DOUBLY REFRACTING FORCE OF CALCAREOUS SPAR. 241 



at great incidences. At the polarising angle, the light polarised 90° is scarcely 

 perceptible. The residual light is reddish, and it is obviously blue light that is 

 polarised 90°. 



Light polarised + 45 — 45°, becomes polarised +36 — 36. 



With Alcohol. 



The prism image is completely polarised, and the spar image E equally so 

 in the same plane, but at an incidence 3° or 4° greater. 



With oil of sassafras, cassada balsam and water, there is no change in the 

 plane of polarisation. 



D. Face inclined 22|° to Axis. 



Azimuth 0° and 180°. In common light, the change of polarisation is 90° to 

 the left. It decreases to 20° in azimuth 90°, becomes 0° in azimuth 113°, and in 



Azimuth 180°, it again becomes 90°, decreasing to 0° in azimuth 247°, becoming 

 20 in azimuth in 270°, and increasing to 90 in azimuth 360°. 



Azimuth 0° and 180°. Light + 45 —43°. E' is polarised 45* to left and O' 

 62f ° to right. 



Azimuth about 15°. Light +45° — 45°. O' is polarised 85^° to left, E vanish- 

 ing with O and E the prism images. When O' vanishes E' has reappeared, and 

 is red. 



Azimuth 38°. Light +55° — 45°. E is polarised 50° to the right, and O' 

 771° to the left. 



Azimuth 20°. E' is polarised 17° to left, and O' 83° to right. 



Azimuth 113. Light +45° — 45'. No change, E' and 0' being both polarised 

 in the plane of reflexion. E' nearly vanishes when 0' vanishes ; but by increasing 

 the incidence it vanishes completely, showing the usual red and blue at the point 

 of evanescence. 



Azimuth 135°. Light +45° -45°. E' is polarised 22° to right, 0'=0 and 

 vanishing with 0. 



Azimuth 142°. Light +45°— 45°. E is polarised 31' to the right, 0' vanish- 

 ing with 0. 



Azimuth 160°. Light + 45° — 45°. E' is polarised 66° to right, and 0' 22° to 

 right. 



Azimuth 218°. Light +45° — 45°. E' is polarised 11° to left, and 0' 77° to 

 left. 



Azimuth 90°. Light +45° — 45°. E' and O' are both polarised 28° to the 

 left. 



Azimuth 270°. Light +45° = 45°. E' and 0' polarised 18' to right. 



VOL. XXIV. PART II, 3 U 



