and the Rotatory Dispersion Formula. 413 



a steel tube containing sodium vapour in an atmosphere of 

 hvdrogen, under which condition the absorption spectrum 

 lias a most beautiful shaded appearance, being made up o£ 

 fluted bands, each band containing a very large number of 

 fine lines. If the hydrogen is removed, and the sodium vapour 

 formed in vacuo, the fluted appearance disappears almost 

 entirely, although the individual lines remain. Many new 

 lines appear, however, which are broader and more distinct 

 than the others, and these lines appear to coincide with the 

 bright lines in the rotation spectrum. This point will be 

 more fullv investigated in the future, the work on the absorp- 

 tion spectrum not having been completed at the present 

 time. 



Returning now to the measurements obtained in the vicinity 

 of the D lines, we will discuss them in their bearing on the 

 theory of magnetic rotation. 



The rotation has been measured over a considerable range 

 of wave-lengths in the case of very dense vapours, by the 

 method described in the previous paper. In brief this method 

 consisted in rotating one of the nicols through various angles 

 and measuring the position of the two dark bands which move, 

 the one up, the other down the spectrum. The dispersion of 

 the 1.4-foot grating was too great for this part of the work, 

 and a large plane grating was accordingly combined with a 

 pair of telescope objectives of six-foot focal length. A very 

 brilliant spectrum was obtained in this way, and the dark 

 bands were not so broad but that the cross hairs of the 

 micrometer could be set on the centre of the. band with a 

 fair degree of accuracy. Readings of the rotation were 

 obtained throughout, the region comprised between \=5840, 

 and \ = 5932. The dark bands cannot be pursued with 

 accuracy to greater distances from the D lines, on account of 

 their increased breadth. It will be understood from what 

 has been said in the previous paper, that these dark bands 

 retreat from the D lines when one of the nicols (originally 

 parallel) is rotated towards the position of extinction. The 

 bands enter the red and green regions when the nicols are 

 very near the crossed position, under which circumstances 

 the bright lines appear, and completely obliterate all trace of 

 the dark bands, which by this time have become very broad. 

 The rotation for the wave-lengths corresponding to the dark 

 bands, is the angle through which one of the nicols has been 

 rotated, measured from the crossed position. 



The values found with vapours of various density are 

 recorded in the folio wing table. The tables are designated by 

 the magnitude of the rotation midway between the D lines. 



