CELESTIAL DISSOCIATION. 



195 



copious emission of light of a few wave-lengths only, the 

 others being missing. The result of this- is that instead of 



Candle-flame spectrum. 



Straight slit. 



Ring slit. 



Spirit 



lamp flame 

 with salt. 



Fig. 45. A continuous and a discontinuous spectrum. 



a broad band of colour, we see a series of lines of definite 

 colour and separated by definite spaces. Such a series of 

 definite lines is called a discontinuous or line spectrum. No 

 two elements emit the same light, and hence no two ele- 

 ments yield the same line spectrum. It is therefore possible 

 to recognise each element by the spectrum of its lines when 

 sifted out by the prism and this process is called spectrum 

 analysis. It makes no manner of difference whether the 

 element exists on the earth, in the sun or in the farthest 

 "fixed " star. Its existence may be recognised beyond dis- 

 pute wherever light shines from it to us. Fig. 46. In the 



B 



Fig. 46. Parts of the spectra of (A) barium and (B) iron (from a 



photograph). 



