﻿Magnetic Rotation Spectra of Sodium Vapour. 505 



The spectrum stimulated by white light I have named the 

 " complex fluorescent spectrum," for it has been found that 

 it is a superposition of a number of simpler spectra, any one 

 of which can be independently excited by suitably controlling 

 the wave-length of the stimulating light. Indications of 

 something of this sort were found last year, and were 

 described in the preliminary paper. An insufficient number 

 of photographs were obtained, however, at the close of the 

 university year, to make anything like a complete analysis 

 of the complex spectrum possible. 



During the past winter and spring a careful study has 

 been made of the relations existing between the complex 

 fluorescent spectrum, the absorption spectrum, and the bright- 

 line rotation spectrum described in the earlier paper. The 

 fluorescent spectrum has at last been photographed with the 

 twelve-foot concave grating, enabling a study to be made of 

 its more minute structure. 



Some very remarkable effects have been observed with 

 monochromatic stimulations obtained by the isolation of 

 certain lines from metallic arcs, which yield comparatively 

 simple fluorescent spectra made up of widely separated sharp 

 lines, placed in many instances at nearly equal intervals 

 along a normal spectrum. A given series of lines can be 

 brought out by stimulating with light of any wave-length 

 corresponding to that of some line in the series, but when 

 the stimulations occur at certain points, some of the lines may 

 be absent, gaps appearing in the series. The most con- 

 spicuous example is the case of stimulation with the cadmium 

 line 480, which will be considered in detail presently. It 

 will be remembered that certain lines are absent in each series, 

 in the magnetic spectrum. 



The apparatus employed in the experiments was essentially 

 the same as that used in the earlier work. It consisted of a 

 seamless tube of thin steel three inches in diameter and thirty 

 inches long, with a steel retort at its centre in which a large 

 amount of sodium could be stored. The retort was made by 

 fitting two circular disks of steel to a short piece of tubing, 

 just large enough to slip snugly into the larger tube. The 

 circular ends of the retort were provided with oval apertures, 

 as shown in PI. XIII., tig. 1. The retort was half filled with 

 sodium, the molten metal being poured in through one of 

 the apertures. It was then introduced into the tube and 

 pushed down to the centre, after which the plate-glass ends 

 were cemented on, as shown in the figure. This arrangement 

 prevented the rapid diffusion of the vapour, and enabled a 



