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XI. The Spark Spectrum of Nickel under Moderate Pressures. 
By E. G. Bilham, A.R.C.Sc., B.Sc., D.I.C., Research Student at the Imperial College 
of Science and Technology . 
Communicated by A. Fowler, F.R.S. 
Eeceived July 9, 1914. 
[Plates 6-7.] 
1. Introductory. 
The work of Humphreys. Duffield, Gale and Adams, Rossi and others has shown 
that an increase of pressure round a source of light results in a general small 
displacement of the spectrum lines in the direction of longer wave-length, together 
with striking changes in the character of the lines themselves. Thus all lines have 
been found to broaden to a greater or less extent, either symmetrically or 
unsymmetrically, and many lines are reversed. In the present investigation the effect 
of pressures up to ten atmospheres above the normal atmospheric pressure, on the spark 
spectrum of nickel, has been studied with a view to obtaining accurate measurements 
of the displacements over that range of pressure which has been found to possess the 
greatest interest from an astrophysical standpoint. The chief problem which has to 
be considered in this connection is that of the pressure of the solar reversing layer. 
It is well known that wave-lengths of spectrum lines, determined from measurements 
in the solar absorption spectrum, exhibit small discrepancies when compared with the 
same lines derived from terrestrial sources. The solar wave-lengths are in general 
slightly greater than the terrestrial. 
Ascribing the solar displacements to a difference of pressure, Humphreys, Mohler, 
and Jewell # deduced a mean pressure of seven to eight atmospheres in the reversing 
layer. Later determinations of wave-lengths by Fap»RY and BuissonI using the 
interferometric method have indicated a somewhat lower figure, namely five to six 
atmospheres. 
* ‘Astrophysical Journal,’ III. (1896), p. 138. 
t ‘Astrophysical Journal,’ XXXI. (1910), p. 97. 
Published separately, October 9, 1914. 
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