420 



Natalie C. B. Allen and T. H. B, Laby : 



values cf the exposure. For the exposures we have used we fincF 

 that the inertia of a plate is independeut of the time of develop- 

 ment. In the table below, J)^ and r)._> are the densities resulting: 

 from exposures of 8.71 x 10^ and 4.10 x KH vol f"^. ampere. sec. cm '"^ 

 on llford X-ray plates. 



Time of Development. 



l>i 



D., 



D2/D1 



3 minutes 



1.88 



2.49 



1.36 



4 minutes 



2.52 



3.42 



1.39 



5 minutes 



2.57 



3.56 



1.36 



Since the ratio D._,/Di is independent of the time development^ 

 it follows that the exposure corresponding to a density of 0, i.e.^ 

 the inertia, is independent of the time of development. Hurter- 



jla Soo J«o y^o Tio J 00 J.^o jifO 2 do - j oa dZa Sg/f 



Fig. 3 



and Drifheld*!^ have proved it to l>e independent also of the kind of 

 developer used. The so-called speed of a plate is a quantity in- 

 versely proportional to the inertia. 



y, or the increase in D corresponding to an increase of 1 in? 

 log^E, is the contrast of the plate. Its value depends on the- 

 development, and reaches a maximum when the plate has been: 

 developed to its maximum density. 



In the table below are tabulated (1) the inertia of the plate (iir 

 volt^.ampere.sec.cm''^), (2) the contrast, and (3) the minimum value 

 obtained for the fog density. 



1 Hurter and Driffield. .Jour. Soe. Chem. Ind., .'^ 1890, p. 455. 



