Rotation of Sodium Vapour at the D Lines. 149 



values obtained by visual observations made with vapour of 

 the same density, and were found to agree to within 0*1 A.E. 

 for wave-lengths outside of the D lines. 



Fisr. 1. 



02 01 00 99 98 97 96 95 _ 94 93 92 91 90 89 88 87 86 Q 5 84- 

 The results of the observations are recorded graphically in 

 fig. 1, as a better idea of how the rotation changes with 

 varying vapour density can be obtained in this way. 



