﻿Magnetic Rotation Spectra of Sodium Vapour. 509 



which are capable of exciting fluorescence have wave-lengths 

 5086, 4800, and 4678. Colour screens and the Fuess mono- 

 chromatic illuminator, as well as the thin crystals of chlorate 

 of potash (described in the Phil. Mag. for June), were tried; 

 also a block of quartz, cut perpendicular to the axis, placed 

 between two nicols. The arrangement which gave the best 

 results, and appeared to be accompanied with the least loss of 

 light, is the one figured on PI. XIII., fig. 2. One vertical tube 

 of the lamp is used as the source, the light from which, after 

 collimation, passes through a large bisulpbide-of-carbon prism, 

 and is focussed upon the retort by an achromatic telescope 

 objective with an aperture of 12 cms. and a focal length of 

 2 metres. The dispersion of the prism was sufficient to 

 separate completely the monochromatic images of the lamp, 

 any one of which could be thrown into the aperture, the light 

 passing by the edge of the 90° prism by means of which the 

 fluorescent light was reflected through a lens and thence upon 

 the slit of the spectroscope. 



The sources of light which have thus far been successfully 

 employed in stimulating the fluorescence of the vapour are 

 the following : quartz arc-lamps containing cadmium, zinc, 

 and thallium ; ordinary arcs between lead, silver, bismuth, 

 and copper electrodes ; lithium, sodium, and barium arcs, and 

 vacuum-tubes containing helium and hydrogen. 



Unfortunately the quartz lamps are very expensive, and 

 become almost useless after a run of about thirty hours, the 

 surface becoming granular and an opaque black deposit 

 forming on the inner walls. As exposures of eight hours 

 are sometimes necessary, it will be seen that lamps at thirty 

 dollars apiece, with an average life of thirty hours, make the 

 investigation an expensive one. 



The photographs of the fluorescent spectra obtained with 

 monochromatic stimulation are reproduced on PI. XII. 



Alter each exposure the D lines were recorded on the plate, 

 so that the different spectra could be brought into coincidence 

 for purposes of comparison : the D lines will be found at the 

 extreme right in each spectrum. The photographs have 

 been reproduced as negatives, and the point or points coin- 

 ciding with the wave-lengths of the stimulating light are 

 indicated by arrows. A large-scale drawing or chart of the 

 most interesting of these spectra, together with others too 

 faint for reproduction, has been made on cross-section paper, 

 the points of excitation being indicated by arrows, as in the 

 photographs. (Folding Plate XV.) 



Drawings of the complex spectrum and the magnetic- 

 rotation spectrum made from the photographs obtained with 



