468 



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



The following tables show the validity of the expression for different 

 batches of plates : 



Table XL 



0-0460 

 0-0562 

 0-0572 

 0-0608 

 0-0578 

 0-0602 

 0-0541 

 0-0480 



Mean 0-0550 



Time. 

 2'0 mins. 

 2-5 

 4-0 

 5-0 

 8-0 



Table XII. 



Y. 



0-684 



0-840 



1-23 



1-63 



1-90 



3-40 



0-0495 

 0-0500 

 0-0500 

 0-0470 

 0-0470 



Mean . . K = 0-0487 



This shows that both 7^ and K vary for different batches ; for the 

 same developer, y x and K are the characteristic development constants 

 of the emulsion. 



The factor 7 is of considerable importance in practical photography, 

 as it measures the gradation or degree of contrast in a negative. 



Influence of Concentration on y. As might be expected, this is simply, 

 the time taken to reach a given factor is inversely as the concentration 

 of the ferrous oxalate. 



Thus for N/12-5 and N/25 ferrous oxalate 



N/25 5-0 mins y = 073 



N/12-5 2-5 7 = 0-72 



N/25 5-0 7 - 1-54 



N/12-5 10-0 7 = 1-59 



Over the range N/5 to N/40 



(A) N/5 1-5 mins. (C) N/20 6'0 mins. 



(B) N/10 3-0 (D) N/40 12-0 



