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



in all cases, the rotations being found by multiplying them 

 by the constants a and b as determined for a particular 

 density, and adding the two products. 



On fig. 3 will be found two curves, A and B, for centre 

 rotations of 50° and 360°. The values of S calculated for 

 the curve A, will be seen to fall in most cases exactly upon 

 the curve of observed values. 



Fig. 

 density 



Fig. 

 vapour 



4 shows that the agreement is also good for a 

 oivina' a rotation of 450° at the centre. 



5 represents the rotatory dispersion of 

 investigated by visual methods. The 



Kir. 5. 



the densest 

 bright band 





5850 5860 



5870 



5&QO 



5890 59CJ 



between the D lines had disappeared completely owing to 

 absorption. This corresponds to a rotation at the centre of 

 about 1500, since 1440 is the largest rotation actually 

 observed. The observed values in this case were obtained 

 with the plane grating and long focus telescope-lenses of five 

 inches aperture. The positions of the broad, dark bands were 

 measured with a micrometer eyepiece for different settings of 

 the nicol, and readings were also taken on the narrow bright 

 and dark bands lying between the broad ones and the D 

 lines, whenever they were distinct enough. In this case, the 

 calculated values lie on a curve slightly lower than the 

 observed curve. This may be due to the fact that the value 

 of X for which the rotation is equal to that of the nicol may 

 not lie exactly at the centre of the band. On the whole, the 

 formula may be said to represent the rotatory dispersion, 



