384 Dr. Wollaston on the oblique 
theory also, the section of the spheroid is in this case a circle, 
and every semidiameter (FC) the same, since the plane is at 
right angles to the minor axis. 
Exp . 6. If a plane surface be formed bisecting an obtuse 
angle of the spar, and applied to a prism, the same minimum of 
refraction 1,488, is found in a direction that coincides with the 
preceding plane, and therefore with the major axis of the gene- 
rating ellipse; but, as the direction is varied, it increases so 
rapidly as soon to exceed the power of glass, and to be no longer 
ascertainable by the angle of incipient reflection. 
Exp. 7. The regular refraction of this spar is also too great 
for examination by means of any prism, for want of a medium 
of union of sufficient density ; but, by trial in the usual method, 
it measured, on an average of several experiments, 1,657, or its 
reciprocal 0,6035. 
By assuming, as Huygens has done, the equality of this 
power with the maximum of the oblique refraction, we have 
sufficient data for construction of the spheroid by which the 
refractions are regulated; for we have 0,67204 [Exp. 1.) as 
major axis of the generating ellipse, and 0,6035 (Exp. 7.) will 
be the minor axis, parallel in position to the short axis of the 
spar. 
The angle of inclination of this axis to the surfaces of the spar, 
if supposed to be equilateral, may be computed by spherical 
trigonometry, from any other angle that has been ascertained 
by measurement. 
The measures that I have taken are not exactly those of 
Huygens; but I nevertheless hold them in equal estimation, 
from the conformity which I find they bear to each other, by 
assistance of his theory. 
