Refraction of Iceland Crystal . 385 
Exp. 8. I measured with care, an angle at which two surfaces 
of the spar are inclined to each other, and found it to be 10 f 5'. 
Hence, the greater angle of the surfaces themselves may be 
computed to be 101° 55'; and the angle which the short axis 
makes with each plane surface is 45° 23' 25". 
If GSMP (Fig. 2.) be a plane, bisecting an obtuse angle of 
the spar, the section of the spheroid in that plane passes through 
the axis CS, and therefore is the generating ellipse. By calcu- 
lating from the known dimensions of its major axis CP 0,67204, 
its minor axis CS 0,6035, and the angle GCS = 45 0 23' 25", 
CG will be found* to be 0,6365, of which the reciprocal is 
1,5736, differing but little from 1,571, as it appeared by mea- 
surement. (Exp. 4.) 
Again, if ABDE (Fig. 4.) be one of the natural surfaces, and 
P Gp the ellipse formed by that section of the spheroid, PC 
being as before 0,67204, and CG 0,6365, the reciprocal of 1,571 
found by measurement, (Exp. 4.) then the semidiameter CT, 
parallel to the side AE, which makes an angle TCP 39 0 2|-', 
will be found to be 0,6573, instead of 0,6587, and its reciprocal 
1,5215, instead of 1,518. (Exp. 2.) 
The semidiameter also, in the direction of CL, perpendicular to 
the side, at an angle LCP 50° 57^', is found by calculation 0,650, 
and its reciprocal 1,539, instead of 0,6506 and 1,537. ( Exp. 3.) 
From the foregoing data, the course of a ray perpendicular to 
the surface of the spar may likewise be computed ; for, since the 
sine of incidence is then nothing, the ordinate of refraction 
must be also nothing, and the ray will be refracted along the 
semiconjugate diameter CM. (Fig. 2.) 
* (Fig. 3.) CS : CP tang. PCG ; tang. VCp. 
sec. P Cp ; sec. PCG ;* CP ; CG. 
