I.J PRINCIPLES AND METHODS 9 



So much then in general for the radiations given out by light 

 sources, and the manner in which the spectroscope shows them and the 

 student records their positions. 



Spectrum analysis was established when experiment proved that no 

 two substances which give a line spectrum give the same order of lines 

 from one end of the spectrum to the other ; in other words, the line 

 spectrum of each chemical substance differs from that given by any 

 other. 



Here then is one of the secrets of the new power of investigation 

 of which the spectroscope has put us in possession : we can recognise 



FIG. 7. Angstrom's grating spectrometer. 



each element by its spectrum, whether that spectrum is produced in 

 the laboratory or is given by light travelling earthwards from the most 

 distant star, provided the element exists both here and there. 



It is in this way that spectrum analysis helps us with regard to 



FIG. 8. Parts of the spectra of (A) barium and (B) iron (from a 

 photograph). 



