DOUBLY REFRACTING FORCE OF CALCAREOUS SPAR. 245 



the deviation from that plane varying with light of different colours, and with the 

 angle of incidence. 



3. In reflecting polarised light, they change its plane of polarisation, some- 

 times as in refraction, and sometimes as in reflexion. 



4. In certain azimuths, and at certain incidences when the pencil is not com- 

 pletely polarised, much light, apparently unpolarised light, is left ; and in many 

 cases, upon all the surface, A, B, C, D and E, the reflected pencil consists of two 

 oppositely polarised pencils, as in the reflexion of common light from the surface 

 of murexide, chrysammate of potash, and a few other bodies. 



5. The changes in the planes of polarisation, both of common and polarised 

 light, are related to the axis of double refraction, that is, to the short diagonal of 

 the primitive rhomb of calcareous spar. 



VOL. XXIV. PART II. 3 X 



