210 Dr Brewster 07i the reflectioii and decomposition of 



is always less the nearer the refractive indices of the media 

 approach to equality ; and when they are exactly equal, re- 

 flection 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 rela- 

 ted to one another; and that in media unequally refractive, 

 the relation between the reflecting and refracting forces is not 

 arbitrary, but that the one is dependent on the other, and in- 

 creases 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 molecules of all bodies on 

 light indifl'erently, 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 defalcation of light will be much greater; and if the sub- 

 stance applied be pitch, softened by heat so as to make it adhere, 

 the second image will be totally obliterated. 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 distin- 

 guished from that reflected at the first surface ; and if we apply 

 mercury or amalgam (as in a silvered looking-glass), the re- 

 flection 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 



