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X. On the Distribution of Intensity in Broadened Spectrum Lines. 
By J. W. Nicholson, M.A., D.Sc., Professor of Mathematics in the University 
of London , and T. Th Merton, D.Sc. ( Oxon ), Lecturer in Spectroscopy in the 
University of London, King's College. 
Communicated by Prof. A. Fowler, F.R.S. 
[Plate 2.] 
Received February 8,—Read March 9, 1916. 
Contents. 
Section Page. 
(I.) Introductory.459 
(II.) The Stark effect in relation to the broadening of spectrum lines.461 
(III.) Methods of measurement.462 
(IV.) Experimental.464 
(V.) Theoretical discussion.466 
(VI.) The effect of dispersion.472 
(VII.) The complex structure of H a when excited by condensed discharges.474 
(VIII.) General theory of a symmetrically compound line.476 
(IX.) Details of the components of H a .482 
(X.) The diffuse series of helium and lithium.485 
(XI.) Ultraspectroscopic analysis by means of the neutral wedge . . 487 
(XII.) Summary.487 
(I.) Introductory. 
It is a curious fact that although our knoAvledge of the structure of the finest 
spectrum lines may now be said to rest on a secure theoretical and experimental 
basis, little is known of the distribution of energy in the broadened spectrum lines 
which are produced under certain conditions of excitation, or the exact circumstances 
which control their broadening. The researches of Lord 11ayleigh, # Michelson,!' 
Buisson and Fabry,} and others have shown that in gases at low pressures, when 
excited by uncondensed electric discharges, the width of the spectrum lines emitted 
can be accounted for completely and satisfactorily by the translatory motion of the 
* ‘ Scientific Papers,’ vol. I., p. 183 ; ‘ Phil. Mag.,’ 29, p. 274, 1915. 
t ‘Phil. Mag.,’ 34, p. 280, 1892 ; ‘ Astrophys. Journ.,’ 3, p. 251, 1896. 
\ ‘Journ. de Physique,’ vol. 2, p. 442, 1912. 
VOL. CCXYI.—A 547. 3 R 
[Published, June 28, 1916. 
