Transparent Inactive Crystal Plates. 



183 



refraction and the azimuth of the plane of polarization of a 

 transmitted wave, the azimuth of the plane of polarization of 

 the incident wave which produced it is obtained by use of equa- 

 tions (35) for biaxial plates, (43) or (44) for uniaxial plates, and 

 (46&) for isotropic plates. (4) To find the azimuth of the plane 

 of polarization of the emergent wave, provided that of the 

 incident wave which produced the refracted wave be known, 

 equation (37) is useful. This last equation, which is apparently 

 new, states that the azimuth of the plane of polarization of 

 an emergent wave S\, resulting from the refracted wave W„ is 

 90° from the uni radial azimuth of the incident wave e 2 which 

 produced the refracted wave W 2 . 



A detailed discussion of the above formulas is deemed 

 unnecessary in this summary, as they are in large measure 

 standard and the effects of the different factors will appear 

 more clearly in the discussion of the data of observation. 



In the development of these formulas, no account has been 

 taken of the effects of surface films on the rotation of the 

 planes of polarization of transmitted waves. These films have 

 been shown by P. Drude' and others to be occasionally of great 

 influence, especially on plates which have been highly polished, 

 while on freshly cleaved plates they are practically absent. 

 The observations listed in the following pages were made 

 largely on cleavage plates, which, however, were usually ex- 

 posed for a month or more before the observations were finished 

 and may have accordingly suffered some deterioration. 



Part 2. 

 Observations. 



Apparatus. — All observations recorded below were made in 

 sodium light, the crystal plate being mounted on a universal 

 stage on the new model petrographic microscope recently 

 described by the writer. 2 To insure accuracy, the microscope 

 was carefully adjusted and its adjustment tested at intervals in 

 the course of the measurements. The nicol prisms were 

 of the square end Glan Thompson type and were crossed by 

 pointing the microscope, from which all lenses had been 

 removed, directly toward the sun whose rays are parallel and 

 so intense that a rotation of less than 1' of arc from the posi- 

 tion of exact crossing of the nicols is readily discerned. By 

 means o,f the iris diaphragms the sun's rays were sent through 

 the microscope centrally so that no rotatory effect of the nicol 

 surfaces on the planes of polarization of the transmitted waves 

 was possible. The ordinary type of nicol prism with oblique 



1 Wied. Ann., xxxvi, 532, 865, 1888; xxxviii, 265, 1889; xliii, 146, 1891. 



2 This Journal (4), xxix, 407-414, 1910. 



