Mr. Luioyp on Conical Refraction. 157 
emergence, is Just equal to the diameter of one of them, or to the diameter of the aper- 
ture. 1 had then only to measure the aperture itself. ‘This was effected by the aid of 
a micrometer divided to the 1-500th of an inch, placed along with the aperture before 
a compound microscope ; and it was found to be .016 of an inch. ‘This therefore 
was the diameter of the oblique section of the cone made by the surface of emer- 
gence ; and the diameter of the circular section at the same distance was .016 cos. 9°, 
since the axis of the cone makes an angle of 9° with the normal to the faces of the 
crystal. ‘The perpendicular thickness of the crystal was .49 of an inch; and there- 
: : : eee : 49 
fore the thickness estimated in the direction of the axis of the cone was 95° 
From these data the angle of the cone was calculated by the tables, and found to be 
1° 50’; a result which differs from the theoretical angle by 5’ only. 
