LIGHT. 115 



further, would lead us beyond our present limits into the region of 

 solar physics. 



In the case of common light colour is due to absorption ; that 

 is, to the extinction of one or more of the components of white 

 light, and to the transmission or reflexion, as the case may be, of 

 the remainder. The lost rays are transformed mainly into heat. 

 This process is selective, and the selection may extend to any 

 number of the components ; so that the spectra of such colours will 

 show dark bands in the places of the colours absorbed. The 

 bands always retain their po'sition whenever the same substance 

 is used ; hence they may ^be used for detecting the presence of 

 colouring substances in solution. 



A very important instance of absorption is that exhibited by 

 metallic vapours. Vapours of metals at a lower temperature 

 absorb exactly those rays which they emit at a higher. This is 

 the great principle first distinctly enunciated by KirchhofF, and 

 made by him the foundation of modern solar physics. The 

 vapours of metals in a state of incandescence on the surface of the 

 sun float upwards, and thus form a stratum of cooler vapours 

 which exert selective absorption upon the light before it reaches us. 

 The celebrated dark lines of Fraunhofer in the solar spectrum are 

 due to this selective absorption. Much time and pains have been 

 expended upon the construction of accurate maps of these lines by 

 Angstrom, Thalen, Kirchhoff, and others. More recently, Lockyer 

 has applied photography to laying down these lines ; the map of 

 the solar spectrum upon which he is now engaged will probably 

 not only exceed all predecessors in accuracy, but will add to an 

 enormous extent to the number of the ascertained lines in the 

 spectrum. 



A variety of spectroscopes or arrangements of prisms with suit- 

 able telescopes or lenses for observation will be found in the 

 Collection. Some of them have few, or even only one prism, and 

 are intended for feeble light ; others have many prisms. In some 

 the light will be reflected back, and sent twice, or even three 



I 2 



