608 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEinCAL EESEAECHES. 
Series of Experiments V. 
Exp. 1. 
Exp. 2. 
L 
16-08 
13-76 
83-8 mm. 
0-0002 
16-08 
13-99 
11-8 
0-0008 
I 
h 
a 
Exp. 3. 
Exp, 4. 
16-08 
13-57 
27-0 
0-0008 
10-23 
8-99 
17-3 
0-0003 
From the extremely small values which are thus found for a, it is seen that the coeffi- 
cient of extinction of the chemical rays employed may, as far as regards the observation, 
be placed at 0 in columns of water less than 80 millims. in depth ; and therefore that the 
factor 10'“* in formula (7.) becomes =1. According to this method it is possible, 
approximately at least, to determine the coefficient of reflexion of all transparent fluids 
for the chemical rays. For this purpose a drop of the liquid is brought between tsvo 
plates of glass, of which the coefficient of reflexion ^ has abeady been determined, and 
the intensity of the light measured before and after transmission through the moistened 
plates. The values of lo and I thus found are substituted in equation (7.), and as the 
value of h is extremely small, the factor 10'*“ vanishes, and we have for the coefficient 
of reflexion of the liquid, 
iVi-s 
O — 0 _. 
^ 1-g 
A simple relation exists, as has been stated, between the coefficient of reflexion c and the 
l-i 
or 1 = 
1 + 
If 
refractive index i, which is represented by the equation ^ ^ 
we substitute the value g>= 0-0509, found for crown glass, we obtain for the refi-active 
index 1-583. The value of this index of refraction, as obtained by experiment for 
Fraunhopee’s line H lying nearest to the chemical rays, is given in Buff’s ‘ Lehrbuch 
der Physik,’ to be 1-5466 and 1-5794, numbers closely approximating to that foimd bom 
the coefficient of reflexion. 
The coefficients of reflexion and the refractive indices for the chemical rays of all 
substances which can be divided into such thin plates that the absorption may be con- 
sidered = 0, may in this way be ascertained. We have thus determined the coefficient 
of reflexion and the refractive index of North American mica, which we shall make use 
of in a later part of the investigation. 
It has been previously observed that the fraction a=\- (formulae 1 and 6) of the imit 
amount of light which passes through n glass plates, is represented by the equation 
I i-p 
— i)g ’ where g signifles the coefficient of reflexion of the glass. In order to 
see whether the plates absorb any light m then- interior, in which case the formula could 
not be employed, it is sufficient to make the experiment first with one plate, and then 
