On the Spectra of Helium and Hydrogen. 805 



respectively, turn out to differ but little from one another. 

 In fact, 



P Na =9'42140 



R Li = 9*53399, 



so that, in round figures, Rli : Rtf a =l'01. Any attempt to 

 find the physico-chemical meaning of this approximate 

 equality, either with or without the help of such formulae 

 as (49) or (49 a), would not answer the purposes of the 

 present paper. 



We have treated with some detail the properties of what 

 has been provisionally called the simple series, taken by 

 itself. The coexistence of two or more such series, which 

 offers some interesting points, will occupy our attention in 

 the next paper, in which also the few hereabove ignored 

 lower members of spectral series will be taken into consider- 

 ation. Meanwhile I should like to point out only that, as 

 far as terms involving 1/w; 2 are concerned, there is no 

 mutual disturbance of the lines of two coexistent simple 

 series ; their mutual " action," to speak figuratively, is chiefly 

 determined by a term of the fourth order in 1/iii, and the 

 corresponding law is of a very simple nature. 



October, 1915. 



Corrigenda. — In the Second Paper on this subject, Phil. 

 Mag., vol. xxx. July 1915, p. 178, line 5, instead of 

 tan77=+co, read tan?? = 0; in equation (25b) replace the 

 exponent —1 by — 2 and io 2 by to*, and consequently, in 

 line 22, (a/\) 2 by (a/\)\ 



LXXXV. A Comparison of the Positive Rays with the Spec- 

 trum of the Positive Column in a Mixture of Helium and 

 Hydrogen. By Harold Smith, B.A., M.Sc, late Royal 

 (1851) Exhibition Scholar*. 



Introduction. 



WHEN an electric discharge is made to pass through a 

 mixture of gases at a low pressure in a Geissler tube, 

 the relative intensities of the spectra are by no means pro- 

 portional to the relative quantities of the gases present. For 

 instance, when the uncondensed spark-discharge is made to 

 pass through air at a low pressure, nitrogen yields its spectrum 

 very brightly, while no trace of any of the oxygen spectra is 



* Communicated by Sir J. J. Thomson, O.M., F.R.S. 



