ABSORPTION OF LIGHT. 297 



solution, a ruby, emerald, or topaz, or a piece of tinted glass, &c.) 

 is one which absorbs certain kinds of light more readily than 

 others, but does not absorb any one kind to any great extent as 

 compared with merely translucent or opaque substances. 



If white light be made to pass through a sufficiently thick layer 

 of a given coloured medium, it is possible to absorb practically all 

 rays except those of some particular colour, depending on the 

 nature of the medium ; if the light thus treated be now passed 

 through another differently coloured medium, it will often happen 

 that the second medium rapidly absorbs those rays which were 

 able to pass through the first one, so that but little of any kind, 

 or even none at all, passes through the two media jointly. Thus, 

 it is possible to select two pieces of coloured glass (say a bright 

 emerald green and a pure red) which when put together will 

 almost entirely stop all light, so that nothing can be distinctly seen 

 through the two together, although most objects would be pretty 

 readily visible through either separately. If certain shades of 

 purple and deep, orange be used together, the light passing through 

 the two jointly will be red; the purple glass allows red and blue 

 with intermediate shades to pass, absorbing chiefly yellow and 

 green; the orange absorbs green and blue rays, but transmits 

 mostly red and yellow ones, so that the two together filter out, as 

 it were, all colours excepting red. 



Certain media have the peculiar property of stopping all rays 

 visible to the eye, but allowing other kinds of radiant energy to 

 pass freely, or vice versa. Thus, a strong solution of iodine in 

 bisulphide of carbon has such a powerful absorbing action on all 

 kinds of visible light that if a stratum of sufficient thickness be 

 used nothing whatever perceptible to the eye passes through, but 

 radiant heat (Chapter XXIV.) passes freely ; on the other hand, 

 yellow glass allows much visible light to pass through, but stops 

 all actinic rays (Chapter XXV.), whence its use in photography. 

 A plate of solid alum cut from a large crystal, or a trough of alum 

 solution, allows all visible rays to pass through equally freely, so 

 that white light emerges ; but such a plate absorbs radiant heat 

 very rapidly. Various kinds of ordinary white glass act in much 

 the same way. On the other hand, a plate of clear rock salt allows 

 radiant heat to pass just as readily as visible light, being equally 

 transparent to all kinds of rays. 



Expt. 336. Absorption during Reflection. A beam of light 

 impinging on a flat polished plate of silver is mostly reflected 

 (Expt. 332), but little being scattered ; whilst, if a piece of black 

 cloth or a well-smoked fragment of glass or metal be substituted 

 for the polished metal plate, scarcely any light at all is either re- 



