﻿Magnetic Rotation Spectra of Sodium Vapour. 507 



drawing of one of the groups of bands at wave-length 5200 

 (tig. 2). The drawing was made from a print with the aid 



Fijr. 2. 



of a hand-magnifier, and the peculiarities shown are found 

 throughout the entire spectrum. 



The bright lines are sharp, and quite as narrow as the iron 

 lines of the comparison spectrum. We must remember, 

 however, that the slit was not very narrow, 0*2 mm. perhaps, 

 and it is quite possible that a further contraction would not 

 decrease the width of the fluorescent lines. Each bright line 

 is in general accompanied by two lateral wings, which ter- 

 minate quite sharply the narrow spaces between, every two 

 adjacent wings appearing as narrow dark lines. These wings 

 do not in general appear with strictly monochromatic stimu- 

 lation. Jn the work of last year, when studying the re- 

 markable changes which occur in the spectrum stimulated 

 with the fairly homogeneous light furnished by the mono- 

 chromatic illuminator with very narrow slits (see previous 

 paper), I observed that as the wave-length of the light was 

 very gradually altered, the fluorescent lines appeared with 

 wings first on one side and then on the other, the change in 

 the appearance of the line reminding one of a flag riving 

 first on one side of the mast and then on the other. With 

 the strictly monochromatic illumination obtained with the 

 isolated metallic arc lines, the fluorescent lines are usually 

 devoid of wings, thongh in some instances the wings are 

 found, and sometimes the wings appear without the lines. 

 These circumstances appear to indicate that the wings are 

 due to the stimulation of the electron by frequencies slightly 

 greater and slightly less than its own natural frequency. 



I have not yet had time to repeat last year's experiments 

 with the monochromatic illuminator, and plan to make a 

 further study of the changes which accompany very gradual 

 changes in the wave-length of the exciting light. The 

 observations are very difficult and uncertain, as the light 

 furnished by the monochromatic illuminator is not very 

 bright when its slits are made as narrow as possible, and the 

 fluorescence spectrum can only be observed by carefully 



