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



made on the dark lines between as well. A little practice 

 was necessary before a complete series o£ readings could be 

 taken without error, but consistent results were finally 

 obtained. The method adopted was as follows. A current of 

 given strength was turned into the iron spiral, and as the 

 vapour-density increased the alternations of brightness and 

 darkness at the centre were counted. No measurements were 

 commenced until a steady state was reached, which was 

 indicated by a fixed condition at the centre. The positions 

 of the bright lines and dark spaces were then measured, the 

 readings being taken down by an assistant. If at the end of 

 the series any change was found to have occurred at the 

 centre, it was rejected. As soon as the series was completed, 

 the heating current was shut off, and the tube allowed to cool. 

 During this process the alternations of brightness at the 

 centre were counted, and if the number of changes differed 

 from that originally recorded the series was thrown 

 out. The rapid changes which occur as the tube cools, 

 especially an unjacketed tube, are most interesting ; the 

 centre of the system changes from bright to dark with great 

 rapidity. The largest number of complete alternations ob- 

 served was eight, corresponding to a rotation of 1440 degrees, 

 one alternation [i. e. from bright to bright) corresponding to 

 180. The whole thing lasts but a few seconds, the bright 

 band '• winking ,y almost as fast as one can count. We can, as 

 it were, see the plane of polarization actually turning around 

 and around, for four complete revolutions. Beyond this 

 point the absorption becomes too strong to admit of further 

 observations between the D lines, but readings may be taken 

 above and below them in the spectrum, with vapours of very 

 great density. 



When the vapour acquires a considerable density, a most 

 magnificent bright-line spectrum appears in the red and green- 

 blue region. Each bright line corresponds to a dark line in 

 the absorption spectrum, but only a small percentage of the 

 dark lines appear to exercise a rotatory power. Some of the 

 strongest ( absorption-lines are absolutely unrepresented in 

 the magnetic rotation spectrum, which indicates that there is 

 some radical difference in the absorbing mechanism. 



Much time has been spent in a further photographic study 

 of the channeled absorption spectrum, °a piece of work which 

 was commenced three years ago by the writer in collaboration 

 with Mr. J. H. Moore. It has since been found that the 

 presence of hydrogen, nitrogen, or carbonic acid modifies the 

 appearance of the absorption spectrum. The photographs 

 previously obtained were made by passing the light through 



