MAGNETO-OPTICS. The first relation between magnetism and light was discovered by Faraday,[1] who proved that the plane of polarization of a ray of light was rotated when the ray travelled through certain substances parallel to the lines of magnetic force. This power of rotating the plane of polarization in a magnetic field has been shown to be possessed by all refracting substances, whether they are in the solid, liquid or gaseous state. The rotation by gases was established independently by H. Becquerel,[2] and Kundt and Röntgen,[3] while Kundt[4] found that films of the magnetic metals, iron, cobalt, nickel, thin enough to be transparent, produced enormous rotations, these being in iron and cobalt magnetized to saturation at the rate of 200,000° per cm. of thickness, and in nickel about 89,000°. The direction of rotation is not the same in all bodies. If we call the rotation positive when it is related to the direction of the magnetic force, like rotation and translation in a right-handed screw, or, what is equivalent, when it is in the direction of the electric currents which would produce a magnetic field in the same direction as that which produces the rotation, then most substances produce positive rotation. Among those that produce negative rotation are ferrous and ferric salts, ferricyanide of potassium, the salts of lanthanum, cerium and didymium, and chloride of titanium.[5] The magnetic metals iron, nickel, cobalt, the salts of nickel and cobalt, and oxygen (the most magnetic gas) produce positive rotation. For slightly magnetizable substances the amount of rotation in a space PQ is proportional to the difference between the magnetic potential at P and Q; or if [theta] is the rotation in PQ, [Omega]_P, [Omega]_Q, the magnetic potential at P and Q, then [theta] = R([Omega]_P - [Omega]_Q), where R is a constant, called Verdet's constant, which depends upon the refracting substance, the wave length of the light, and the temperature. The following are the values of R (when the rotation is expressed in circular measure) for the D line and a temperature of 18° C.:-- Substance. R × 10^5. Observer. Carbon bisulphide / 1.222 Lord Rayleigh[6] and Köpsel.[7] \ 1.225 Rodger and Watson.[8] Water / .377 Arons.[9] \ .3808 Rodger and Watson.[8] Alcohol .330 Du Bois.[10] Ether .315 Du Bois.[10] Oxygen (at 1 atmosphere) .000179 Kundt and Röntgen (_loc. cit._) Faraday's heavy glass 1.738