DOUBLY REFRACTING FORCE OF CALCAREOUS SPAR. 237 



B With Oil of Anise Seeds. 



Azimuth 0° and 180°. E' and 0' vanish together, E' at all angles less than 

 45°. and a great part of E, viz. the blue light and more is polarised at angles 

 above 45°, a small portion of red light remaining. = E, bluish and ~& yellowish. 



Azimuth 90° and 270°. Change of polarisation 94° to left. Maximum polar- 

 ising angle for E' at angles much less than 45°, polarisation nearly complete. 

 No appearance of polarisation at great incidences. 0' = E'. 



Azimuth 39° and 321°. Change of polarisation 50° to left. The maximum 

 polarising angles commence at the angle of polarisation for O, and the polarisa- 

 tion is complete at angles both above and below 45°. 



Azimuth 219°. Change of polarisation 0°, 0' and E' vanishing at the same 

 time at about 45°. Above 45°, E' is nearly wholly polarised 15° to the right, 

 while below 45°, it is polarised 0°, 5°, 10°, &c to the left as the incidence diminishes. 



With sunlight complete polarisation took place much lower than 45°, and blue 

 light was less in this light. 



B. With Oil of Sassafras. 



Azimuth 0° and 180°. No change in the plane of polarisation. 0' is bluish 

 and = 4 to 5 E'. 



Azimuth 39°. Change of polarisation about 30°. 



Azimuth 219°. Change of polarisation 0°. 



Azimuth about 60°. Change of polarisation 45°, the polarisation increasing 

 at great incidences. 



Azimuth about 70°. Change of polarisation about 50°. Maximum polarisa- 

 tion about 48° of incidence. 



Azimuth 90°. No light seems to be polarised at or above 45°. 



Azimuth about 240°. Change of polarisation from 15° at great incidences to 

 about 25° at small incidences. Light polarised in plane of reflexion is treated 

 nearly as common light, but light polarised 90° out of that plane has a change of 

 polarisation a few degrees greater, owing to its being incident at a less angle ; the 

 one image from the polarising rhomb being higher than the other. 



B. With Castor Oil. 



Azimuth 0° and 180. O' and E' completely polarised without any change of 

 plane. 



Azimuth 39°. Change of polarisation about 10° to left. 

 Azimuth 90°. Change about 20° to left at moderate incidences. 

 Azimuth 141°. Change 0°. Polarisation complete. 

 Azimuth 219°. Change 0°. Polarisation complete. 



VOL. XXIV. PART II. 3 T 



