THE ZEEMAN EFFECT AND DISPERSION. 



A GOOD many years ago, as the years of scientific dis- 

 covery in the nineteenth century are counted, Fara- 

 day observed an action of magnetism on Light. He 

 observed that when a beam of plane polarised light is trans- 

 mitted through a transparent solid along the direction of 

 a magnetic field in which the solid is placed, the plane of 

 polarisation is rotated. This action of matter when mag- 

 netised upon light may be described as due to its trans- 

 mitting a beam of right-handed circularly polarised light 

 at a different rate from a left-handed beam. This difference 

 of rate of transmission is an extremely small part of the 

 whole rate. The rotation of the plane of polarisation is a 

 very delicate test for a difference between the rate of 

 propagation of the right- and of the left-handed beam. For 

 example, we may suppose a substance such that, after 

 passing through one centimeter of it in a certain magnetic 

 field, the plane of polarisation is rotated through a right 

 angle. It would require a very strong magnetic field to 

 produce this effect with the most active transparent body 

 known. Yet the change in the rate of propagation involved 

 would be very small even in this case. A rotation of the 

 plane of polarisation through a right angle would require 

 one of the circularly polarised beams to gain one-half wave 

 length over the other in going through a centimeter. As 

 there are some 17,000 wave lengths in a centimeter of air 

 and in the dense media that are magnetically active about 

 30,000, it is evident that a gain of a half wave length per 

 centimeter means a difference of rate of propagation of only 

 one 60,000th part of the velocity. In most cases the rota- 

 tion per centimeter is very very much less than this, so that in 

 these cases the phenomenon is due to a very much smaller 

 action between the magnetised medium and light. 



It is to be observed that this action is an indirect one. 

 It is not an action of magnetic force directly on light. It 

 is an action of magnetic force on matter, and of the 



