41<i LKCTURTE XXXV. 



In order to obtain the eifects of regular reflection and transmission, wc 

 must have perfectly smooth and polished substances; for all rough bodies, and 

 sometimes even such as to the touch seem tolerably smooth, have their surfaces 

 divided into innumerable eminences and depressions, constituting, in realit}', as 

 many separate surfaces, disposed in all imaginable directions, so that from 

 the e(|uality of the angles of incidence and reflection, with respect 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 continuing the mechanical 

 operation of polishing, we only render these surfaces 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 re- 

 turned 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 wliich the rays of light are caused to approach nearest to 

 the line perpendicular to its surface, is said to have the greatest refractive 

 density. In general there is a considerable analogy between this re&active 

 density and the specific gravity of the substance: thus water is more refrac- 

 tive than air, and glass than water. But inflammable bodies are usually 

 more refractive than bodies of the same specific gravity, which are not in- 

 flammable; and it is well known that from the high refractive power of the 

 diamond, in proportion to its actual density,Sir Isaac Newton 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 combination of oily or inflammable 

 particles, with others earthy or not inflammable. In the order of refractive 



