Light Vibrations in a Magnetic Field. 31 



source of light, which has been recently investigated by Messrs. 

 Humphreys and Mohler. 



The photographs referred to in this communication were taken 

 with a spark source of light placed between the pole pieces of a good 

 electromagnet of the ordinary U-shape, for the use of which I am 

 indebted to the kindness of the Bight Rev. Monsignor Molloy. With 

 this magnet I was able to obtain a field of about 25,000 C.Gr.S. units, 

 and this separated the side lines of the triplets in the violet line 4678 

 of cadmium to a distance of 0*56 mm. apart. The same separation 

 takes place in the triplet given by the violet line 4680 of zinc, so 

 that in the case of these lines we may infer in round oumbers that a 

 magnetic field of 20,000 C.G.S. units strength produces a difference 

 of wave-length of one Angstrom unit between the side lines of the 

 triplet, i.e., a separation equal to one-sixth the difference of wave- 

 length between the two D lines of sodium. For iron the separation 

 is about half this amount in the case of several lines. 



"Note on the Connection between the Faraday Rotation of 

 Plane of Polarisation and the Zeeman Change of Frequency 

 of Light Vibrations in a Magnetic Field." By GEO. FRAS. 

 FITZGERALD, F.R.S., F.T.C.D. Received March 2, 1898. 



(Being Notes of a Contribution to the Discussion of Mr. Preston's Paper above, 

 read January 20, 1898.) 



The rotation of the plane of polarisation of light in a magnetic 

 field is due to the velocity of propagation of light circularly 

 polarised in one direction being different from that of light circu- 

 larly polarised in the opposite direction. The Zeeman effect is due 

 to a difference in the frequency of vibration of these circularly 

 polarised waves. What is required is to connect the frequency of 

 vibration with the velocity of propagation.* 



All modern theories of dispersion connect these two quantities. 

 The velocity of propagation of light in transparent media is now 

 universally considered to be determined in part by what may be 



* From an abstract of a paper by M. Becquerel, in the ' Comptes Rendus ' of 

 last year, I understand him to view the Faraday effect as due to a carrying round 

 of the light vibrations by matter rotations. This is quite in accordance with the 

 view sometimes held as to refraction, namely, that it is due to the waves being pro- 

 pagated through the molecules more slowly than through the ether. The dynamical 

 theory of these views is difficult on account of the smallness of the molecules 

 in comparison with the length of the waves. M. Becquerel's view, as I under- 

 stand it, makes the Faraday effect depend on a change of frequency of rotation of 

 the waves in matter rather than on their velocity of propagation, and is conse- 

 quently quite at variance both with the commonly received theory and with the one 

 put forward in this note. 



