DOUBLY REFRACTING FORCE OF CALCAREOUS SPAR. 239 



These observations were made at angles of incidence considerably greater 

 than 45°, the polarising angle. 



The following experiments were made with polarised light. 



Inclination of Plane of 



Inclination of New 



Polarisation to Plane of Incidence. 



Plane to Plane of Incidence 



0° 



0° 



221 



30 



45 



67-L 



67i 



79 



90 



90 



When E' was polarised 90° and O' 0°, O' was nearly thrice as faint as E', and 

 much redder. 



When the spar and oil image vanishes, red light is seen on one side, and blue 

 on the other side of the vanishing point, as in elliptical polarisation, the rotation 

 being different for different colours. 



The following experiments were made with an equilateral prism of glass. 



In Azimuth 0° and 180°. Light polarised + 45° — 45° is polarised + 67°— 67°, 

 the change of polarisation being 22°. 



Azimuth about 9°. Light polarised 0° and 90°. E' is polarised 87° to the 

 left, and O' and vanish together. Common light is polarised 87° to left. 



Light + 45°— 45°. E' is polarised 59£° to left, and 0' 71 i° to right. E' and 

 are equally bright. 



Azimuth about 17°. Light +45°— 45°. E' is polarised 27|° to left, and 0' 

 80^° to right. E' is very faint and red. 



Azimuth about 40°. Light + 45°— 45°. Both 0' and E' polarised 25° to left. 

 0' is brighter than E', which is faint and bluish. 



Azimuth about 60°. Light + 45 — 45°. O' and E' vanish together a few 

 degrees to the left. 



Azimuth 90°. Light +45° — 45°, and light 0° and 90°, are treated exactly as 

 by common surfaces ; the prism and spar images undergoing the same changes. 



As the plane of light polarised +°45 — 45°, becomes light +0°— 0°, or is all 

 polarised in the plane of reflexion, as the azimuths change from 0° to 90°, the 

 inclination of their planes must diminish from 50° to 0°, or from 135° to 0° ; 

 that is, from + 67^° — 67^°, to + 0° — °0. 



From the observations with common light, it appears that at angles of inci- 

 dence considerably above 45°, it is polarised a few degrees beyond the azimuth of 

 90°, and we have no doubt, that at 45°, it is polarised in that azimuth. Hence, it 

 follows, that the change of polarisation is equal to the complement of the azimuth, 

 or 90 — A. 



