1016 Prof. R. W. Wood and M. G. Ribaud on the 



Re- establishment of light perpendicular to lines of force. 



Cotton has shown that if a sodium flame is, placed in 

 a magnetic field, between crossed nicols, the light traversing 

 it in a direction perpendicular to the lines of force is re- 

 established in the vicinity of the D lines, if the planes of the 

 nicols are at 45° with the lines of force. The same 

 phenomenon has been observed by one of us in the case of 

 the non-luminous vapour of metallic sodium. 



Voigt and Wiechert have studied the spectrum composition 

 of this re-established light under high dispersion, and have 

 given a theoretical treatment based upon the marked 

 Zeeman effect shown by the lines. 



The same experiment has been tried by Cotton with iodine- 

 vapour, and by one of us with bromine vapour, with a more 

 powerful field, with negative results. 



We have, however, obtained a very marked restitution 

 of light, employing the iodine bulb used in the previous 

 experiments, which was unfortunately too feeble to permit of 

 its examination with the echelon. With the crater of the 

 carbon arc as a source the restored light was quite brilliant,. 

 and of the same emerald-green colour as in the lonoitudina 

 experiment. 



Destruction of Fluorescence by Magnetic Field. 



Steubing * has observed a diminution of the intensity of 

 iodine vapour fluorescence in a magnetic field amounting to 

 as much as 30 per cent. We have repeated the experiment 

 with a much more powerful field and have succeeded in 

 almost completely abolishing the fluorescence. 



The effect of the magnetic field in reducing the intensity 

 of the fluorescence becomes more marked as the vapour- 

 pressure of the iodine is diminished. -™ _ 



We found that the form of tube best 

 suited to the study of the phenomenon was 

 as shown in fig. 5. 



A thin-walled tube as free as possible 

 from striae, and 8 mm. external diameter, 

 is blown out at one end into a small bulb. 

 The tube is highly exhausted and sealed, 

 a crystal of iodine having been introduced 

 before drawing down the tube to a capillary. 

 In exhausting iodine bulbs it is important 

 1o cool a portion of the tube leading to the pump with 

 solid C0 2 , to prevent the vapour of iodine from entering 



* Ber. Deut. phys. Ges. 1913. 



