﻿Theory of Photographic Exposure. 2G5 



investigated in this laboratory for their frequency curves, 

 then, with A written for the total area of silver halide, 



But this only by way of illustration. The fundamentally 

 important thing will be formula (7), applicable to each 

 size-class of grains separately. .In fact the experimental 

 verification of the theory now in progress in this laboratory 

 deals, not so much with K but, as it should, microscopically, 

 with k = k a for each class of grains separately, including 

 clumps of grains. 



Before passing to a further discussion and development of 

 the elementary formula (7), but one more remark concerning 

 the presence of more than one layer of grains. The case of 

 two or more layers will at once be seen to be reducible to 

 that of a single layer. In fact, either a grain of, say, the 

 second layer and of size a is not shielded by any of the first 

 layer or else it is thus screened off and only a part b of it 

 remains uncovered. In the former case the grain in question 

 will simply be classified among those of size a of the first 

 layer, and in the latter case among those of size b. This will 

 hold with respect to the exposure to the impinging light- 

 quanta, and b will also be the contribution of the grain in 

 question to the photographic density ; for its covered part 

 will remain inoperative. Similarly for three or more layers. 

 In fine, the presence of a plurality of layers of grains will 

 modify only the frequency curve N a ~f{a) which would 

 otherwise belong to a single layer. We shall henceforth 

 assume that this factor has already been taken into account 

 in constructing the function /(a) or in microscopic counts of 

 the grains within every particular size-class. We disregard 

 here, of course, such factors as a possible absorption of light 

 in additional strata of gelatine. 



5. Dependence upon wave-length. — Once more return to 

 the elementary formula (7) or (4). Denote by s the ex- 

 ponent so that 



N L e ' 



Under the more or less implicit assumption that the trans- 

 versal dimensions of a light-quantum are negligible in 

 comparison to those of a grain, we had s = na. But it will 



