240 SIR DAVID BREWSTER ON THE 



A. With Oil of Anise Seeds. 



Azimuth 0° and 180°. Scarcely any effect is produced upon common light at 

 any incidence ! The spar pencil is brighter than the prism pencil, and yellow. 



Azimuth between 0° and 90°, and 0° and 270°, the pencil is almost wholly 

 polarised at great incidences, and about 90° to the left. 



Azimuth 0° and 180°. Light +45° — 45°. E' was polarised 30|° to left, and 

 36° to right. Instead of widening the planes of E' and O' into +67^ — 6'7|, the 

 anise seeds oil has brought them nearer into 4- 30^° — 36 a . Conceiving, therefore, 

 that an oil of intermediate refractive power might produce little or no change 

 upon the light + 45° — 45°, I mixed 2 drops of oil of cassia with 1 drop of oil of 

 anise, and obtained the following results. 



A. Oil of Cassia and Oil of Anise Seeds. 

 Azimuth 0° and 180°. Light + 45° — 45°. E' is polarised 44° to the left, and 

 0° 44° to the right ; that is, almost no change is produced. 



With common light there is not a trace of polarisation, and yet reflexion from 

 a transparent surface ! 



Light 0° and 90°. O' vanishes with prism image, and A' polarised about 90°. 



Azimuth 90° and 270°. With common light, the spar and prism image vanish 

 together. 



Between Azimuth 0° and 90°. With common light, the polarisation increases 

 to about 45° of azimuth, when it is complete, and then gradually returns into 

 common light at 90°. 



C. Face 'perpendicular to Axis. 

 On the natural surface of the Chaux Carbonate Base. 



Oil of Cassia. 



With common light, the spar image E is polarised 90° out of the plane of 

 reflexion, and is then orange, showing that the light polarised 90° is blue. It 

 becomes whiter at great incidences, and redder at small ones. The prism image 

 O is =2E at 45°. At greater incidences E increases faster than O, and becomes 

 nearly equal to it. At small incidences, E decreases much faster than 0, so that 

 it follows a different law of reflexion. 



At the greatest incidences which the prism allows, the spar pencil is com- 

 pletely polarised. 



Light 'polarised + 45° — 45°. E' is polarised 67° to right, and 0' 67° to left. 



Light polarised 0° and 90°. 0' vanishes with 0, and E is polarised 90° to left. 



Oil of Anise Seeds. 

 In all azimuths about -| or-f of E is polarised 90° out of the plane of reflexion ' 



