A.— .^LATHEAIATICAL AND PHYSICAL SCIENCES. 25 



by Rydberg more than thirty years ago. Groups of this type were furtiicr 

 investigated by Popow and by Gotze/^ and their real structure was 

 deduced from observations of Zeeman effects. It then appeared that 

 such a group was derived from combinations of P terms of the regular 

 series with another set of P terms, or of ordinary D terms with a second 

 set of D terms. The additional types of terms, which are usually distin- 

 guished as ' anomalous terms ' and designated by P', D' . . . , have the 

 same inner quantum numbers and show the same Zeeman effects as 

 ordinary terms of corresponding types ; but in their combinations v«'ith 

 the regular terms they mostly follow the rule AZ;=0, giving the combina- 

 tions PP', DD'. . . . Among themselves, however, the anomalous terms 

 combine in accordance with the ordinary selection rule, Ak=±l, giving 

 such combinations as P'D', D'F'. . . . Such terms are not restricted to 

 the spectra of the alkaline earths, but have been found to be of very 

 general occurrence in all but the simplest spectra. 



Russell and Saunders'^ have since found other terms in the alkaline 

 earth spectra which have the same combining properties as ordinary 

 terms of corresponding types, but are anomalous in the sense that thev 

 do not form part of the regular term sequences ; they have distinguished 

 such terms by the convenient symbols, P", D". . . . 



Further examples from the spectrum of ionised oxygen will conveniently 

 indicate the selection rules applicable to quartet D'P' and D'D combina- 

 tions (Table III). 



Multiplets formed from terms of higher multiplicities thanthose shown in 

 the foregoing examples are built up on the same general plan, but naturally 

 include a greater number of lines. The combination rules for anomalous 

 terms have thus been more or less systematised, but exceptions to the 

 selection rule for azimuthal quantum numbers occasionally occur also 

 among regular terms. Thus in the arc spectrum of sodium there is a 

 series of faint lines represented by 2-P — m'-^P (Lenard's series) and another 

 by 2'-P— wrF, corresponding to Ak=0 and AZ;=2 respectively. Such 

 combination lines are generally faint, except when produced under the 

 influence of a strong electric field, in which case they may be very 

 numerous. It should be observed, however, that the series I'S — »rD, 

 for which Ak — 2, appears in the absorption spectrum of potassium under 

 conditions in which no electric field would appear to be present.'"^ Excep- 

 tions to the inner quantum combination rule are very rare under ordinary 

 conditions of observation, but such lines are especially liable to be excited 

 in strong magnetic fields. Paschen and Back, for example, were able to 

 excite the complete T — ^D ' triplets ' of Ca, Zn and Cd, the usual six 

 lines in each being increased to nine. Among other observations which 

 could be mentioned, extensive experiments on ' forbidden lines ' in 

 zinc, cadmium and mercury have been made by a number of Japanese 

 physicists. ^^ 



1* Ann. d. Phys., vol. 66, p. 285 (1921). 



15 Astrophys. Jour., vol. 61, p. 38 (1925). 



i« Datta, Hoy. Soc. Proc, A, vol. 99, p. 69 (1921) ; Foote, Mohler and Meggers, 

 Phil. Mag., vol. 43, p. 659 (1922). 



1' E.g., Fukuda, Kuyama and Uchida, Sr. Pap. Xo. 56, In'^i. of Phiis. and Chem. 

 Res. (1926). 



