190 Prof. Porter and Dr. Slade on the Fundamental 



Let us suppose that the connexion between D l and log E x 

 is known. It is represented graphically by the characteristic 

 curve usually obtained for a plate (tig. 4). 



Fijr. 4. 



The above equation will enable us to determine the values 

 of D 2 and log E 2 for a plate that will form a suitable positive. 

 Such plates which suit one another as negative and positive 

 we may advantageously refer to as conjugate plates. The 

 former equation (3) asserts that we can obtain a suitable 

 value of D 2 by taking any constant (the same for all points) 

 and subtracting log E T from it. This is equivalent to shifting 

 the origin to any point 0' and measuring D 3 backwards 

 from it ; thus if OM represents a particular value of logE l5 

 then O'M represents the suitable value of D 2 , On the other 

 hand, the second equation (4) asserts that the suitable value 

 of logE 2 is obtained by taking any constant (the same for 

 all points) and subtracting D 1 from it. This is equivalent to 

 still further shifting the origin to any point O" and reckoning- 

 log E 2 doionivards from it. Thus if O'N represents the value 

 of D l5 then 0"N represents the suitable value of logE 2 . 

 Hence the same point Q represents not only the corresponding 

 values of Dj and log E l5 but also, when measurements are 

 made as indicated from the new origin 0", it represents the 

 corresponding values of D 2 and log E 2 . Since this can be 

 said for every point Q in the characteristic curve for the 

 negative, it follows that this curve suitably interpreted gives 



