863 
The measurements of the grain of the plates developed with 
glycin show that the grain is pretty well constant, and oscillates 
within the errors of measurement round a value of 0.96 u with a 
blackening of 1.3 to 0.15 (measured for green); for slighter blacken- 
ings the valne descends to 0.9 u. 
No measurements have been possible for greater blackenings, 
because the grain itself seems to be broken up into smaller pieces, 
when the layer of gelatine is pressed out. 
In table II *) is found the ratio of blackening and silver content 
for the three colours used. It is seen that this ratio decreases with 
the blackening. 
SUMMARY. 
1. A method has been given to measure blackenings of photo- 
grapbic plates up to a blackening of 0.001 for different wave-length 
with an accuracy of 0.7°/, for the blackenings of from 2.0 to 0.1, 
and of 3.3°/, for the blackenings of from 0.1 to 0.001. 
2. It is found that the blackening of the blackened photographic 
plate depends in a very great degree on the colour of the radiated 
light; hence when used as reducer the plate must first be gauged 
for the different colours. 
3. Morr’s extinction meter has been applied to the silver analysis 
of the photographic plate. The method has been used from 0.5 < 10—3 
to 156 X 10-% grams of silver per Liter. The accuracy amounts to 
3°/, on an average. | 
4. The mean cross-sections of the grains of the examined plates 
have been determined. As the ratio of blackening and silver content 
present per unit of area in the photographic plate must be constant 
for grains of the same size and nature, the curves of this ratio and 
of not constant grain have been plotted. 
Besides this ratio has been determined for plates with grains of 
constant cross-section of the grains. 
With great pleasure we acknowledge here our indebtedness to 
Prof. L. S. Ornstein, Dr. W. J. H. Morr, and Dr. H. C. Buregr 
for the great encouragement and assistance they gave us in the 
execution of these researches. 
1) The observations for collargol have been put together at the end of 
Table II. 
