DOUBLY REFRACTING FORCE OF CALCAREOUS SPAR. 



235 



A 



Al 



D 



B 



Bl 



B2 



E 



El 



C 



Inclination to Axis 



0° 





4 



40' 



22 



30 



45 



23 



50 



51 



57 



31 



67 



30 



84 



30 



90 







Observed Polarising 



Angle.* 



54° 



3' 





54 



16 





55 



22 





57 



12 





59 



16 





59 



38 





59 



59 





B. Surface of Rhomb, Inclination to Axis 45° 23'. 



1. With Oil of Cassia, on an Artificial Face. 



Azimuth 90°. Obtuse angle to the right. 



Light polarised 0° and 90°. E' and O'f vanish simultaneously 42|° to the left 

 of the plane of incidence, E' being reddish and O' yellowish. 



Light polarised + 45° and — 45°. E' is polarised 45^° to the left. O' vanishes, 

 but a little blue light is left, which disappears along with E and O. O' vanished 

 more completely by turning the rhomb that gave the images E, 0, 9° or 10° farther, 

 from 45° to 55°. 



Azimuth 270°. Obtuse angle to left. 



Light polarised 0° and 90°. E' and 0' vanish simultaneously 33 1° to the right 

 of the plane of incidence. A little blue light remains in E' and 0' at the point of 

 evanescence. 



Light polarised + 45° — 45° 0' is polarised 35° to the right. E' vanished 

 completely. 



2. With Oil of Cassia, on a Natural Face of Cleavage. 



Azimuth 90°. Light polarised 0° and 90°. E' and 0' vanish simultaneously 

 471° to the left. 



Light polarised + 45° — 45°. E' is polarised 46° to the left. 0' is scarcely 

 visible. 



Azimuth 270°. Light polarised 0° and 90°. E' and 0' vanish simultaneously 

 42^° to the right. 



Light polarised + 45° — 45°. 0' is polarised 44^° to the right. E' is nearly 

 invisible. 



Common light is polarised 44° to the right. 



N.B. — The evanescence is not complete either on the glass or on the spar sur- 

 face ; but more complete on the natural than on the artificial face of the spar. 



Azimuth 38°. Obtuse angle to right. In common sun's light, E' and 0' vanish 

 simultaneously at 45° of incidence, and in a plane 82^° to the right of the plane 



* In the plane of the principal section. See Phil. Trans., 1819, p. 158. 



f E' and 0' are the extraordinary and ordinary images from the spar, and E and the same 

 ' from the prism surface. 



4 



