A.— MATHEMATICAL AND PHYSICAL SCIENCES. 43 



electric furnace so effectively employed at Mount Wilson by A. S. King, 

 have brought the spectrum of an element under more gradual control, 

 so that a valuable aid in the classification of the lines of a complicated 

 spectrum is provided by the order of their appearance as the temperature 

 is raised. From other experiments by A. S. King,*- it would appear that 

 an arc with a current of the order of 1,000 amperes, at 100 volts, has the 

 advantage, from the present point of view, that it exhibits with relatively 

 greater intensity the high temperature lines which are weak in ordinary 

 arcs ; moreover, the fact that the widenings and reversals of lines, which 

 are a prominent feature of this kind of arc, are of the same type in the same 

 multiplet, promises to be of distinct value in the analysis of complicated 

 spectra. 



A valuable method of producing or modifying the spectra of certain 

 gases by admixture with helium was introduced by Merton" in his 

 experiments on the band spectrum of hydrogen. Among the most 

 remarkable results obtained by this method was the production, almost 

 free from enhanced lines, of spectra attributed to atoms of neutral carbon 

 and neutral nitrogen when a trace of one of these elements was present 

 in helium at a pressure of several centimetres.** Little was previously 

 known of these spectra, apparently because carbon compounds and 

 nitrogen, in ordinary vacuum-tube observations, mainly break down 

 directly from the molecular state (giving band spectra) to ionised atoms, 

 the spectra of which were already well known. Other observations have 

 shown that while the presence of helium is not essential for the develop- 

 ment of these spectra, the mixture method has the advantage of bringing 

 out the lines with greater completeness and intensity. 



A further important application of this method to oxygen has been 

 made by McLennan and Shrum,** resulting in the appearance of a 

 previously unrecorded oxygen line at 5577 -35, apparently agreeing in 

 position with the well-known green line in the spectrum of the aurora. 

 Here again, the line could be obtained, but with much lower intensity, in 

 the absence of helium. A continuation of the experiments may be 

 expected to reveal other members of the singlet system, which probably 

 forms part of the spectrum of oxygen in company with the already known 

 triplet and quintet systems. 



Among numerous other methods of controlling the spectra of certain 

 elements which have been successfully adopted, it will suffice to mention 

 the electrodeless ' ring discharge ' of Sir J. J. Thomson, in which different 

 spectra of the same element may be excited by varying the voltage and 

 the pressure in the bulb or tube. Zeeman and Dik,'"^for example, in this 

 way obtained the second spectrum of potassium entirely free from 

 arc lines, and, as expected from the displacement law, found it to be of 

 the same general character as that of argon. McLennan^' similarly found 

 it possible to obtain spectra of potassium apparently corresponding to the 



*2 Astrophys. Jour., vol. 62, p. 238 (1925). 

 *3 Roy. Soc. Proc, A. vol. 96, p. 382 (1920). 



** Merton and Johnson, Boy. Soc. Proc, A. vol. 103, p. 383 (1923). Merton and 

 Pilley, Roy. Soc. Proc, A, vol. 107, p. 411 (1925). 

 *5 Roy. Soc Proc, A, vol. 108, p. 501 (1925). 

 *« Proc. Kon. Acad. Amst., 1922, 1925. 

 *' Roy. Soc. Proc, A, vol. 100, p. 182 (1921). 



