460 
PROF. J. W. NICHOLSON AND DR. T. E. MERTON ON THE 
radiating particles, in accordance with Doppler’s principle. Measurements of the 
width of such lines are carried out with the interferometer, the measurement 
consisting of a determination of the limiting order of interference at which fringes 
can be seen. This limiting order of interference is given by the equation 
N = K (M/T) 1/a where N is the limiting order of interference, M the mass of the 
luminous particle in terms of the hydrogen atom, T the absolute temperature and K 
a constant. This equation is derived from a consideration of the Doppler effect 
produced by a distribution of the velocities of the radiating particles in accordance 
with Maxwell’s law, and its experimental verification by Buisson and Fabry 'snows 
not only that under the conditions specified the widths of the lines are completely 
accounted for, hut also that the distribution of intensity in the lines is given by the 
well-known probability law. Under these conditions it is further shown that the 
limiting order of interference is constant for all lines and all series of the same 
element. Thus the same value of N is found for the helium and the parhelium 
series, and in the same manner for the Balmer series and lines of the secondary 
spectrum of hydrogen. 
With any departure from the specified conditions of low pressure and excitation by 
uncondensed electric discharge the law breaks down, and the lines broaden in an 
apparently anomalous manner. The characteristics which lead Ilyin; erg to adopt 
the terms diffuse and sharp series appear, the higher members of a series undergoing 
the greater degree of broadening. Matters are complicated by the fact that the 
broadening is in many cases unsymmetrical, and in addition we must take into 
account the fact pointed out by Boyds, # that different members of the same series 
may be unsymmetrically broadened in opposite directions. Thus ill the case of the 
first subordinate “ triplet ” series of barium, the members consisting of triplet and 
satellites at AA 5819-5424 are all unsymmetrically broadened towards the red, whilst 
the members of the succeeding triplet (and satellites) and AA 4493-4264 are unsym¬ 
metrically broadened towards the violet. Boyds shows that similar phenomena occur 
in the spectra of calcium and strontium, and points out the importance of the 
phenomena in relation to the pressure shifts of the lines in question. There can be no 
doubt of the intimate relation between the direction of the asymmetry and the 
pressure shift, since it has been shown by St. John and Ware, Fabry and Buisson, f 
and by Gale and Adams | that iron lines which broaden unsymmetrically towards 
the red are displaced by pressure towards the red, whilst lines which are unsym¬ 
metrically broadened towards the violet are displaced by pressure towards the violet. 
It is well known that broadening of spectrum lines can be produced either by an 
increase of the pressure of the luminous gas or by the use of highly condensed 
discharges as a means of excitation, and although the two conditions are different 
* ‘ Astrophys. Journ.,’ 41, p. 154, 1915. 
t ‘Astrophys. Journ.,’ 36, p. 14, 1912; 31, p. Ill, 1910. 
| ‘Astrophys. Journ.,’ 37, p. 391, 1913. 
