The Magneto- Optics of Sodium Vapow 



409 



accurately measured. Very remarkable effects have been 

 observed when the vapour is illuminated with a very narrow 

 band of approximately homogeneous light, the lines in the 

 fluorescent spectrum changing their position and appearing 

 to dance about in the liveliest manner with the slightest 

 change in the wave-length of the exciting light. The motion 

 is of course only an illusion, lines disappearing and others 

 reappearing, like the sparks of a spinthariscope. Stokes's 

 law is violated in a most flagrant manner, bright lines coming- 

 out on both sides of the excited region. The behaviour of 

 the spectrum indicates that we are dealing with a number of 

 groups of electrons, each group containing a large number 

 of vibrators. The excitation of one of the vibrators sets 

 the whole group agoing, but does not start disturbances in 

 the other groups. These and other remarkable phenomena 

 will be fully discussed in the following Number of this Journal; 

 they are mentioned here only on account of the apparent 

 relation between magnetic rotation and the fluorescence. 



In the earlier experiments referred to above, the sodium 

 w r as heated in a tube of hard glass, the ends of which were 

 closed with thin plate glass. The tubes lasted but a short 

 time, owing to the chemical action of the metal, and in fact 

 usually cracked on a second heating. In the present work a 

 tube of thin steel has been used, the ends of which projected 

 from the helices of the magnet. It was found that the field- 

 strength within the steel tube did not differ greatly from 

 that obtained when glass tubes were used. 



The arrangement of the apparatus is shown in fig. 1. A 

 piece of thin seamless steel tubing of such a diameter as to 



Fiff. 1. 



E 



&k» 



Z3 



slip easily through the hollow cores of the electromagnet, 

 from which the conical pole-pieces have been removed, is 

 procured. A short piece of small brass tubing is brazed into 

 one end, through which the tube is exhausted. It has been 

 found that a good vacuum is essential, all trace of the rotation 

 disappearing in hydrogen or nitrogen at atmospheric pressure. 

 In the earlier work this fact was not known, and the tubes 



