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XX. — On the Influence of the Doubly Refracting Force of Calcareous Spar on 

 the Polarisation, the Intensity, and the Colour of the Light which it Reflects. 

 By Sir David Brewster, K.H., F.R.S. 



(Read 15th February 1864.) 



It was the opinion of Malus, and adopted by Arago, Biot, and other philoso- 

 phers, that the surfaces of regularly crystallised bodies acted upon light in the 

 very same manner as the surfaces of ordinary bodies, whether solid or fluid ; or, 

 in other words, that the reflecting forces extended beyond the limits of the forces 

 that produced double refraction and polarisation. Having been led to question 

 this opinion, I undertook an extensive series of experiments on crystalline re- 

 flexion, as exhibited in calcareous spar, a crystal peculiarly fitted for this pur- 

 pose, from its perfect transparency and great double refraction ; and I published 

 the results of these experiments in the Philosophical Transactions for 1819. 



In these experiments, the peculiar action of crystalline surfaces which I had 

 expected was placed beyond a doubt. The angle of complete polarisation on the 

 surface of the primitive rhomb was found to vary with the inclination of the 

 plane of reflexion to the principal section of the crystal ; and with different sur- 

 faces the variation of that angle depended on the inclination of the surface to the 

 axis of the rhomb. 



As the doubly refracting force thus modified the polarising angle produced by 

 superficial reflexion, it became probable that the polarised ray might suffer some 

 change from the same cause ; but, after the most careful observation, I could not 

 discover the slightest indication of such an effect. Conceiving, however, that 

 the change which I expected might be masked by the powerful action of the 

 ordinary reflecting force, I thought of reducing it till it was overpowered by the 

 doubly refracting force. With this view, I introduced a film of oil of cassia 

 between the larger surface of a rectangular prism of plate glass and the surface 

 of the spar, and by inclining the prism at a small angle, as in the Lithoscope, I 

 was able to separate the image of the sun, or any other light formed by the com- 

 mon surface of the prism and the oil, from the image formed by the common 

 surface of the spar and the oil, and to examine the properties of the last of these 

 images. 



VOL. XXIV. PART II. 3S 



