20 Larmor, Physical Aspect of the Atomic Theory. 



it is hardly surprising that the law connecting the over- 

 tones (so to speak) with the fundamental in each spectral 

 series is of a type that is not met with in ordinary 

 dynamics. A probably easier problem, as yet unravelled, 

 is the mode of genesis of banded spectra ; here the law 

 connecting the frequencies of the series of lines which 

 constitute a band is of type not unfamiliar,* but the 

 known conditions in which these relations occur seem 

 rather complex for an ordinary molecule. The facts that 

 increased density of the surrounding medium does not 

 shift the bands, and that the Zeeman magnetic effect is 

 absent in bands, are very pertinent to the problem : it 

 has been thought that these facts are somehow correlated : 

 it may well be that the former indicates close concentra- 

 tion of the steady aethereal vibration into the interatomic 

 spaces in the molecule.f 



But though the problem of the dynamics of the 

 spectrum has not hitherto yielded much under the 

 accumulation of knowledge, the primal property of the 

 spectrum as an analytical agent remains unimpaired. It 

 is still true that the occurrence of a definite line marks 

 the presence of a definite substance. With variation of 

 the conditions of excitation that substance may or may 

 not emit the line in question : but wherever the line is 

 seen the inference backward is still valid, though high 

 dispersion may of course be requisite to distinguish it 

 from closely adjacent lines. The inference as to the 

 presence of a substance is easy ; but it would be far more 

 difficult to establish its absence. 



Among the remarkable results of recent research in 

 this field are those of R. W. Wood on the fluorescent 

 spectrum of sodium vapour, and the conditions of its 



* Cf. "Ency. Brit.," ed. ix. supplement (1900), Art. 'Radiation.' 

 t Cf. Astrophyskal Journal, 1907, p. 120. 



