﻿the Reciprocity Law in Photography. 907 



The following are the values obtained : — 



Exposure 



(mius.). 



30 



t (mm.). 



3310 



■ 32-96 



t^ (mm.). 

 1479 

 14-65 



P> 

 00754 

 0-0764 



30 



3492 



16-34 



0740 



2-5 



34-87 

 34-72 



15-80 

 15-51 



00733 

 0-0728 



2-5 



36-72 

 35-66 



18-64 

 16-48 



00773 

 0-0729 



25 



34-08 

 33-98 



16-23 

 15-24 



0-0783 

 0-0746 



2-5 



35-54 



17-74 



0-0725 



2-5 



46-07 



27-34 



0-0746 



In the case of the last of the above results the plate used 

 was a Wratten " Instantaneous," which is considerably faster 

 than the Paget. The mean of the above is 0071:7, with a 

 probable error of +0*0004. 



The mean obtained is somewhat higher than the value 

 given by the manufacturers, Messrs. Ilford,in their calibration 

 of the wedge used, but this is accounted for by the fact that 

 the wedge constant increases in the violet and ultra-violet. 

 Toy & Ghosh*, who investigated this point, found that the 

 value of p begins to rise at a wave-length of about 4500 A, 

 at which point the density of their wedge was 1'25. The 

 density at 4000 A they found to be 1*6 and at 3000 A about 3'5. 

 A usual method of calibration is to focus the (small) filament 

 •of an electric lamp on various parts of: the wedge, and to throw 

 the spot of: light passing through on to a white screen. Light 

 from a similar source is passed through two Nicol prisms on 

 i;o an adjacent portion of the same screen, and the two spots 

 adjusted by means of: the nicols to the same intensity. 

 Since the normal photographic range extends further into 

 the ultra-violet than the visual range, the value for p given 

 by this method will be lower than that obtained by the 

 photographic method. 



Determination of the Schwarzschild Constant. 

 Three types of plate were used, so chosen as to give a large 

 variation of properties. They were the Paget " Half-Tone/' 

 the Wratten "Instantaneous," and the Imperial "Eclipse." 

 The Paget " Half-Tone" is a process plate, about one-third 

 faster than wet collodion plates, and giving great contrast. 

 The Imperial "Eclipse " is one of the fastest plates obtainable, 

 and the Wratten " Instantaneous' 5 is intermediate between 

 the two, being a medium slow landscape plate. 

 * Phil. Mag. Dec. 1920, p. 775. 

 ^3N2 



