CHAP, ix.] ELECTRICITY AND MAGNETISM. 353 



this is the number of degrees through which a polarised 

 ray of green light (of thallium flame) will be rotated by 

 traversing unit difference of potential. For rays of 

 different colours the rotation is not equal, but varies 

 (very nearly) inversely as the square of the wave-length ; 

 the rotation by bisulphide of carbon of red, green, and 

 blue light (rays "C," "E," and "G"), being respectively 

 as *6o, i-oo, and 1-65. H. Becquerel, who gave this law, 

 also found that for substances of similar nature the rota- 

 tion depends on the refractive index, but in rather a com- 

 plicated relation, being proportional to ^ (ju? - i) ; 

 where ^ is the refractive index. 



Gases also rotate the plane of polarisation of light in 

 a magnetic field with varying amounts ; coal-gas and 

 carbonic acid being more powerful than air or hydrogen ; 

 oxygen and ozone being negative. The rotation is in all 

 cases very slight, and varies for any gas in proportion to 

 the density that is to the quantity of gas traversed. H. 

 Becquerel has lately shown that the plane of the natural 

 polarisation of the sky does not coincide with the plane 

 of the sun, but is rotated by the influence of the earth's 

 magnetism through an angle which, however, only reached 

 59' of arc at a maximum on the magnetic meridian. 



388. Dr. Kerr showed in 1877 that a ray of polarised 

 light is also rotated when reflected at the surface of a 

 magnet or electromagnet. When the light is reflected 

 at a pole the plane of polarisation is turned in a direction 

 contrary to that in which the magnetising current flows. 

 If the light is reflected at a point on the side of the 

 magnet it is found that when the plane of polarisation is 

 parallel to the plane of incidence the rotation is in the 

 same direction as that of the magnetising current ; but 

 that, when the plane of polarisation is perpendicular to 

 the plane of incidence, the rotation is in the same 

 direction as that of the magnetising current only when 

 the incidence exceeds 75, being in the opposite direc- 

 tion at lesser angles of incidence. 

 2 A 



