PEOFESSOE BFNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 605 
As the thick plate of crown glass, which we used for the determination of a, was 
somewhat dark-coloured, and contained bubbles of air, it was to be expected that the 
plates of white mirror glass, which we employed for our transparent cylinders, would 
not give so large a coefficient of extinction. The follovring experiments confirmed this 
opinion, showing that the amount of light a, absorbed by the plates of white glass 
4*7 millims. in thickness, was not more than that absorbed by the thin plate 0’3 millim. 
thick ; hence that the coefficient of extinction of our plates was so small, that, without 
incurring any perceptible error, the product a. 4-7 in formula (2.) becomes equal to 0 ; 
or in other words, the absorption in the interior of the glass may be neglected without 
any appreciable error. 
Series of Experiments III. 
Exp. 1. 
Exp. 2. 
Exp. 3. 
Exp. 4. 
Exp. 5. 
Exp. 6. 
lo 
9-58 
8-88 
9-91 
10-36 
11-87 
11-91 
I 
8-62 
7-99 
8-97 
9-25 
10-85 
10-66 
h 
0‘30 mm. 
4-70 
4-70 
4-70 
4-70 
4-70 
p 
0-0514 
0-0514 
0-0487 
0-5552 
0-0439 
0-0540 
The last horizontal division contains the values of the coefficient of refiexion calculated 
according to formula (5.), upon the assumption, proved correct by the last experiment, 
that no appreciable absorption of the rays of light occurs in the plates. The mean value 
of § is found to be 0'0509. When the coefficient of refiexion for glass is knovm, the 
amount of light a, transmitted by n plates, is found from the formula 
1-g 
1 + ( 271 - 1 )^—“' 
( 6 .) 
If the mean value of § 0’0509, as found in the last experiment, be substituted in this 
formula, we obtain a value «i = 0'823 for the amount of light passing through two 
plates. This value, 0‘823, plays an important part in the calculation of the coefficient 
of extinction, so much so, that we have thought it necessary to corroborate this calcu- 
lated result by fui’ther experiment. For this purpose a series of direct observations were 
made with the cyhnders closed with two plates of glass. In these, as well as the suc- 
ceeding absorption experiments with these cylinders, the accuracy of the result de- 
pended entirely on placing the cylinder before the insolation-vessel (i, fig. 2, Part II.), 
so that none of the rays from the luminous source were obstructed by the edge of 
the cylinder. This aiTangement was effected by illuminating the insolation-vessel, and 
placing the transparent cylinder in such a position that when the observer moved 
his eye near the source of light, he saw that the insolation-vessel was covered by the 
screen, and not by the edge of the interposed cyhnder. If the cone of rays from a 
convex lens was employed, the aperture of the lens was made smaller than that of the 
cylinder, and lens and cylinder were brought as near as possible together. Having duly 
observed these precautions, we determined the intensity of the chemical rays proceeding 
from a flame of coal-gas, before and after their passage through our transparent cylinder, 
MDCCCLVII. 4 K 
