2059. The theory of the transmission of light through transparent bodies is derived from a well known law in optics, that the influence of the sun's rays on any surface, both in respect to light and heat, is directly as the sine of the sun's altitude; or, in other words, directly as his perpendicularity to that surface. If the surface is transparent, the number of rays which pass through the substance is governed by the same laws. Thus, if 1000 rays fall perpendicularly upon a surface of the best crown glass, the whole will pass through, except about a fortieth part, which the impurities of even the finest crystal, according to Bouguer, will exclude; but if these rays fall at an incidental angle of 75ﾦ, 299 rays, according to the same author, will be reflected. The incidental angle, it will be recollected, is that contained between the plane of the falling or impinging ray, and a perpendicular to the surface on which it falls.
