1899.] on Magnetic Perturbations of the Spectral Lines. 163 



frequently happened that small and obscure effects, obtained after 

 much labour and difficulty, have led to results of the highest import- 

 ance, while very pronounced and striking effects which have forced 

 themselves on the attention of the observer have proved comparatively 

 barren. It was by a determined effort of this kind, founded on a 

 correct appreciation of the importance of small outstanding differ- 

 ences — so small as to be despised or passed over by all other 

 observers — that Lord Eayleigh discovered a new gas in our atmo- 

 sphere, added argon to our list of elements, and initiated the attack 

 which led to the brilliant capture by Prof. Eamsay of several new 

 terrestrial substances. 



Viewed from this standpoint I hope I am to some extent justified 

 in occupying your attention this evening with the consideration of 

 the action of magnetism on light, for although the effect produced is 

 small and not easy to observe, yet it is likely to prove an important 

 instrument of research in the study of matter, and it is not inappro- 

 priate that a public account of what has been already achieved should 

 be given in this Institution, in which the enquiry was first begun by 

 Faraday, and in which his spirit still lives. 



IT. P.l 



