188 Prof. Porter and Dr.. Slade on the Fundamental 



When the plate is exposed to the subject (exposure = E 2 ) 

 a certain density of deposit is produced and the negative has 

 a certain transparency Tj in consequence after development. 



Fig. 1. 



£l 



■ : V-"V; -VV:"ij V*X\\"-; Negative 



Taking stage. 



In the second place a print is made by exposing the positive 

 plate (or paper) through the negative. Let the printing 

 light be P. Where the transparency is T r the light getting 

 to the negative is PT 1 = E 2 ; this exposes the positive pro- 

 ducing a transparency T 2 after development. 



Fig. 2 

 P 



>tage 





St^S 



> pa five 



PT,=E 2 





W^M- 



si RVe 



Printing 5 





Finally, this positive is viewed under an illumination V 

 (the viewing light) and the light that issues from it is 

 VT 2 =I. 



Fig. 3. 



V 



■'2.-.V.V.V. 



Viewing stage. 



Since for a true rendering of contrast it is necessary to 

 liave the same ratio between light emitted from two portions 

 of our final picture as that which fell in the camera from the 

 two corresponding portions of the subject, we must have 

 I = KE where K is a constant reduction factor. Now this 



