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, unclianged 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 impi'obable. 



" 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 mercuiy 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. 



