855 
The points marked in fig. 2 have been obtained by taking the 
mean everywhere of the values A and 5*) recorded in table I 
(see the end of this communication). 
It appears convincingly from fig. 2 that the blackening found is 
a function of the wave-length with which the blackening has been 
measured. 
As we hope to demonstrate further in a following communication 
and as appears from the measurements in § 3 — the strong deviation 
of H, for blackening from O to 0,6 will have to be attributed to 
the influence of the size of the grain of the silver. 
160 — en : 
Lb = Hydrochinon, utr 
| ra EL 
140 Gy = elycine wivaroot 
(p= : geel 
0.60 05 10 
Fig. 3. 
In order to make the survey of the efficiency of the photographic 
plate as light-reducer clearer, the transparency for the measured 
blackenings from fig. 2 has been derived in ratio to the transparency 
for green, by the aid of the relation: 
df? D 
10%6—*d il (1) 
Ti ee 
in which D represents the transparency and the index 6 and d 
means ultra-red and green. 
(1) results immediately from the definition of the blackening. 
1) Finally the same measurements b, c and d have been carried out for 
collargol. As analogues of the blank plate serves a dish filled with pure water, 
as analogues of the blackened plate serves a same dish filled with collargol- 
solution. These observations are indicated in fig. 2 by C. 0.65 X 10-3 grams 
of collargol are taken per cm° of water. 
