322 LECTURE XXXV. 



black as to reflect no light at all, and to be perfectly invisible in a strou^, 

 light ; although at the surface separating two very rare bodies, ffs two 

 kinds of gas, the reflection is too faint to be perceptible ; but in this case 

 the separation is seldom perfectly abrupt. 



The angles of incidence and reflection are the angles made by a ray of 

 light, before and after its reflection, with a line perpendicular to the re- 

 flecting surface ; and these angles are always equal to each other ; conse- 

 quently the inclination of the rays to the surface remains also the same. 

 The quantity of light reflected, when other circumstances are equal, 

 appears to be always greatest when the difference of the optical or refrac- 

 tive density of the two substances is greatest. Thus the reflection from 

 the common surface of glass and water is much weaker than from a surface 

 of glass exposed to the air. Metals in general reflect a great proportion 

 of the light falling on them, and even the reflection from the common 

 surface of glass and mercury appears to be but little weaker than the 

 reflection from the surface of mercury immediately exposed to the air, so 

 that the optical density of the metals must be exceedingly great. 



It appears also that a portion of the light falling on a reflecting surface 

 is always transmitted, at least to a certain depth, notwithstanding the 

 apparent opacity of any large masses of the substance. Thus, if we cover 

 a small hole of a window shutter with the thinnest leaf gold, we shall find 

 that it transmits a greenish light, which must have passed the reflecting 

 surface, but which, if the gold had been but one ten thousandth of an inch 

 in thickness, would have been wholly intercepted, and probably almost in 

 the same manner as by passing through 700 feet of water. In transparent 

 substances, however, the greater part of the light penetrates to all distances 

 with little interruption, and all rays of the same kind, thus transmitted by 

 the same surface, form with the perpendicular an angle of refraction which 

 is ultimately in a certain constant proportion to the angle of incidence ; 

 that is, for instance, one half, three fourths, or two thirds, according to the 

 nature of the surface. Thus, if the refractive properties of the substance 

 were such, that an incident ray, making an angle of one degree with the 

 perpendicular, would be so refracted as to make an angle of only half a 

 degree with the same line, another ray, incident at an angle of two degrees, 

 would be refracted, without sensible error, into an angle of one degree. But 

 when the angles are larger, they vary from this ratio, their sines only pre- 

 serving the proportion with accuracy : for example, if the angle of inci- 

 dence at the supposed surface were increased to 90, the angle of refraction 

 would be 30 only, instead of 45. Rays of the same kind are in general 

 distinguished by the same colour, although some rays which differ from 

 each other in refrangibility, have scarcely a discernible difference of colour ; 

 and it is possible, on the other hand, to find a surface at which the ratio of 

 the angles is the same for rays of all kinds. (Plate XXVI. Fig. 369, 370.) 

 In order to obtain the effects of regular reflection and transmission, we 

 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 reality, as many separate surfaces, disposed in all imaginable directions, 



