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



It is especially noteworthy that many, in fact most, of the 

 strongest absorption-lines are not represented at all in the 

 rotation spectrum. This fact is of fundamental importance, 

 for it indicates that the absorbing mechanism is different in 

 the two cases. Just wherein the difference lies cannot as yet 

 be definitely stated. On either hypothesis as to the cause of 

 the magnetic rotation, the effect will diminish as the mass 

 of the electron increases. The lines of the rotatory spectrum 

 may correspond to the negative electrons of small mass ; the 

 other lines in the absorption spectrum which show no rotatory 

 power may be due to positive electrons, or at all events to 

 some form of vibrator having a much larger mass than 

 the negative electrons. The fact that the bright lines 

 in the fluorescence spectrum coincide with the lines of the 

 rotatory spectrum appears in accord with this assumption ; 

 for the agitation of the electrons by the light vibra- 

 tions will be greatest for those having the smallest mass. 

 It will doubtless be possible to speak more definitely 

 in regard to this point, after the fluorescence spectrum 

 has been more carefully studied. The matter will be 

 discussed further in the paper on the fluorescence of the 

 vapour. 



The rotation spectrum in the red and orange region is 

 more brilliant than the green-blue one, but it was found 

 impracticable to photograph it with the 14-foot concave 

 grating. Excellent photographs of it were secured with a 

 small concave grating of about a metre radius. One of these, 

 together with the absorption spectrum, is reproduced in fig. 3, 

 Plate V. As the red lines could be seen without difficulty 

 with the 14-foot grating, a method was devised for mapping 

 them which may prove useful in other lines of work. A 

 brass rod, on which a short brass tube had been fitted, was 

 fastened into the grating camera just below the groove which 

 held the plate. To the movable tube was soldered a second 

 tube of 2 mm. bore, into w r bich was fitted a short piece of 

 brass rod. This rod carried a needle point. The whole 

 arrangement is shown in fig. 6. A plate of thin glass was 

 carefully smoked over a gas-flame, and then wiped clean with 

 the exception of a narrow strip 4 mms. wide along the centre. 

 This plate was inserted in the camera in place of the photo- 

 graphic plate, and the spectrum brought into such a position 

 that the lines crossed the smoked strip. The positions of the 

 lines could be accurately recorded by bringing the needle- 

 point in contact with the glass exactly on the centre of a line 



