﻿the Reciprocity Law in Photouraphy. 905 



Blackening" the following relation : 



v\ here S is the degree of blackening, 1 the intensity of the 

 incident light, T the time of exposure, and k and p are 

 constants. The constant k is fully discussed by Plotnikow *, 

 and depends on several factors, mainly on the thickness and 

 composition of the film. The exponent p is known as the 

 Schwarzschild constant. Its value was determined by 

 Schwarzschild for Schleussner plates, which he exposed to 

 the light of a normal benzene lamp at various distances for 

 varying lengths of time, measuring the depth of blackening 

 by comparison with a scale of blackness standards made by 

 means of a Scheiner sensitometer. He found jt? = 86. 



Other workers in this field are Becker and Werner f, 

 Leimbach J, Helmick § who used a Lemon spectrophoto- 

 meter, Stark || who used a Konig-Martin spectrophotometer, 

 and Plotnikow who used a Kruss polarization colorimeter. 

 Apart from the fact that all these workers have obtained their 

 results by methods depending on the comparison of light and 

 shade by eye, and that their numerical results vary some- 

 what, no one appears as yet to have suggested any physical 

 meaning for the constant p. It is, in fact, uncertain whether 

 it depends on the density, and whether it is constant for a 

 given make of plate under all conditions. Stark has shown 

 that it is subject to considerable variation over abnormal 

 ranges of exposure, and he and others have shown that it 

 varies somewhat with the wave-length of the light used. 

 Schwarzschild (loc.cit.) used both light and heavy blackening, 

 and found it to remain constant, but some further investigation 

 seemed desirable. 



It was decided to use a method which did not depend on 

 visual comparison of varying shades of blackness, and to this 

 end a Goldberg wedge screen was introduced between the 

 plate and the source of light. This was carried out as 

 follows : — 



A brass plate, having in it a slit about three inches high 

 and three millimetres wide, was let into a wooden hoard, 

 having at the back a recess of such size that a quarter-plate 

 wedge screen fitted in closely, in such a position that the 

 slit was opposite the middle of the wedge. Immediately 

 behind the wedge was placed the plate-holder, consisting of 

 * Lehrbuch d. Photochem. p. 667. 

 t Zeit.f. luiss. Phot, x. p. 882 (1907). 

 X Zeit.f. iviss. Phot. x. p. 137 (1909). 

 $ Plrys. Rev. xvii. 2, p. 142 (1921). 

 ] Ann. d. PJnjs. xxxv. 3, p. 461 (1911). 



Phil. Mag, S. 6. Vol. 44. No. 263. Nov. 1922. 3 N 



