22 SECTIONAL ADDRESSES. 



structure of complex spectra was furnished by the investigations of 

 Catalan, who was then working at the Imperial College. Catalan first 

 made an extended study of the spectrum of manganese," in which series 

 of triplets of somewhat peculiar character had already been partially 

 disentangled by Kayser and Runge, and discovered that, while the 

 principal and sharp series consisted of simple triplets, the members of the 

 diffuse series each consisted of nine lines in place of the six which had up 

 to that time been considered to characterise a diffuse ' triplet.' It followed 

 that the D terms had five values, as against three values in calcium and 

 other elements of the second group. Besides lines forming regular series, 

 Catalan also identified several complex groups which he called ' multi- 

 plets,' one of which included as many as fourteen lines. In each multiplet 

 the lines were of similar character and generally of the same class in King's 

 temperature classification, and the lines could be arranged on a simple 

 plan to show the regularity of their distribution. 



The essential feature of Catalan's work was the discovery that in the 

 arc and spark spectra of manganese, and in the arc spectrum of chromium, 

 there were terms of greater complexity than the triple terms which had 

 previously been recognised. It was this discovery that opened a way to 

 the analysis of complex spectra in general. It has been pursued with 

 amazing success by Catalan himself, Walters, Laporte, Meggers, Sommer, 

 and others, and the main features of the structure of many spectra as 

 complicated as that of iron have been revealed. 



It is not necessary to go into all the intricate details of the spectra, 

 because the general results can now be very simply summarised in conse- 

 quence of the theoretical developments which have gone hand in hand with 

 the experimental investigations. Bohr and Sommerfeld had already 

 established certain ' selection rules ' for the combination of the terms of 

 the simpler spectra on a quantum number basis, and, immediately following 

 the work of Catalan, Sommerfeld showed that the scheme of ' inner quantum 

 numbers ' which he had devised for the simpler spectra could be extended 

 so as to fit the observations empirically. As other spectra came to be 

 disentangled, an assignment of quantum numbers which appears to be 

 adapted to all spectra was completed by Lande.''' 



In accordance with the work of Bohr, Sommerfeld and Land^, a spectral 

 term may be represented by four quantum numbers, written in the form 

 nlj or 'n^j. Here n is the principal quantum number, increasing by 

 unity for successive terms of the same sequence"* ; k is the azimuthal 



quantum number and has the values 1, 2, 3, 4, 5 for the term 



tyjaes S, P, D, F, G, ; j is the inner quantum number, having one 



8 Phil. Tram., A, vol. 223, p. 127 (1922). 



» Zeit.f. Phys., vol. 15, p. 189 (1923). 



''■" For descriptive purposes the initial values assigned to n in the respective 

 sequences of terms are of no great importance. In the Ritz-Paschen system of 

 numeration, the first terms are IS, 2P, 3D, 4F ; in the Rydberg system adopted in 

 Fowler's ' Report on Series ' they are IS, IP, 21) (occasionally ID), 3F. The latter 

 system has the advantage that the term numbers are usually the integral parts of the 

 ' effective quantum numbers,' i.e. the values of 7t* in the expression R/(n*)- for the 

 terms. Paschen's numeration, however, has been extensively used. In theoretical 

 investigations the values assigned to n are definitely associated with the corresponding 

 values in the hydrogen spectrum. 



