﻿the Reciprocity Law in Photography. 



911 



despite the alterations of intensity and duration of exposure. 

 The points represented by -I- A etc. refer to the different 

 plates measured, and the values plotted are over the same 

 range of density so as to show the similarity in the shape of 

 the curves. From this it is evident that the Schwarzschild 

 constant has a definite value for each type of plate, and 

 that this value remains constant over a range of density 

 extending from zero to approximately unity, since the density 

 of the wedge used is about 0*075 per mm. and the portion of 

 the lines considered in the curves given is about 14 mm. 



long. 



Fi£. 1, 



It is further evident from a consideration of these curves 

 that the value of the Schwarzschild constant is the same at 

 the " threshold," i. e. at the point where the plate is just 

 darkened, as at greater densities. Owing to the "cutting. 

 of the plates used in determining the constant, the value 

 given by them was not that at the " threshold," but 

 the measurements made with the photoelectric cell extend 

 to the extreme ends of the lines, and the curves coincide 

 perfectly at this point also. 



