ON THE ACTION OF MAGNETISM ON LIGHT. 335 



The Action of Magnetism on Light ; with a critical correlation of 

 the varioxLS theories of Light-propagation. By Joseph Larmor, 

 M.A., D.Sc, F.R.S., Fellow of St. John's College, Cambridge. 



[A communication ordered by the General Committee to be printed 

 among the Reports.] 



Part I. — Magnetic Action on Light. 



Discovery of Magnetic Rotation. 



1. The redaction of light and heat, and of electrical phenomena, to a 

 common cause has been a cardinal subject of physical speculation from 

 the earliest times. More recently Oersted ' fully persuaded himself, on 

 somewhat wider knowledge of fact, ' that beat and light are the result of 

 the electric conflict,' and saw in his great discovery of the gyratory 

 action of an electric current on a magnet the explanation of the phenomena 

 classed under the name of polarisation of light. But it was reserved for 

 Faraday to make the first eifective entrance into this domain of know- 

 ledge. 



2. After failure in 1834 to discover any direct relation between light 

 and static electi'ification, and after repeated attempts in other directions, 

 he at length discovered" the fact that when plane polarised light is passed 

 through a transparent body along the direction of lines of magnetic force, 

 its plane of polarisation undergoes rotation by a specific amount character- 

 istic of the medium traversed. He thus succeeded ' in magnetising and 

 electrifying a ray of light, and in illuminating a magnetic line of force.' 

 After observing that when the ray is oblique to the lines of magnetic 

 force it is the component of the force in the direction of the ray which 

 appears to be effective in producing the rotation (a law which has since 

 been exactly verified by Verdet and more recently by Dn Bois), he proceeds 

 to inquire into the condition of the active medium with the following 

 results : 



' 2171. I cannot as yet find that the heavy glass when in this state, 

 i.e., with magnetic lines of force passing through it, exhibits any increased 

 degree, or has any specific magneto-inductive action of the recognised 

 kind. I have placed it in large quantities, and in different positions, 

 between magnets and magnetic needles, having at the time very delicate 

 methods of appreciating any difierence between it and air, but can find 

 none. 



'2172. Using water, alcohol, mercury, and other fluids contained in 

 very large delicate thermometer-shaped vessels, I could not discover that 

 any difference in volume occurred when the magnetic curves passed 

 through them.' 



The rotation was in general right-handed with respect to the magnetic 

 force; and in the case of (2165) ' bodies which have a rotative power of 

 their own, as is the case with oil of turpentine, sugar, tartaric acid, tar- 

 trates, etc., the effect of the magnetic foi'ce is to add to, or subtract from, 

 their specific force, according as the natural rotation and that induced by 



' Hans Christian Oersted, Exjjerimenta circa Effect urn cojiflictiis Electrioi inAcum 

 Magneticam, Hafniae, 1820. 



- M. Faraday, Experimental Researclies, 19th series j Phil. Trans., 1845. 



