CHAP, ix.] ELECTRICITY AND MAGNETISM. 351 



in which it would itself polarise light. Faraday caused 

 a polarised ray to pass through a piece of a certain 

 " heavy glass " (consisting chiefly of borate of lead), 

 lying in a powerful magnetic field, between the poles 

 of a large electromagnet, through the coils of which a 

 current could be sent at pleasure. The emerging ray 

 traversed a second Nicol prism which had been turned 

 round until all the light was extinguished. In this posi- 

 tion its own plane of symmetry was at right angles to the 

 plane of polarisation of the ray. On completing the cir- 

 cuit, light was at once seen through the analysing Nicol 

 prism, proving that the ray had been twisted round into 

 a new position, in which its plane of polarisation was no 

 longer at right angles to the plane of symmetry of the 

 analyser. But if the analysing Nicol prism was itself 

 turned round, a new position could be found (at right 

 angles to the plane of polarisation of the ray) at which 

 the light was once more extinguished. The direction of 

 the magneto-optic rotation of the plane of polarisation is 

 the same (for diamagnetic media) as that in which the 

 current flows which produces the magnetism. Verdet, 

 who repeated Faraday's experiments, using powerful 

 electromagnets of the form shown in Fig. 127, dis- 

 covered the important law that, with a given material, 

 the amount of rotation is proportional to the strength of 

 the magnetic force H. In case the rays do not pass 

 straight along the direction of the lines of force (which 

 is the direction of maximum effect), the amount of rota- 

 tion is proportional to the cosine of the angle ft between 

 the direction of the ray and the lines of force. It is also 

 proportional to the length I of the material through 

 which the rays pass. These laws are combined in the 

 equation for the rotation d ; 



& = w- H cos /, 



where w is a coefficient which represents the specific 

 magnetic rotatory power of the given substance, and is 

 known as " Verde? s constant." Now, H cos /3 is the 



