﻿Quantum Theory of Photographic Exposure. 255 



sulphuric acid. The strips thus obtained contain the unde- 

 veloped grains, and by taking the difference the number of: 

 developed grains is calculated. 



The data given below cover only the first or highest 

 density step of a Hurter and Driffield sensitometric strip *. 

 20 fields on each of 3 strips are employed to determine the 

 developed grains. To determine the number of grains and 

 clumps in the original one grain layer plate before sensi- 

 tometric exposure and development, 10 fields on each of 

 4 strips were used. By taking this large number of fields 

 on several strips we obtained a much better average. The 

 results in both the above cases are reduced to a number of 

 grains or clumps per square centimetre of one grain layer 

 plate. Then as the dilution is known, one may, with certain 

 restrictions, refer back to the original plate. 



All photomicrographs were made at a magnification of 

 2500 diameters and these negatives enlarged 4 times in 

 printing. On the prints the grains and clumps are measured 

 and counted, and then classified in class sizes (areas). The 

 class sizes (areas) are to 0*2 /* 2 , 0'2 to 0*4 yu 2 , 0*4 to 0"6//, 2 , 

 etc. The light source is a point source from a Pointolite 

 lamp which is screened with a YVratten (H) blue filter to 

 restrict the wave-length range and therefore increase the 

 resolving power of the microscope. A cell containing 

 copper sulphate solution absorbs heat rays and a cell con- 

 taining a solution of quinine bisulphate excludes the ultra- 

 violet light. The optical system is built up as follows : 

 Cedar oil immersion condenser and objective, aplanatic 

 condenser of numerical aperture 1*4, and Bausch and Lomb 

 objective 1*9 mm. numerical aperture 1*3 in combination 

 with a No. 6 compensating ocular. 



In the following is a table of our results. Column 1 

 contains the number of grains in each clump, column 2 the 

 average area of grains in corresponding clumps, column 3 

 the number of grains times 10 -3 per square centimetre of 

 original one-grain layer plate, column 4 the number of 

 grains times 10 ~ 3 per square centimetre of developed one- 

 grain layer plate, and column 5 gives the proportionate 



k 

 number ^ of clumps affected. 



* The five remaining steps each corresponding to one-half the exposure 

 of the preceding are now being counted and mapped out, and we hope 

 to be able to publish the results obtained with them in the course of 

 one or two months. 



