452 Messrs. S. E. Sheppard and C. E. K. Mees. [Dec. 20, 



The means of all series are as follows : 



1 0*01035 Gravimetric. 



II 0-01012 Volumetric. 



Ill 0-01037 



IV 0-01029 



V... 0-01040 



Total mean 0'01031 = P for plates employed. 



Wratten ordinary emulsion and ferrous oxalate. Curve I shows the 

 directly proportional ratio between the density D = Iogi and the 

 mass of silver per 100 cms. 2 . The mean value differs materially from that 

 found by Hurter and Driffield, but this may be due (a) to a constant error 

 in their photometer ; (b) to the plates used, as it seems probable that the 

 emulsifi cation of the silver haloid has considerable influence on the 

 "covering power" of the silver subsequently deposited. This 

 u covering power " is, of course, the quantity P or photometric 

 constant. 



