, ON THE THEORY OF OPTICS. 323 



co that from the equality of the angles of incidence and reflection with 

 resp'&At to each of these surfaces, the light must be scattered every way, 

 and no regularity can be observed in its direction. It is true that by con- 

 tinuing the mechanical operation of polishing, we only render these sur- 

 faces more minute and more numerous ; but when they are so much 

 reduced in magnitude as not to be elevated or depressed more than about 

 the millionth part of an inch, they appear to become, for some physical 

 reason, incapable of acting separately, and only to conspire in the general 

 effect. 



In* all cases of refraction, as well as of reflection, if the ray of light 

 return directly backwards in the same line to the surface, it would proceed, 

 after a second refraction or reflection, in the direction precisely opposite 

 to that in which it first was incident, so that the same lines would mark its 

 path in both cases. Thus, if we stand before a looking glass, with one eye 

 shut, and cover its place on the glass with a finger, the same finger will 

 hide the other eye as soon as it is shut and the first is opened in its place ; 

 and a similar effect might be observed if the glass were under water, or 

 behind any other refracting substance. (Plate XXVI. Fig. 371.) 



The medium, in which the rays of light are caused to approach nearest 

 to the line perpendicular to its surface, is said to have the greatest refrac- 

 tive density. In general there is a considerable analogy between this 

 refractive density and the specific gravity of the substance : thus water is 

 more refractive than air, and glass than water. But inflammable bodies 

 are usually more refractive than bodies of the same specific gravity, which 

 are not inflammable ; and it is well known that from the high refractive 

 power of the diamond, in proportion to its actual density, Sir Isaac New- 

 ton most ingeniously conjectured that it was combustible, as more modern 

 experiments have actually shown it to be. It is still more singular that 

 he also imagined, from the same analogy, that water consists of a combi- 

 nation of oily or inflammable particles, with others earthy or not inflam- 

 mable. In the order of refractive density, beginning from the lowest, or a 

 vacuum, we have airs and gases of different rarities, water, which is the 

 least refractive of all liquids, and which is still less refractive when frozen 

 into ice : alcohol, oils, glass, and lastly the diamond ; but probably some 

 metallic substances are much more refractive than even the diamond. 



The refractive powers of different substances, are usually estimated by a 

 comparison of the refractions produced at their surfaces in contact with 

 the air, which, in all common experiments, has the same sensible effect as 

 a vacuum or an empty space ; the ratio of the angles of refraction and inci- 

 dence, when small, and that of their sines, in all cases, being expressed by 

 the ratio of 1 to a certain number, which is called the index of the refrac- 

 tive density of the medium. Thus, when a ray of light passes out of air 

 into water, the sines of the angles are in the ratio of 3 to 4, or of 1 to , 

 which is, therefore, the index of the refractive density of water. In the 

 same manner, for crown glass, the ratio is that of 2 to 3, and the index 

 l^ ; but for flint glass it is somewhat greater, the ratio being nearly that 

 of 5 to 8. 



It may easily be shown that a refractive substance, limited by parallel 



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