1899.] on Magnetic Perturbations of the Spectral Lines. 153 



either by removing the air altogether, and so causing the beam to 

 traverse a vacuum, or we may replace the air by some dense trans- 

 parent substance such as glass or water. Under these new conditions 

 we still fail to detect auy influence of the magnetic field on a beam of 

 ordinary light. This negative result might arise from the field of 

 force being too weak to produce an observable effect, or it might be 

 that the effect (if any effect really does exist) may be of such a cha- 

 racter that it is impossible to detect it with ordinary light. In common 

 light the vibrations take place indifferently in all directions around 

 the ray, and follow no law or order as to their type. They possess 

 no permanent relation to any direction around the ray, so that if the 

 magnetic action should happen to be a twisting of the vibrations 

 round the ray, it will be impossible to detect this twist in the case of 

 ordinary light. 



As a matter of fact it is a twist of this kind that actually happens, 

 and this is probably what Faraday anticipated. In order to detect 

 it, therefore, it is necessary to employ a beam of light in which the 

 vibrations are restricted to a single plane passing through the ray. 

 Such light is said to be plane-polarised, and may be obtained by 

 transmitting common light through a doubly refracting crystal. 

 Faraday found that when a beam of this plane-polarised light is 

 passed through the magnetic field, in the direction of the lines of 

 force, a distinct effect takes place, and that the effect is a twisting of 

 the plane of polarisation of the light vibrations as they pass through 

 the magnetic field — or, to be more precise, as the light passes through 

 the matter occupying the field. 



This is the Faraday effect. Its magnitude depends on the strength 

 of the field and upon the nature of the matter, through which the 

 light passes in that field. This latter is an important fact that 

 should not be lost sight of in reasoning upon the nature of this effect. 

 The presence of matter in the field appears to be necessary. The 

 effect is not observed in a vacuum, but becomes greater as the field 

 becomes filled with matter of greater density. It is, therefore, not a 

 direct action of the magnetic field on the light vibrations, but rather 

 an indirect action exerted through the intervention of the matter 

 which occupies the magnetic field. 



This action, as we have said, is a rotation of the plane of polarisa- 

 tion of the beam of light, and it arises from the circumstance that, in 

 passing through the magnetic field, vibrations which take place from 

 right to left do not travel forward with the same velocity as those 

 which take place from left to right. There is no change in the 

 periods of the vibrations : it is essentially a change of velocity of 

 propagation that occurs. If we examine the transmitted light with a 

 spectroscope, we find that the wave-lengths are unaltered, but that 

 the amount of rotation of the plane of polarisation is different for 

 waves of different lengths. The law which governs the effect is that 

 the rotation of the plane of polarisation varies inversely as the square 

 of the wave-length of the light employed. 



