Stoney — Analysis of the Spectrum of Sodium. 205 



K is the " inverse-iuave-length,'^ i. e. IO7A. It is the number 

 of wave-lengths-in-air which occupy one-tenth of a milli- 

 metre. 



T is the periodic time of the waves of a ray of light, expressed 

 in micro-] ots. Hence T = /nX, in which fx is the refractive 

 index of air for the ray of wave-length X. 



iVis the oscillation-frequencj/ in each jot of time, i.e. the number 

 of the oscillations of the waves of a ray in each jot. 

 It = kIjx. 



Of these quantities iVis obviously the one which is best adapted 

 to theoretical investigation. It is, however, A that is observed. 

 From this k can be accurately deduced, but iV = /c//i cannot be 

 accurately obtained till we know the value of fx for different parts 

 of the spectrum. We may, however, in a theoretical investigation, 

 use, instead of iV", any quantity proportional to it, e. g. n^N, where 

 ^1 has the value assigned to it above. We shall call this n. It is 

 the oscillation- frequency in each " air-jot " of time. Accordingly 



n is the oscillation-frequency in each air-jot of time. It = niN 



= K.fXi/lU. 



Now Xetteler's observations on the dispersion of air, though 

 not sufficient for the general determination of fx, are enough to 

 satisfy us that fi does not anywhere differ more than a very little 

 from fxi, its mean value. And accordingly, in comparing our 

 results with observation, we shall regard ju,//^ as unity, and 

 treat k (the quantity furnished by observation) as if it were 

 identical with n (the quantity required by theory]. With this we 

 must be content until adequate determinations of /x for the various 

 parts of the spectrum shall have been made. 



Method of Analysis. 



The spectrum of sodium, so far as it has been explored, consists 

 of 14 pairs of lines, and of 8 others which have not yet been seen 

 to be double. Professor Rydberg and Professors Eayser and 

 Runge independently discovered that all the observed lines — with 

 one remarkable exception — lie in three definite series, somewhat 

 similar to that which Dr. Huggins had found in the Spectrum of 

 Hydrogen ; and they have worked out empirical formulae which 



