Law for True Photographic Rendering of Contrast. 189 



relation between the -first and last lights concerned in the 

 process involves a connexion between the relative transpar- 

 encies of the positive and negative plates. Hurter and 



-n -a: u j c j ±u rv Light transmitted 



JJrimeld denned the transparency 1 as — fV— j — = — r, — r — 



r " Light incident 



They also make use of a correlative quantity, the density 

 D of a plate, the connexion between these quantities being 



lo g f = density = D. 

 It should be observed that since T is always less than 



unity, its reciprocal ^ is greater than unity and its logarithm. 

 is positive. 



Now I = VT 2 from the definition of transparency. 



But if also T_T7-p 



we 



have 



Similarly 

 whence 



Ex 



T 2 ~ 



V 

 ~K = 



- constant 



E 2 = 



= PT! 



> 



E 2 

 TV 



= F = 



constant 



(1> 



P) 



The first of these equations states that the transparency of 

 the positive must bear a constant proportion to the light 

 from the subject at all corresponding points. The second 

 equation states, what is only the direct result o£ definition, 

 that the light transmitted by the negative is proportional to 

 the transparency of the negative itself. Both of these 

 equations must be caused to be true simultaneously by a 

 suitable choice and treatment of the two plates. 



To examine these equations further it is convenient to 

 take the logarithms (to base 1U) of both sides of each and 

 write 



Dj = log TjT , L> 2 = rjT~ > 



whence from (1) y 



log E^ Jog g 



and from (2) log E 2 = log P-D,. . (4) 



ogE^log^-D* (3) 



