ON THE' SPECTRUM OF HYDROGEN. 
371 
that a much more striking change can be brought about by the admixture of Helium. 
It was found that in the presence of Helium some of the lines were greatly enhanced 
and that a number of new lines appeared ; another class of lines were apparently 
unaffected, whilst a third class showed a marked reduction in intensity.* 
There are two other methods by which the lines have been classified. Dufour 
(‘Ann. Chim. et Phys.’ (9), 361, 1906; ‘ Journ. de Phvs.’ (4), 8, p. 258, 1909) has 
investigated the Zeeman effect for the secondary spectrum, and has found that a large 
number of the lines are not affected in the magnetic field. This classification of the 
lines has been shown by Fulcher ( loc. cit.) to be related to the results which he obtained 
by varying the velocities of the exciting cathode rays, and to the regularities which he 
found in the spectrum. The Stark effect, the resolution of the lines into components 
in an electric field, has been studied by Takamine and Yoshida (‘ Mem. Coll, of Sci. 
Kyoto,’ 2, p. 321, 1917), by Nitta (ibid., 2, p. 349, 1917) and by Takamine and Kokubu 
{ibid., 3, p. 271, 1919), who have found that the effect is exhibited by 54 lines in the 
spectrum. Such investigations and those of Dufour (loc. cit.), relating to the Zeeman 
effect, are necessarily restricted to the stronger lines of the spectrum, and their value 
as a means of classification is greatly increased when they can be correlated to changes 
in the spectrum of a kind which permit of observation for all the lines. 
The wave-lengths of the lines in the secondary spectrum have been measured by 
Hasselberg (‘ Mem. Acad.,’ St. Petersburg (7), 30, No. 7, 1882 ; ibid. (7), 31, No. 14, 
1883 ; ‘ Phil. Mag.’ (5), 17, p. 329, 1882), Ames (‘ Phil. Mag.,’ 30, p. 33, 1890), Frost 
(‘ Astrophys. Journ.,’ 16, p. 100, 1902), Watson (' Hoy. Soc. Proc.,’ A, 82, p. 189, 1909), 
Porlezza (‘ Atti Accad. Lincei,’ 20 (2), p. 178, 1911), Porlezza and Norzi (ibid., 20 (1), 
p. 822, 1911), and Croze (‘Ann. de Phys.’ (9), 1, 48, 1914), but the results obtained by 
these investigators differ widely in their estimates of the relative intensities of the lines, 
which is greatly dependent on the particular conditions under which the spectrum is 
produced, and it would appear also that the tables are by no means complete, more 
especially in the yellow green regions of the spectrum, for which it has only recently 
been possible to obtain photographic plates of a sufficiently high degree of sensibility 
for recording lines of low intensity with a moderately high dispersion. 
There has been much difference of opinion as to whether the secondary spectrum is 
to be attributed to the Hydrogen atom or to the molecule. To the theoretical physicist 
this is a question of vital importance, for there appears to be little prospect of explaining 
the origin of the spectrum as due to the Hydrogen atom on the views which are at present 
accepted with regard to its structure. Evidence on this question has been sought in 
investigations of the Doppler effect in positive rays by Stark (‘ Astrophys. Journ.,’ 25, 
pp. 23 and 170, 1907), Wilsar (‘Ann. der Phys.,’ 37, p. 1251, 1912) and Fulcher 
(‘Astrophys. Journ.,’ 35, p. 101, 1912), and more recently by Thomson (‘ Phil. Mag.’ 
* Experiments on the effect of Argon on the secondary spectrum are now in progress. It would appear 
that if the presence of Argon gives rise to any changes similar to those produced by the presence of Helium, 
they are at any rate very much less conspicuous. 
3 F 2 
