DOUBLY REFRACTING FORCE OF CALCAREOUS SPAR. 243 



Azimuth 9°. Light + 45° = 45°. E' is polarised 67° to the right, and 0' 55° 

 to the left. 



Azimuth about 16°. Light + 45° — 45°. E is poarlised 71° to right, and 51° 

 to left. 



Azimuth 45°. Light + 45° — 45°. E' is polarised 90° out of the plane of re- 

 flexion, and 0' a few degrees to the left of and E. 



Azimuth about 60°. E' is polarised 76° to left. 0' vanishing with and E. 



Azimuth 90°. Light + 45° - 45°. E' is polarised 70° to left, and 0' 53°. 0' 

 is very faint when the prism images vanish. 



Azimuth 180°. Light + 45° — 45°. E' is polarised 68° to the left, and 0' 63° 

 to the right. 



Azimuth 0°. Light polarised 0° and 90°. E' is polarised 90° to the left. 0' 

 which is reddish, vanishes with 0. 



Azimuth 180°. Light polarised 0° and 90°. E' is polarised 85° to the right. 

 0' vanishes with 0. 



With Oil of Anise Seeds. 



Azimuth 0° and 180°. There is no polarisation at great incidences. One-half 

 the spar pencil seems polarised 90° to the left, red light being polarised in the 

 plane of reflexion. 



Azimuth 90°. Light is almost wholly polarised about 100° to the left at the 

 greatest incidences. At less incidences it is half polarised, red light being left. 



With Alcohol. 



In Azimuth 0°, 90°, and 180°, there is no change of polarisation. The spar 

 image is polarised at a less angle of incidence than the prism image. 



In order to observe the changes of polarisation in passing from one plane to an- 

 other, I had three artificial faces made, slightly inclined to the three principal 

 planes C, B, and E. 



E 1. Face inclined 5° \ to C. 



This face was used a few minutes after it was polished. 

 Polarising angle in principal section, 59° 38'. 



Polarising angle perpendicular to it 58° 25', much unpolarised light being 

 left. 



With Oil of Cassia. 



Azimuth 0°. Light + 45°— 45°. E' is polarised 61° to right, and 0' 73° to left. 

 Azimuth 180°. Light + 45° — 45°. E' is polarised 67° to right, and O' 76° to 

 ■ left. 



