NOTES TO BOOK IX. ~>1 I 



Dr. Faraday's recent experiments have shown that 

 the magnetic polarity is directly connected with that op- 

 tical polarity by which the plane of polarization is affected. 

 When the lines of magnetic force pass through certain 

 transparent bodies, they communicate to them a certain 

 kind of circular polarizing power ; yet different from the 

 circular polarizing power of quartz, and certain fluids 

 mentioned pp. 388 and 506. 



Perhaps I may be allowed to refer to this discovery 

 as a further illustration of the views I have offered in 

 the Philosophy of the Inductive Sciences respecting the 

 Connexion of Co-existant Polarities. (B. v. Chap, ii.) 



(NA.) p. 484. I believe Fresnel did not deduce the 

 phenomenon from the mathematical formula, without the 

 previous suggestion of experiment. He observed ap- 

 pearances which implied a difference of retardation in the 

 two differently polarized rays at total reflection ; as Sir 

 D. Brewster observed in reflection of metals phenomena 

 having a like character. The general fact being observed, 

 Fresnel used the theory to discover the law of this retar- 

 dation, and to determine a construction in which, one 

 ray being a quarter of an undulation retarded more than 

 the other, circular polarization would be produced. And 

 this anticipation was verified by the construction of his 

 rhomb. 



As a still more curious verification of this law, 

 another of FresneFs experiments may be mentioned. He 

 found the proper angles for a circularly polarizing glass 

 rhomb on the supposition that there were four internal 

 reflections instead of two ; two of the four taking place 

 when the surface of the glass was dry, and two when 

 it was wet. The rhomb was made; and when all the 



