A.— MATHEMATICAL AND PHYSICAL SCIENCES. 45 



Apart from the question of sources for spectroscopic study, Zeeman 

 effects, with the rules of Laude" for their interpretation, provide the 

 observer with a powerful means of determining the types of terms in many 

 spectra. 



Enough has been said, I hope, to give some idea of the main lines of 

 development and present trend of spectroscopy. The analysis of spectra 

 with which I have been chiefly concerned is a fascinating pursuit, and the 

 establishment of a beautiful order out of an apparent chaos of spectrum 

 lines brings great satisfaction to the investigator. I have endeavoured 

 to show, however, that the analysis of spectra is not an end in itself, but 

 that under the guidance of quantum theory it has fundamental con- 

 tributions to make to our understanding of atomic structure and of the 

 periodic classification of the chemical elements. It appears not at all 

 improbable that some of the mysteries of chemical valency may also find 

 a solution in the classification of spectrum lines, and there are indications 

 that the conceptions of spectroscopy may ultimately extend our knowledge 

 of the structure of matter in the liquid and solid states. 



It may be that in the future the theory of spectra will be so far 

 developed that it will become possible to calculate the positions and 

 intensities of the lines composing the spectrum of an element with greater 

 accuracy than they can be observed. We are, however, still very far 

 from this ideal, and meanwhile experiment and theory must go hand in 

 hand towards a better understanding of the problems that lie immediately 

 before us. 



*' Back and Lande, Zeemaneffeki vnd Multipletistrvkiur, Berlin, 1925. 



