- 
stone’s bridge. wo of the other arms were made up of 
any time more than a few millimeters from 265. 
The resistance of the connections was determined at the close 
of the series by substituting for the plate a sheet of silvered 
copper of negligable resistance. — 
The sensibility, or number of scale divisions traversed by the 
index for the introduction into the circuit of one millimeter of 
rheostat wire, was also determined for each set of observations 
in the following manner. The micrometer screw atthe end 0 
the rheostat slide was so adjusted as to allow the slide a 
movement of just one millimeter. Ten readings of the index 
were then taken, the slide being alternately at its extreme 
positions. The mean of the differences between every two 
as approximately parallel, the ratio of the resistance of the 
illuminated portion to that of the whole plate is sensibly that 
of their areas. - 
- The percentage of change of resistance in the lighted portion 
of the plate corresponding to a movement of the galvanometer 
index one scale division evidently 
S| 100 
“piecn Ri«€ 
eo 
the plate, and “a” the fraction of the plate covered by the 
light spot. It is assumed that the observer could detect 4 
movement of the index amounting to one-fourth of a scale 
division. 
The method, then, was as follows: The plate resistance and 
sensibility having been determined, the index was balanced 
and the circuit closed. When the spot was perfectly still, the 
light was suddenly thrown on the plate and the movement 0! 
the index, if any, was noted. The spot had generally a steady 
FN ee atten gs as, 2 
; 
a 
3 
ee 
3 
ia 
