ASTRO-PHYSICS 279 



gas-burner. More recent experiments have shown 

 that traces of impurities may modify the spectrum 

 considerably, while, in some cases, the presence 

 of one substance will completely mask the 

 spectrum of another. 



Again, when an atom is ionized, that is given 

 an electric charge, some of its spectral lines 

 are often found to become much more intense. 

 These ** enhanced" lines are very important in 

 the interpretation of solar and stellar spectra. 



The possibilities introduced by all these 

 effects naturally complicate the interpretation of 

 solar and stellar spectra. On the other hand, 

 the very complications greatly increase the interest 

 of the luminous messages, and the investigation 

 of the connection between the external conditions 

 and the nature of the spectra in the physical 

 laboratory opens an almost limitless field to 

 profitable research. Co-operation between the 

 laboratory and the observatory doubtless will 

 elucidate gradually the fascinating problems of 

 the nature of the celestial bodies. 



The spectra of various substances differ 

 widely in complexity. Some consist of a few 

 lines, some of very many. Iron, for instance, 

 emits light of at least two thousand different 

 wave-lengths. Of recent years, as explained on 

 page 248, order has been introduced into our 

 knowledge of complex spectra by the discovery 

 that fairly simple relations hold between the 

 wave-lengths, or rather the number of vibrations 

 in a centimetre. Simple formulae have been 

 devised, which in a general way express the 

 connection between the vibration number of one 

 fundamental line and its companions, somewhat 



