﻿908 Mr. R. A. Mallet on the Failure of 



The approximate relative speeds of these plates were 

 obtained by exposing a specimen of each behind the wedge 

 to light of the same intensity for the same length of time, 

 and measuring the lengths of the lines obtained as nearly as 

 possible without further treatment of the plates after develop- 

 ment. Then, at the end of each line, 



I'(ST)*e-P* = e* 



where S is the speed of the plate and e k is a constant. 



Since p is not identical for each plate, it is necessary to 

 reduce all lines to a standard length, and take the ratio 

 of the actual time to the time for that length as proportional 

 to the speed. If t u t 2 , etc. are the actual lengths of the lines, 

 and t the standard length, then 



f= logI -*+;?log(ST) 

 P 



lo gIo-* + plog(ST ) . 



ana Cq — > 



P 



log™- = ~(t- t ) == log /cS, 

 J-o V 



Si p 



.,l 0g |i = | ( , 1 - <2 ) + (£._^ (f2 _ a . 



^2 ^1 x Pi Pi/ 



For Paget plates, /o/p = 0*0864, * = 21'3 



„ W ratten plates pjp = 0*0884, * = 38"0 



„ Imperial plates p/p = 0'0ti84:, £ = 46*7. 



t was taken as 35, which is approximately the mean value 

 of t. From these figures the relative speeds of the plates 

 are roughly : — Paget 1, Wratten 29, Imperial 165. 



The Schwarzschild constant was determined by photo- 

 graphing lines on the plate with long and short exposures 

 alternately, the intensity of the light being kept constant 

 throughout, and the duration of the long exposures being 

 usually one hundred times that of the short. The actual 

 times of exposure are given in the table below. In the case 

 of the Wratten and Imperial plates, the intensity of the 

 incident light was reduced to a convenient extent by inter- 

 posing one or two neutral screens between the source of 

 light and the wedge, the screen being let into the front 

 of the box in which the light was enclosed at a distance 

 of about 5 cm. from the filament. This was necessary, 

 as, owing to the sensitivity of these plates, the exposures 

 would otherwise have had to be made too short to admit 



