POLARIZED LIGHT AT THE SURFACE OF A UUIAXAL CRYSTAL. 
603 
In the spar prism used one face coincided very closely with a cleavage plane. The 
other face was inclined to this plane at an angle of 39° 17' 20", and the edge of the 
prism—that is, the intersection of the two faces—was nearly coincident with that of 
two cleavage planes. 
Let 0 E : , 0 E 2 , 0 E 3 (fig. 3), be normal to the three cleavage planes; O P, 0 Q 
to the faces of the prism. The incident light fell on the face normal to 0 P. Then 
experiment proved that P, Q, R 3 , E x were very closely indeed in the same zone; for 
the present we shall treat them as if they were accurately so, and this zone will 
therefore be the principal plane of the prism. 
Fig. 3, 
A series of observations on August 25, 1880, gave the values for the angles. 
P 0 E 1 = 105° 54' 
E x 0 Q= 34° 48' 
Q O Eg— 40° 8' 
P 0 E 3 =180° 50' 
Each observation was the mean of four or five closely concordant ones. The prism 
was reset and relevelled in November and another series of measurements taken, 
which agreed with the above to within 1'. 
Let the optic axis and the edge of the prism meet the sphere in X and B respectively 
(fig. 4). Then B E 2 X is a great circle which bisects the arc E L E 3 in M say. 
Fig. 4. 
Let P 0 produced backwards meet the sphere in P'. Then 
E 1 E 2 =E 2 E 3 BgBj;= 74° 56' 
