34 TRANSACTIONS OF THE 
REVERSALS IN PHOTOGRAPHIC PLATES. 
BY C. ARTHUR SMITH. 
(Abstract). 
The density of a photographic plate increases as 
the logarithm of exposure, up to a certain maximum. On 
further exposure the density is found to diminish to 
a minimum. On still further exposure it begins to 
increase again. These points of maxima and minima are 
called critical points and vary with the kind of plate 
exposed. 
The relation of density to exposure is best shown by 
the curve fig. 1. B and C are the critical points. The rever- 
the curve, fig. 1. B and C are the critical points. The rever- 
sal of the plates begins at B. It is seen that the curve 
between C and D has approximately the same slope as 
that between A and B. The same is true of the slope 
between B and C and A and B. The curve has the 
appearance of having reached a second maximum at D, 
but time did not permit taking data beyond that point. 
It is possible that had we continued our exposures we 
would have found a succession of maxima and minima 
points, but the character of the curve beyond that shown 
is problematical. It might settle down to a regular sine 
curve, or it might obey the decay law and finally disap- 
pear entirely, or it might become wholly irregular. 
Just what is the cause of reversals in a photographic 
plate does not seem to be well understood. There are 
two theories regarding it. It is thought by some to be 
due to a chemical change produced by the action of light 
on the silver salt. By others it is believed to be due to 
a photoelectric effect upon the silver grains. This theory 
is based on the now established fact that light incident on 
a metallic surface will produce an electric discharge from 
the surface of the metal. 
