188 
DR. BREWSTER ON REFLECTED LIGHT. 
with one another,) the intensity of reflection at their common surface is always 
less the nearer the refractive indices of the media approach to equality ; and 
when they are exactly equal, reflection ceases altogether, and the ray pursues 
its course in the second medium, unchanged either in direction, velocity, or 
intensity. It is evident from this fact, which is general, that the reflective or 
refractive forces, in all media of equal refractive densities follow exactly the 
same laws, and are similarly related to one another ; and that in media un- 
equally refractive, the relation between the reflecting and refracting forces is 
not arbitrary, but that the one is dependent on the other, and increases and 
diminishes with it. . This remarkable circumstance renders the supposition 
of the identity of form of the function expressing the law of action of the mole- 
cules of all bodies on light indifferently, less improbable. 
“ To show experimentally the phenomena in question, take a glass prism or 
thin wedge of a very small refracting angle (half a degree for instance : almost 
any fragment of plate glass indeed will do, as it is seldom the two sides are 
parallel), and placing it conveniently with the eye close to it, view the image 
of a candle reflected from the exterior of the face next the eye. This will be 
seen accompanied at a little distance by another image reflected internally 
from the other face, and the two images will be nearly of equal brightness, if 
the incidence be not very great. Now apply a little water, or a wet finger, or 
still better, any black substance wetted, to the posterior face, at the spot where 
the internal reflection takes place, and the second image will immediately lose 
great part of its brightness. If olive oil be applied instead of water, the defal- 
cation of light will be much greater ; and if the substance applied be pitch, 
softened by heat so as to make it adhere, the second image will be totally ob- 
literated. On the other hand, if we apply substances of a higher refractive 
power than glass, the second image again appears. Thus with oil of cassia it 
is considerably bright. With sulphur it cannot be distinguished from that 
reflected at the first surface ; and if we apply mercury or amalgam (as in a 
silvered looking-glass), the reflection at the common surface of the glass and 
metal is much more vivid than that reflected from the glass alone. The de- 
struction of reflection at the common surface of two media of equal refractive 
powers explains many curious phenomena, &C.”* 
* Treatise on Light, § 547, 548. 
