1906.] Professor Zeeman on Magneto -Optics. 289 



WEEKLY EVENINCx MEETING, 



Friday, March ;J0, 1906. 



His Grace the Duke of Northumberland, K.G. D.C.L. F.R.S., 

 President, in the Chair. 



Professor P. Zeeman, Hon. Mem. R.I., of the University, 

 Amsterdam. 



Recent Progress in Magneto-Optics. 



It is my intention this evening to give you a general review of the 

 experimental researches which have occupied me during the last few 

 years. They all refer to the relation between magnetism and light, 

 a relation the first and fundamental example of which was discovered 

 in this very Institution, by Faraday, in 1845. 



Surely every physicist should feel inspired by the idea of having 

 the privilege to address an audience in the same lecture room, where 

 so often some of Nature's deeper mysteries were revealed ; and I feel 

 the uplifting force of this inspiration all the stronger, as my own 

 work for many years has been so closely connected with one of Faraday's 

 discoveries. Faraday discovered that the plane in which the vibra- 

 tions of light take place, rotates whenever a ray of light is propagated 

 parallel to the magnetic lines of force through some substances, such 

 as Faraday's own heavy glass ; this fact we now indicate by the name 

 of the magnetic rotation of the plane of polarisation. The discovery 

 of this fact opened the chapter of magneto-optics. 



Faraday's mind again and again returned to the relation between 

 magnetism and light, and incessantly he sought for closer and more 

 intimate connections ; in one experiment in March 1862 (which is 

 said to have been his last) he tried to observe a change in the 

 spectrum of a flame, when acted on by a magnet. The entry in 

 Faraday's note book, preserved in this Institution with pious care, 

 concludes with the words, " not the slightest effect on the polarised 

 or unpolarised ray was ol3served." As we now know, the means of 

 Faraday's time were not powerful enough to observe the effect sought 

 for. Yaiious physicists since Faraday have sought in the siime 

 direction ; some have recorded their negative results, others have 

 not, for most physicists have an almost invincible dislike for the 

 publication of negative results, though a collection of such unsuccess- 

 ful attempts, if precisely stated, would be most interesting, and should 

 afterwards prove very valuable. 



Vol. XYIII. (No. 100) u 



