[ 408 ] 



XLIX. The Magneto- Optics of Sodium Vapour and the 

 Rotatory Dispersion Formula. By R. W. Wood, Pro- 

 fessor of Experimental Physics, Johns Hopkins University*. 



[Plate V.] 



IT has been shown in a previous paper f that the vapour 

 oil metallic sodium is an ideal substance for investigating 

 the effect of a strong absorption-band on the magnetic 

 rotation of the plane o£ polarization. The preliminary work 

 was not very satisfactory, however, as the method employed 

 did not admit of very accurate determinations of the wave- 

 lengths, and the verification of the rotatory dispersion 

 formula could only be considered as approximate. Improve- 

 ments in the methods of observation and design of the 

 apparatus have been accompanied by an increase in accuracy, 

 which could scarcely have been hoped for, as will be seen by 

 the following comparisons. 



In the preliminary work no readings of the rotation for 

 wave-lengths between the sodium lines could be obtained, 

 while in the present case accurate readings have been made 

 for as many as nine different values of X between T) 1 and D 2 . 

 Rotations as great as 1440° (four complete revolutions) have 

 actually been observed, and this with a 10 cm. column of not 

 very dense vapour, in a field of only 2000 C.G.S. units. 

 This was for a wave-length midway between Dj and D 2 . In 

 the earlier work it was only with the greatest difficulty that 

 the bright lines which appeared in the region of the red and 

 green channeled absorption spectrum could be seen. They 

 have now been photographed to the number of about 160 

 with the 14-foot concave grating. Comparatively few of the 

 absorption-lines show any trace of magnetic rotation, scarcely 

 one per cent. ; and the fact seems to have been established 

 that these lines coincide with the bright lines of the fluores- 

 cent spectrum of the vapour, which has been found to be by 

 no means the exact complement of the absorption spectrum 

 as was at first supposed. 



In the present paper the magneto-optics of the vapour for 

 light travelling along the lines of force will be discussed. 

 The double-refraction of the vapour, which occurs when the 

 rays are perpendicular to the field, has also been studied, and 

 will be reported in a subsequent paper. An exhaustive study 

 of the fluorescence of the vapour has been made, and 

 the lines of the spectrum to the number of several hundred 



* Communicated by the Physical Society : read June 30, 1905. 

 f Wood and Springsteen, " The Magnetic Rotation of Sodium Vapour,'' 

 Phys, Review, July 1905. 



