298 SCIENTIFIC AMUSEMENTS. 



fleeted or scattered. In the latter case, practically all the incident 

 light is absorbed by the cloth or lampblack without being re- 

 emitted in the form of visible rays ; even with the most highly 

 polished reflecting surfaces some amount of absorption always 

 takes place, the brilliancy of an object illuminated by reflected 

 light being less than it would be if directly illuminated from the 

 same source at such a distance as would cause the rays of light 

 falling on the object to traverse equal amounts of space in each 

 case. Silver and white metals generally absorb much the same 

 amounts of all kinds of coloured rays, so that the reflected light is 

 substantially white ; though, even with such metals as polished 

 aluminium, silver, tin, and zinc, slight but perceptible differences 

 in whiteness are observable, aluminium and zinc giving a bluer 

 shade, and tin a yellower one, than silver. Polished copper 

 absorbs red and orange rays less readily than green and blue ones ; 

 so that when white light falls upon copper the reflected rays contain 

 a larger proportion of red and orange than white light, and conse- 

 quently appear coloured; and similarly with gold and other 

 coloured metals. By causing the rays to be reflected several times 

 in succession from two or more polished surfaces, the absorption of 

 certain coloured rays is intensified and a much deeper colour is 

 produced. Thus, a gold cup looked into obliquely appears of a 

 much deeper orange red at the bottom than at the rim, on ac- 

 count of the greater absorption, owing to repeated reflection taking 

 place with rays from the lower part. A polished sheet of copper 

 rolled up into a short wide tube, and looked into obliquely, 

 exhibits an analogous deepening in tint for the same reason. 



Expt. 337. Total Internal Reflection. A somewhat remark- 

 able result is brought about when an incident ray is inclined at 

 a particular angle to a refracting surface, when the ray tends to 

 pass from a denser to a rarer medium. As shown in Expt. 333 

 the refracted ray will be bent away from the normal in a direction 

 given by the law of sines. Now, suppose that the index of refrac- 

 tion relatively to the passage from the denser to the rarer medium 

 is represented by the letter n, obviously ?^<l; if the angle of 



incidence be such that its sine is -, then the sine of the angle of 



% 



refraction will be n x - = 1 ; that is, the angle of refraction is a 



71 



right angle, so that the emergent ray passes into the rarer medium 

 at right angles to the normal at the point of incidence, that is, 

 glances along the common surface of the two media. But now, 

 suppose that the incident ray falls upon the surface at such an 

 angle that the sine of the angle of incidence is numerically greater 



