THE SIGNIFICANCE OF SPECTROSCOPY 231 



a striking fact that, for all series proceeding from electrically 

 neutral atoms, N has approximately the same value — 109675. 

 It appears to be almost a constant quantity for all elements, 

 and is called the " series constant." This number, and the 

 limits of the series, are, however, the only quantities in the 

 general series formula that can be said to be known with any 

 confidence, and the formula is, perhaps, best given as 



__ . N 



where f{m) is some function of m as yet undetermined. Another 

 fact of great importance is that the limit of a series is itself a 

 term of another series in the same spectrum. We can there- 

 fore generalise our conception of series somewhat by saying 

 that, associated with a given spectrum, there are a number 

 of terms, and that the wave-numbers of the lines in the spectrum 

 are differences of pairs of these terms. When the greater term 

 of the pair is common to a number of lines, the lines form a 

 series, with that term as limit. 



The detection of all this regularity, in what appears at 

 first to be a meaningless confusion of lines, has a profound 

 bearing on the problem of the final structure of matter. For 

 the spectrum is an atomic phenomenon, and its order and 

 arrangement are the outward and visible expression of the 

 order and arrangement of the fundamental material units. 

 The spectrum is, in fact, a housetop, on which are proclaimed 

 the workings of the secret chambers of the atom. Unfor- 

 tunately, the proclamation is made in a language which we 

 have hardly yet begun to understand — which seems, indeed, 

 to express ideas for which the tongue of classical mechanics 

 has no equivalent. The spectrum analyst, with an ear for 

 style, can identify the herald, but he has no faculty for trans- 

 lation. Nevertheless, of late years, a few fitful shreds of mean- 

 ing have been discovered to us. A word here and a word there, 

 put together and pondered over, have conjured up to the 

 mind's eye the picture of an atom, beautiful in its simplicity, 

 but, withal, so opposed in its workings to the tried and trusted 

 laws of mechanics that, were it not for its almost uncanny 

 harmony with the facts of the spectrum, and suggestions of 

 similar violations of mechanical laws coming from other branches 

 of physics, it would be laid to rest at once among the many 

 fond delusions with which the too speculative have sought to 

 fathom the mysteries of the universe. We are led to conceive 

 of an atom, containing a central nucleus possessed of a net 

 positive electrical charge, round which revolve a number of 

 electrons whose total charge just balances that on the nucleus. 

 The spatial dimensions of the nucleus are negligible, but its 



