﻿Characteristic Curve of a Photographic Emulsion, 303 



The highest value of N is about 1, which corresponds to 

 less than 70 per cent, of grains changed, whereas actually 

 the percentage changed corresponding to this value of N was 

 about 90. Tbis is because the partial development has not 

 been sufficient to show up all the nuclei, and it is very 

 difficult to do this, since before this stage is reached, nuclei 

 which initially were distinguishable from one another have 

 grown together into a single mass of silver. It is, however, 

 very unlikely that even if every nucleus could be observed 

 the general form of the curve in fig. 8 would be changed. 



Fte. 8. 



+00 



N, 



O QSfr 



op 



025- 



-e- 



0-2S 



0-50 



0-75 



■00 



The most natural assumption to make is that longer develop- 

 ment would merely result in an increase of the number of 

 visible nuclei in proportion to the number already observable, 

 and that this is the case is shown by the following experi- 

 ment : — Two plates were given the same exposure under the 

 wedge and partially developed, one for 15, and the other for 

 18 seconds. The values of N were then found for widely 

 different intensities, with the following results : — 



(1) I = 1*00, N for 15 seconds development = 0*613, 

 N for 18 seconds = 0'980, whence (N ) 18/(N )15= 1-59. 



(2) I = 0*27, (N )15 = 0-153, (N )18 = 0*240, whence 



(N ) 18/(N ) 15 = 1*57 ; so that this ratio is practically 

 constant, and the general form of the curve is indepen- 

 dent of the development. 



