36 TRANSACTIONS OF THE 
AN EQUATION OF THE DENSITY EXPOSURE 
CURVE OF A PHOTOGRAPHIC PLATE 
TAKING REVERSAL INTO ACCOUNT. 
BY ORIN TUGMAN. 
(Abstract). 
The equation derived by Hurter and Driffield 
accounts only for a curve which never reverses. This 
equation was derived on the laws of absorption and that a 
silver bromide grain in a photographic plate remained 
exposed. 
This equation assumes that after a silver halide grain 
absorbed a sufficient energy to be exposed, further 
absorption will render the grain unexposed. After that 
more energy will be required to expose the grain again 
and so forth. 
The equation is an infinite series of exponential 
terms. 
