THE BLACKENING OF A PHOTOGRAPHIC PLATE AS 
A FUNCTION OF INTENSITY OF MONOCHRO- • 
MATIC LIGHT AND TIME OF EXPOSURE 
P. S. HELMICK 
{Abstract) 
By an empirical modification of Hurter and Driffield’s^ and 
Ross’^ formulae, it has been possible to express the blackening of 
a photographic plate exposed to monochromatic light as a function 
of intensity of light and time of exposure with an accuracy within 
the limits of the experimental data at hand. By means of mono- 
graphic charts the constants of the equations can be computed by 
semi-graphical methods. 
The equations used were: 
L” 
b-ib-l)e -aO^^/B + Clog,,/)" 
and 
= « — 1 S + Clogj(, /) “ 
M S ^ 
S—0 
£)=- logg 
a 
where D is the density of plate 
density equals log^^ 
incident light 1 
transmitted light J * 
/ is the intensity of monochromatic light, and t is the time of ex- 
posure to light. 
The p of Schwarzchild’s ^ formula for equal blackenings of a 
photographic plate: equals constant; can be found from the 
above formulae as {B C log^o 
The; Physical Laboratory, 
The: State University oe Iowa. 
1 Journ. Soc. Chem. Ind. 9, 455; 1890. 
2 Journ. Optical Soc, of Am., 4, 261; 1920. 
3 Astrophys. Journ., 29, 154; 1909. 
