PEOPESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 609. 
with a greater number. If both experiments give the same value for §, it is clear that 
the disturbing influence of absorption within the plate is inappreciable. 
The following series of experiments were made with plates of mica, which were only 
some himdredths of a millimetre in thickness. 
Series of Experiments VI. 
A. With one plate of Mica. 
Exp. 1. 
Exp. 2. 
Exp. 3. 
Exp. 4. 
lo 
I 
I 
lo 
12-69 
10-33 
12-37 
10-33 
12-83 
10-17 
12-41 
10-17 
0-814 
0*835 
0-793 
0-816 
Mean value of ^ = 
•^0 
P = 
0-8143 
0-1024 
B. With three plates of Mica. 
1 
! 
Exp. 1. 
Exp. 2. 
Exp. 3. 
Exp. 4. 
lo 
I 
I 
lo 
12-91 
7-80 
0-602 
13-08 
7-80 
0-596 
12-89 
7-65 
0-596 
12-84 
7-65 
0-596 
Mean value of 
P = 
0-5969 
0-1011 
The mean of the two values found for § is 0T0I7, and this gives, when substituted in 
the formula as a mean refractive index of mica for the chemical rays of a 
coal-gas flame, the number 1-936. 
Another important element in the investigation of photo-chemical extinction, is the 
law according to which the optical coefficient of extinction varies with the density of the 
absorbing medium. In order to determine this law for the chemical rays we have 
employed coloured solutions, knowing that for a considerable depth water may be con- 
sidered as perfectly diactinous. We made use of columns of water of various lengths, 
each containing the same quantity of a dilute solution of common red ink. The follow- 
ing experiments, VII., show the dependence of the extinguished chemical rays upon 
the density of the extinguishing medium. I^ signifies the amount of the chemical rays 
before transmission through the cylinder, I the amount remaining after transmission, 
b=Y transmitted fraction of the unit amount of incident light, h the length of the 
columns of liquid in millimetres, e the density of the colouring substance, that is, the 
