861 
of the great blackening. For this reason we have repeated the 
measurements, after the grains of the plates have been mechanically 
pressed out into thinner layers. After this operation we find 0.48u 
for the cross section of the grain. It seems therefore that the pressing 
out does not change the cross-section in the plates developed with 
hydroquinone. 
The size of the grain of the strongly blackened plates 9 and 10 
has been determined in the same way. 
O = zarteng voor ultraood 
@ = mgAg per 100 cm 
0.1 02 03 04 0.5 06 
Fig. 5. 
Fig. 5 gives the measured cross-sections of the grain of the plates 
4--10 plotted against the quantity of silver found in § 2 in mg. 
per 100 cm’, and against the blackening measured in $1 for ultra- 
red. The lefthand vertical line is the axis of ordinates for the quan- 
tity of silver, the righthand vertical line that for the blackening. 
When of the plates N°. 3 and 2 the quantity of mg. of silver 
per 100 cm.’ and the blackening of ultra-red are derived from table 
1 (see the end of this paper), and the values A and B averaged, 
two values of the size of the grain are found of every plate by 
substitution in the curves of fig. 5, which must both pass through 
the origin, and have therefore been continued to the origin. 
The mean of this is 0.05 u for plate 3 and 0.03 u for plate 2. 
As these quantities have been obtained by interpolation, they are 
printed in italics in table I. 
For the theory and the understanding of the action of the 
