﻿Spectrum of Iodine Vapour. 333 



emission spectrum are shown in register with the fluorescence 

 spectrum. The dark bands of the latter appear to be pushed 

 further towards the red end than in the magnetic-rotation 

 spectrum, while in the temperature spectrum they are still 

 further displaced. The magnetic spectrum shows much more 

 structure than either of the other two. 



Some visual observations were made of the fluorescence of 

 the vapour when stimulated with fairly homogeneous light 

 furnished by the monochromatic illuminator. The colour 

 of the fluorescent light changed in a marked manner when 

 the wave-length of the exciting radiation was altered. It 

 was bluish-green when stimulated with blue light, yellow- 

 green with green stimulation, and orange with yellow sti mix- 

 tion. It was only with great difficulty that the spectrum 

 could be observed under these conditions, and about all that 

 can be said at present is that the fluorescence spectrum covers 

 the exciting region and extends to a short distance on the 

 red side of it. 



The Magnetic-Rotation Spectrum. 



A glass bulb, 3 cms. in diameter, containing a small flake 

 of iodine, was exhausted, sealed, and mounted in a small 

 section of brass tubing placed between the poles of a powerful 

 magnet. Two small holes in the brass tube permitted the 

 passage of light, along the lines of force, through the bulb. 

 Parallel light from an arc was passed in succession through a 

 Nicol prism, the bulb, and a second nicol. When the latter 

 was set at extinction, the crater of the arc could be barely 

 seen, and appeared of a faint grey colour. On exciting the 

 magnet, an intense blaze of emerald-green light at once 

 appeared, wholly different in colour from that of the fluorescent 

 light. 



The spectrum of this light was photographed without diffi- 

 culty with the 12-foot concave grating, with an exposure of 

 eight hours. The absorption spectrum and the iron arc- 

 are shown on the same Plate, together with a scale of wave- 

 lengths. These spectra are reproduced on Plate IV. (lower 

 figure), the upper absorption spectrum taken with dense, the 

 lower with rare vapour. At certain points the magnetic 

 spectrum appears to be the complement of the absorption 

 spectrum, the bright lines of the former coinciding with the 

 dark lines of the latter. This is especially noticeable at wave- 

 length 5125. Below this point the magnetic spectrum shows 

 more structure than the absorption, though both were taken 

 with the same width of slit. The absorption-lines in this 



