506 FRAUNHOFER'S LINES. 



cloud may be reflected by the mirror horizontally into the spectro- 

 scope. The mirror should first be held in the hand, and by turning 

 it about and looking into the spectroscope the proper position for it 

 may thus be first approximately found ; it is then clamped in the 

 fork in that position, and finally adjusted by turning it and moving 

 the champ. 



The preceding experiments have led to the following 

 results : first, a single colour emitted by an incandes- 

 cent vapour produces a single image of the slit, that is, 

 a coloured line ; second, the continuous spectrum pro- 

 duced by the white hot carbon particles of a flame is 

 an uninterrupted series of all the variously coloured 

 images of the slit which can possibly be produced ; 

 third, each single colour has its definite position in the 

 spectrum. It follows from these results, that dark 

 lines must occur in a spectrum which does not include 

 all colours, that is, in which definite colours are absent. 

 The existence of Fraunhofer's lines in the spectrum of 

 the sun is hence a proof that, of all possible coloured 

 rays, some are wanting in the light of the sun. 



Since the spectra of all luminous solid and liquid 

 bodies are continuous, and those of incandescent gases 

 consist of single bright lines, we cannot obtain a spec- 

 trum intersected by dark lines by merely heating any 

 known body until it becomes luminous. But a spec- 

 trum containing dark lines like that of the sun may be 

 obtained if the light emitted by a luminous body is 

 allowed to pass through a luminous vapour, and under 

 these circumstances the remarkable fact is rendered 

 evident that in the spectrum of the solid luminous 

 body those colours are wanting which the interposed 

 vapour emits, by itself, when in an incandescent state. 



By holding a piece of lime in the flame of a common 



