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VIII. ON THE LAWS OF REFLEXION FROM CRYSTAL- 

 LIZED SURFACES. 



[From the Philosophical Magazine, VOL. vin., 1835.] 



To SIR DAVID BREWSTER. 



DEAR SIR I have great pleasure in sending you an account 

 of the laws by which I conceive that the vibrations of light are 

 regulated when a ray is reflected and refracted at the separating 

 surface of two media ; especially as the only guide which I had, 

 in my inquiry after these laws, was your Paper on the action of 

 crystallized surfaces upon light, published in the Philosophical 

 Transactions for the year 1819. The observation which I found 

 there, that the polarizing angle was the same fdr a given plane 

 of incidence, " whether the obtuse angle of the rhomb [of Iceland 

 spar] was nearest or furthest from the eye, or whether it was to 

 the right or left hand of the observer," disappointed me at first, 

 being contrary to what I had anticipated from principles ana- 

 logous to those which had been employed by Fresnel in the 

 problem of reflexion from ordinary media. I then sought other 

 principles, and the observation is now a result of theory. 



Assuming, as a basis for calculation, that Fresnel's law of 

 double refraction is rigorously true, I have been obliged to 

 make an essential change in his physical ideas. Conceive an 

 ellipsoid whose semiaxes are parallel to the three principal direc- 

 tions of the crystal, and equal respectively to its three principal 

 indices of refraction, and let a central section of the ellipsoid 

 be made by a plane parallel to the plane of a wave passing 



