16 
CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
Further, it must be observed that if we find from the photographs, we can at 
once find /x for the visual intensity ; for since 
fii _ ^ 
^ “ 105 ~ 105 ’ 
therefore 
105 , 
^ — 308 ^ 
Now 
jj, — kA~^ 
and 
therefore 
= •341 /x' or /X = /x' nearly, 
in Lord Rayleigh’s formula, 
p-i = 
105 ~ 308 ■ 
Thus by finding either /x or /x' the absorption of every ray of the spectrum can be 
determined. 
XXXI .—Absorption of the Blue Glass. 
As the platinum paper was exposed through blue glass, it becomes necessary to 
know the coeflicient of absorption of the glass for the photographically active rays. 
Experiments have been made with this view, and exposures have been made simul¬ 
taneously through the blue glass, and without any glass intervening. The results 
obtained were that the same blackness would be produced if exposure to light through 
blue glass to that without were as 2'15 to 1. In the tables ol exposure, which are 
annexed, it follows that to obtain the true photographic intensity of total sunlight, the 
values given must be multiplied by 2’15. 
Table of Sensitiveness of Platinum Paper to the different parts of the Solar Spectrum 
when the exposure was made through blue glass, as used in the apparatus with 
which the observations were made. 
Scale number. 
Relative sensitiveness. 
Scale number. 
Relative sensitiveness. 
7 
0 
13 
86 
8 
4 
13-5 
70 
8-5 
10 
14 
56 
9 
20 
14’5 
42 
10 
50 
15 
30 
10-5 
66 
16 
16 
11 
84 
17 
6 
11-5 
95 
18 
2 
12 
100 
19 
1 
12-5 
97 
20 
0 
H is at 13'8 ; G at IPi ; Li at 9 ; and D at 4‘3. 
Scale No. 12 has a wave length of 4260. 
