﻿Characteristic Curve of a Plwtograpliic Emulsion. 360 



the paper he compares the variation of N with grain size 

 for grains which have been exposed to light with the variation 

 when the exposure is to X-rays, and suggests certain deduc- 

 tions as regards the absorption of light and X-rays by the 

 silver halide from the difference which he finds. 



Now, from fig. 10 we see (since A=N ) that the manner 

 in which N varies with grain size depends on the intensity 

 to which the grains have been exposed ; we can select an 

 intensity such that N varies in almost any manner we please. 

 Thus, unless the difference between Sved berg's results and 

 those found here is due to the different emulsion used, there 

 seems to be no justification for making deductions from the 

 relation which is found between N and the size of grain at 

 one fixed arbitrary exposure. 



The theory which has been advanced here is capable of 

 explaining an important fact which appears quite inexplicable 

 on such a theory as Allen's (ibid.). It is well known that the 

 sensitivities of the grains in an emulsion depend to a great 

 extent on the conditions of precipitation and ripening; and 

 that, in different emulsions, sets of equal-sized grains may 

 have quite different sensitivities, and even different maximum 

 slopes for their characteristic curves. If, as Allen suggests, 

 the nucleus is really a simple molecule of silver halide which 

 has lost an electron, its characteristics will be the same 

 whatever the emulsion, and it is difficult to see why grains in 

 one emulsion should be more sensitive than those of the same 

 size in any other emulsion. If, however, the nucleus is not 

 silver halide, it is very probable that the conditions of pre- 

 cipitation and ripening do play an important part in deter- 

 mining its characteristics. 



Thus, on Ren wick's theory, the condition of the colloidal 

 silver which is produced will certainly depend on such factors 

 as the kind of gelatin, conditions and time of ripening, etc., 

 and the ease with which colloidal silver particles can be 

 coagulated will be affected by the amount of gelatin present, 

 since this is a protective colloid. The great difficulty 

 in accepting Renwick's theory as it stands is this : — It is 

 known that an unprotected silver sol is very stable to the 

 action of light. Therefore, if a protective colloid is present, 

 it will be still more difficult to effect its coagulation and 

 precipitation by light, whereas in the case of our most 

 sensitive silver halide grains the energy necessary to make 

 them developable is exceedingly small. 



Luppo-Cramer (ibid.) believes that the mechanism of the 

 formation of the latent image is not the same for the most 

 sensitive and very insensitive emulsions, and he claims that 

 Phil. Mag. S. 6. Vol. 44. No. 260. Aug. 1922. 2 B 



