606 PEOFESSOE BUNSEN AND DE. H. E, EOSCOE’S PHOTO-CHEAnCAJL EESEAECHES. 
closed with two plates of glass*. The experimental numbers, and the values of a^. as 
calculated with the formula «, = are thrown mto the followiag Table. 
Series of Experiments IV. 
Exp. 1 . 
Exp. 2 . 
Exp. 3 . 
1 Exp. 4 . 
Exp. 5 - Exp. 6 . 
lo 
17-76 
17-18 
17-66 
1 18-55 
11-39 18-37 ' 
I 
13-41 
13-67 
13-69 
1 14-49 
9-56 15-04 ! 
Ti 

15-84 
17-76 
17-19 
18-26 
11-37 ^ 18-77 
Mean of and IJ 
16-80 
17-47 
17-42 
18-40 
11-38 1 18-57 

0-798 
0-783 
0-784 
0-788 
0-840 0-810 
The mean of these values, O'SOO, is slightly less than the value 0’823 as calculated from 
formula (6.). In all the following experiments we shall employ the mean of these two 
determinations, 0'8II, as the most exact value for the coefficient of transmission of our 
cylinder plates. 
At the commencement of this section we have shown that when light passes through 
a number of media, the amount of transmitted hght remains the same, m whatever 
order the media be placed ; thus a film of ah’ between two glass plates would allow 
exactly as much light to pass through, were the air in front of or behind the plates. 
This assumption is however only true when all the media possess the same coefficients 
of reflexion. If the refractive index ^, and hence the coefficient of reflexion p, diflers 
according to the order in which the media are placed, the values of ? and p var}' in a 
relation dependent upon the arrangement of the media. In this case therefore allow- 
ance must be made for the order in which each limiting siu’face of a medium occurs. 
This is best explained by an example. We select for this pm-pose the determination of 
the coefficient of extinction for water which we have occasion to employ in oiu’ subsequent 
experiments. 
Let us first examine what occurs when a ray of light passes through two glass plates 
(GG,, fig. 2) surrounded by air, but so thin that, hke the glasses of oiu’ cy finders, they 
may be regarded as perfectly diactinic. The fraction i of the unit amount of incident 
fight falling perpendicularly upon the medium 
Which passes the first surface at r is . . . (1— f) 
Which passes the second surface at r, is . . (I — 
Which passes the third surface at r, is . . (1 — f)* 
Which passes the fourth surface at >’3 is . . (I — g')'*. 
It we suppose that a film of water be placed between the plates 
instead of the air, the coefficient of reflexion from glass and water 
will no longer be but a new quantity, ; so that the unit amount 
of incident fight becomes 
Fig. 2. 
^ G 
* See Philosophical Transactions, 1857, Plate XVII. fig. 1. 
