Sensitiveness, of Photographic Plates to X-rays. 425 



A plate of green glass was placed in the path of each beam of 

 iight so as to eliminate any difficulty due to difference of colour 

 in the two halves of the field of view. The analysing Nicol was 

 rotated until the two halves of the field of view matched; the 

 -angle between the two Nicols was read, and the intensity of the 

 light from S was therefore proportional to cos-^. The photo- 

 graphic plate B was now placed in position, and the analysing 

 Nicol was rotated until the field again appeared uniform. If the 

 angle between the two Nicols was now 0', the intensity of the light 

 -transmitted through the plate was proportional to cos-^'. Hence 



tlie density of the plate was obtained as Jo^^ -, or lo<^ cos-^ 



^^If "^"cos-^' 



Ihe density of the fog strip was also measured and was subtracted 

 from the measured density of each of the other strips to give the 

 true density due to the exposure. 



AVhen, for any given plate, the density was plotted against the 

 logarithm of the exposure, the curve obtained Avas similar in 

 <;haracter to that for light. In the region of under exposure the curve 

 -Nvas convex to the axis of log E (see Fig. 2), but for densities greater 

 than about 1.0 a straight line was obtained, so that in this neigh- 



bourliood the density of a plate is given by D = y.log^^. The 



quantity i is the inertia of the plate, and is the exposure corre- 

 sponding to the density of if the straight line law held for all 



2t 



Log,.E 



U-O 



A^ 



to ■ ' ■ 1 1 



' " 1 'Ml 



1 



1 I ' I 1 ^ 





1 \ 1 1 1 







1 



- - -- -1-+ -^ ^ ' \ i ! 



1 



/ 



1 



/ 





y 



1 1 



i |l/^ 1 



h. 





*t- MM 



\ J^ 1 M t 





^n 1 M 



111 1 



r\ ' lilt" 



M 



' 1 / M ' ' I i 



' 1 1 



\j^ 1 Mill 



^T '" 







f 1 1 j ' ' ! t 



i " I ! 1 r 



r 1 



1 i 



^r i 1 ! ' ' ' ' M ! 



•} 



^ I ! ' 1 1 1 ' 1 ■ M 



^ \ 



X ! 1 M 1 M ! ' ! 





;^ ' i ' M M ! 



■ 4: " If: i p ' :^* 







Ml" 



' Jr 



1 , 



1 ^Si 





^' \ 







1 1 



r\ hM4, 1 1 1 liTl l.rl 1 1 1 IT 



::±i::-:: :::_::::-:::_:£::: 



Fig. 2 



