238 



SIR DAVID BREWSTER ON THE 



Azimuth 270°. Change about 1 0° to right at considerable incidences. 

 Azimuth 321°. Change 10° to right, and polarisation complete. 



Azimuth 39°. 

 Azimuth 90°. 

 Azimuth 141°. 

 Azimuth 219°. 

 Azimuth 270°. 

 Azimuth 321°. 



Azimuth 39°. 

 Azimuth 90°. 

 Azimuth 141°. 

 Azimuth 219°. 

 Azimuth 270°, 

 Azimuth 321°. 



B. Oil of Cajeput. 



Change about 5° to left, and polarisation complete. 

 Change 7° to left at moderate incidences. 



Change 0°. Polarisation complete. 



Change 0°. Polarisation complete. 



Change 5° to right at considerable incidences. 



Change 5° to right. Polarisation complete. 



B. With Olive Oil 

 Change about 5° to left. 

 Change about 100° to left. 



Change 0°. Polarisation complete. 



Change 0°. Polarisation complete. 



Change 8° to right. 



Change 5° to right. Polarisation complete. 



B. With Alcohol highly rectified. 



Azimuth 39°. Change about 1° or 2° to left. The two images vanish at 

 different incidences, S at greater than P. 



Azimuth 90°. Change about 1° or 2° to left, spar pencil at angle greater than 

 prism pencil. 



Azimuth 141°. Change 0°. 



Azimuth 270°. Change about 1° or 2° to left. 



Azimuth 280°. Change 0°. Polarisation complete. 



Azimuth 321°. Change at 1° or 2° to right. 



A. The Face parallel to the Axis. 



With Oil of Cassia. 

 The following observations were made with a plate of glass placed on the 

 surface of the spar, the plate being inclined about 5° in the Azimuth 3 parallel 

 to the axis. 



Azimuths. 



Change 



of Polarisation. 



0° 





93°* 



10 





85 



20 





80 



221 





74 



30 





55 



40 





59 



45 





55 



Azimuths. 



Change 



of Polarisation 



50° 





47° 



60 





37 



67* 





33 



70 





32 



80 





20 



90 









* The change was 90° at the polarising angle of the spar and oil surface. 



