PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 603 
and hence 
in which represents the intensity before transmission, I the intensity after transmission 
through a medium of the thickness A, and ^ the thickness of absorbing medium by pass- 
ing through which the amount of light has diminished tp ono-tenth. 
The values of a for chlorine, or the chlorine detonating gas, may be determined by 
allowing the rays from a constant source of light to pass through layers of the gas of 
various degrees of thickness ; whilst the intensity of the incident and transmitted rays 
is measured. As the gases are enclosed between plane plates of glass, the loss of light 
in the passage through these plates must be considered. This loss of light is made up 
of the light reflected from the surfaces of the glasses, and that absorbed in the mass of 
the glass. For the purposes of our investigation, it is necessary to know what fraction 
of the loss of Hght is to be ascribed to the reflexion, and what fraction to the absorp- 
tion in the mass of glass. The following was the method adopted in order to determine 
this point. 
Let Ij represent the portion of incident hght which enters the medium, and let 
represent the amount of light which remains after the ray has traversed a thickness h of 
the medium, and let ^ represent the depth of the medium, by passing through which the 
original amount of light I, is reduced by extinction to one-tenth of its intensity, we then 
have Ip=I^. 10”*“, where a. represents a constant, which we call the Coeflicient of Ex- 
tinction, dependent upon the nature of the medium. We can only measure the amount 
of light lo falhng upon the medium ; the light which enters the medium can, however, 
as the following considerations show, be calculated, at least for rays perpendicular to the 
reflecting surface. Suppose that the unit amount of light falls in 
the direction Mj, flg. 1, perpendicular to the surface AA of the medium, 
a portion § of this light will be reflected at and a portion 1—^ will 
enter the medium. This ^ is a constant, dependent on the nature 
and surface of the transparent medium, which may be called the 
Coefficient of Eeflexion. When the light 1— f has passed through 
Fig. 1. 
A 
the thickness h of the medium, and reached the second surface 
the intensity has diminished to (1 — ^)10”*°', in consequence of the extinction which it 
has suffered. At the second bounding surface the fraction § of this light, that is, 
gi(l — is reflected; so that the amount of light which passes this second reflecting 
surface is (1— ^(1— ^)10”*“, or (1— In the case of our experiments, 
the further reflexions are not considered, as their influence on the result would be inap- 
preciable, and hence we have the following equation representing the relation between 
the incident light Ij and the transmitted light I : — 
I 
