152 Mr. A. B. Basset on an 



electrified or connected to the earth. Small spirit-lamp made 

 of metal and put on the positive conductor of the electrical 

 machine is extinguished by the repulsion, but not so easily 

 when it is put on the negative conductor. 



XVI. An Electromagnetic Theory of Quartz. 

 By A. B. Basset, M.A., F.R.S* 



1. f 1 1HE rotation of the plane of polarization of light by a 

 J_ magnetic field, although presenting several features 

 in common with the rotation of the plane of polarization by 

 syrup or quartz, is a phenomenon of a totally distinct cha- 

 racter, which depends upon the existence of an external 

 magnetic force, and disappears as soon as the magnetic force 

 is removed. A theory of magnetic rotation is given by Max- 

 well f, and some extensions have been made by Fitzgerald % ; 

 but it is, I think, pretty obvious that Maxwell's theory will 

 not apply to syrup or quartz; for the additional terms consist 

 of the products of the magnetic force and the differential 

 coefficients of the displacements, and are therefore terms of 

 the second order when the magnetic force is produced by 

 optical and not by external causes. 



On the other hand, rotations produced by syrup and quartz 

 appear to be phenomena of an essentially similar character, 

 and therefore a theory which explains one ought to explain 

 the other ; and the object of the present paper is to endeavour 

 to show how the electromagnetic theory may be extended so 

 as to account for these phenomena. 



2. Let' us conceive a medium which possesses three rectan- 

 gular planes of symmetry, and which produces rotatory pola- 

 rization when a plane wave of polarized light travels through 

 the medium parallel to any one of the axes of symmetry, and 

 elliptic polarization when the wave travels through in any 

 other direction, the values of the rotation per unit of length 

 being different for each of the three axes. The physical pro- 

 perties of the medium, so far as rotatory polarization is con- 

 cerned, will be specified by three constants, p v p 2 , p 3 , which 

 depend upon the amount of rotation produced when the direc- 

 tion of propagation is parallel to any of the three axes. In 

 an isotropic medium the three jo's will be equal, since the 

 rotation is independent of the direction of the wave ; whilst 

 in quartz two of the p's must be zero, since no rotation will 



* Communicated by the Author. 



t Electricity and Magnetism, vol. ii. chap. xxi. 



% Phil. Trans, part ii. 1880, p. 691. 



