﻿370 Characteristic Curve of a Photographic Emulsion. 



this is supported by his experiments. He found that the 

 sensitivity of a very fast emulsion was decreased considerably 

 by treatment with chromic acid, but that the sensitivity of a 

 very slow emulsion remained unchanged. He explained 

 this by the existence on the surface of the sensitive grains of 

 colloidal silver, formed during the ripening process, which 

 was not present in the insensitive grains, and which was 

 removed by the chromic acid. 



It is very difficult to imagine that the fundamental light 

 action varies with the kind of emulsion, and that considering 

 a whole series of emulsions, from the most sensitive to the 

 most insensitive, there is a transition region where an entire 

 change of mechanism takes place* Strong evidence against 

 Liippo-Cramer\s view is that Svedberg (ibid.) has shown that 

 in one of the slowest emulsions the reduction centres are 

 distributed amongst the different grains according to the 

 same law as has been shown here to hold for their distribution 

 in the case of one of the fastest commercial emulsions. 

 This is in favour of the view that for all kinds of emulsions 

 the process of the formation of the latent image is the same. 



The existence of this chance distribution of developable 

 " centres " in the grains does not conclusively prove that 

 they are the kind we have considered in this paper, and there 

 are at least three other possibilities. Assuming a discrete 

 structure of the radiation, the centres may, as suggested by 

 some, be the points of impact of light quanta on the grains, 

 but the fact that the majority of these centres are located on 

 the edges of the grain is strongly against this view. Also 

 within the crystal there maybe a chance concentration of the 

 light energy at certain points, and both these possibilities are 

 being tested in this laboratory. Again, this chance dis- 

 tribution may be due merely to the fact that the grain as a 

 whole is changed by the light, but the developer reaches 

 some points of it sooner than others. If this is so, there 

 appears to be no reason why the average number of centres 

 per grain, considering only developable grains, should in- 

 increase, as it does, in a regular manner with the light 

 intensity. The author believes that the evidence so far 

 obtained is mainly in support of the theory discussed in this 

 paper. 



In conclusion, the author wishes to express his thanks to 

 Dr. T. Slater Price, Director of Research of the British 

 Photographic Research Association for much valuable 

 criticism and advice. 



