Sensitivenses of Photographic Plates to X-Rays. 429 



1.63, 1.85, 1.71; the average departure from the mean value of 

 1.71 was .10. 



(2) A Paragon X-ray plate (size 10 in. x 8 in.) was given an 

 exposure of 2760 volt^-ampere-seconds per cm. 2 After development 

 its density was measured at twenty points, the values obtained 

 being:— 3.58, 3.94, 4.08, 4.21, 4.73, 4.73, 4.59, 4.33, 4.14, 3.74, 

 4.08, 4.21, 4.14, 3.50, 4.04, 4.21, 4.59, 3.50, 3.91, 4.08; the 

 average departure from the mean value of 4.12 was .26. 



Direct measurements of the mass of silver salts per square centi- 

 metre of each plate verified the existence of these inequalities. 



Mr. Jones, of the Kodak (Australasia) Ltd. kindly prepared 

 plates of the same type as the Austral Standard (Extra Rapid) 

 platos, but having a double coating of emulsion. This plate is 

 tabulated as Austral Standard (Extra Rapid) II. As can be seen 

 from the exposure Hues, this led to an increase in speed, though 

 the contrast remained practically unaltered. Unfortunately, how- 

 ever, the fog-density was also increased, and, as the plate was 

 always somewhat stained by the developer, it was difficult to 

 measure its density. 



In obtaining the mass of silver i salts on the plate, the emulsion, 

 after having been dissolved off in boiling water, was digested in 

 nitric acid and filtered through a Gooch crucible. Two measure- 

 ments were made for each plate and the differences between the 

 values obtained confirmed the existence of inequalities in the 

 thickness of the emulsion. The total mass of emulsion and gelatine 

 together was also measured, and the percentage of silver salts in 

 the emulsion was calculated. 



The table below gives the two values obtained for the mass of 

 silver salts per unit area of the plate, and also the average value of 

 the percentage by mass of silver salts in the emidsion. From these 

 figures it appears that the speed of a plate does not depend 

 -entirely on the mass of silver present in the emulsion : — 



I 





Mass of Silver Salts per ein2. Mass of 



plaf> 





^ 



Silver Salts 



rl9,\iC. 



'■ 





~^ perlOOmtrnis. 





1 



2 



of Emulsion. 





inpfnis. 



ingins. 



rngms. 



Paragon X-ray - 



24 



2.6 



48 



Wellington X-ray 



2.2 



2.4 



39 



Wratten X-ray - 



2.5 



2.6 



38 



Ilford X-ray 



2.4. 



2.6 



45 



Imperial X-ray - - - 



3.2 



2.9 



38 



Austral Standard (Sun)- 



1.3 



1.5 



— 



Austral Standard (Extra Rapid) 



1.0 



.8 



29 



Austral Standard (Extra Kapid) II. 



2.5 



2.9 



45 



