MAGNETISM AND LIGHT 325 



and OL will be equally rotated, and the analysing Nicol must 

 be turned through the same angle to secure equality of the two 

 halves of the field once more. The angle is read on a divided 

 circle with which the analyser is provided. 



The result which Lord Rayleigh obtained for the Verdet 

 constant for sodium light for bisulphide of carbon at 18 

 measured in minutes per unit difference of potential is 



0-04202. 



Hodger and Watson* made determinations of the Verdet con- 

 stants for sodium light in carbon bisulphide and in water over a 

 range from C. to 40 C. for the former, and over a range from 

 C. to 100 C. for the latter. Their values for the constants 

 were: for carbon bisulphide 0*04347-737rf/10 7 , and for water 

 0-01311 -4 //10 7 -4 t*/\0*. Their formula for bisulphide of carbon 

 would give the value of the constant at 18 as 0*4214, which only 

 differs from Lord Rayleigh's value by 1 in 350. 



Constants for many other materials were determined in terms of 

 that of carbon bisulphide by H. Becquerel, and these may be turned 

 into absolute values by using the constant just given for that liquid. 

 The following values are extracted from a table of Becquerers results 

 u r iven by Gordon, I.e. : carbon bisulphide, 1; water, 0'308 ; 

 benzene, 0'636 ; rock-salt, 0*843 ; crown glass, 0*481; bichloride 

 of titanium, - 0*358. f 



Rotation by gases. In 1879 both H. Becquerel and Kundt 

 and BdotgenJ succeeded in measuring the rotation by gases. 

 Hv.jiu-!vl pa-x-d a ray several times to and fro along a copper 

 tube with gla>s ends surrounded by a coil. Kundt and Rontgen 

 aKo ti>ed a copper tube with glass ends surrounded by a coil. 

 Into this the gas to be dealt with was compressed to 250 atmo- 

 spheres. The glass ends were so seriously strained by the internal 

 jirr^iire that they became doubly refracting. It was therefore 

 nece>sarv to put the polariser and analyser within the tube, each 

 being fixed to the tube at the end at which it was placed. The 

 polariser end was clamped in a fixed position. The analyser end 

 c.mld be twisted round, subjecting the tube to torsion, and the 

 angle of twist could be measured. It was found that the rotation 

 was proportional to the density of the gas, so that it could be 

 reduced to normal density at C. and 760 mm. In all cases 



i mined the rotation was positive. The following rotations at 

 N'lT in terms of that of bisulphide of carbon were obtained : 



CS 2 O N II C0 2 



1 0-000109 0000127 0*000132 0-000232 



* /V//7. 7/v///., A., 180, Part II. p. 621 (1895). 



f A nti'iiber of values by different workers will be found in Wiukelmann's 

 IfnndhtK'h, vol. v. 



* Som<- in/count of both researches is given by Gordon, I.e. 



