410 Prof. Wood on the Magneto- Optics of Sodium Vapour 



were exhausted and then sealed off from the pump. The 

 hydrogen liberated from the sodium must have raised the 

 pressure to at least 15 cms. in all of these experiments. In 

 the light of what is now known, it is surprising that any 

 results at all were obtained under these conditions. 



A lump of sodium the size of a walnut is melted in an iron 

 crucible, and poured out into a Y-shaped trough made of 

 thin sheet iron. As soon as the bar is solid it is placed in the 

 iron tube, one end of which has been previously closed with 

 a small piece of plate-glass cemented on with sealing-wax. 

 The tube is introduced into the magnet, the sodium bar 

 pushed to a position midway between the helices, and the 

 other end closed with a piece of glass in a similar manner. 

 The ends of the tube should be coated while hot with sealing- 

 wax before the introduction of the sodium. One has then 

 only to wave a Bun sen flame over them and press on the 

 piece of glass, previously heated; the sealing-wax should 

 come into optical contact with the glass to insure an air-tight 

 joint. The tube is now connected with an air-pump which 

 will produce a vacuum of a millimetre or two. If the air- 

 pump leaks, it is a good plan to place a glass stopcock 

 between the pump and tube to prevent the entrance of traces 

 of air after exhaustion. For purposes of demonstration it 

 is sufficient to heat the tube gradually with a Bunsen burner 

 turned down low. In the present work, however, where 

 constancy of temperature was essential, electrical heating was 

 invariably used ; the tube was wrapped with a thin sheet of 

 asbestos board, around which was wound a spiral of iron wire, 

 and the whole subsequently covered with an asbestos jacket. 



The light from an arc-lamp, made parallel by a lens, is 

 passed through a Eicol prism, the steel tube, and a second 

 nicol, after which it is brought to a focus by means of a 

 second lens upon the slit of a spectroscope. In the present 

 case, the instrument in question was a concave grating of 

 14-foot radius, the observations being made both visually and 

 by means of photography. 



We will now consider briefly the phenomena which are 

 presented when the sodium vapour is formed in the magnetic 

 field. The nicol s are crossed and the spectrum vanishes 

 completely. The magnet is now excited and the Bunsen 

 burner placed under the tube, the tip of the flame barely 

 touching it. In a few minutes we see two bright yellow lines 

 exactly in the position of the D lines. The light constituting 

 these lines comes, however, from the crater of the arc, as we 

 can easily see by intercepting the beam. The lines are in 

 reality double, though they appear single with low resolving 

 powers, while even with the 14-foot grating their duplicity 



