AND DISPERSION IN ICELAND SPAR. 
435 
To determine \ we require to know the position of the optic axis with reference 
to x. 
The optic axis is equally inclined to ly Ih R 3 (fig. 2). Hence each of the angles 
subtended at S by the arcs R 1 R 3 , Ih R 3 , lb R 3 is 120°. 
Therefore R 3 S x is 60 ° 
sin R,£C= sin SR.-, sin R.^Sa; 
sin SRo = —7= sin R.-,a; 
v o 
2R 2 x= U° 55' 35" 
whence 
SR 3 =44° 36' 57".(6) 
=SR 3 =SR 1 
Again, from the right-angled triangle R 3 x R 3 
cos R 3 R 3 = cos R 3 x cos ccR 3 
and 
R 3 R 3 = 2 / i = 2 Rye 
cos R 3 cc= cos 2 ijl sec p 
Substituting the value 
2p=74° 55' 35" 
we have 
R 3 ^e=70° 52' 28" 
. \ S^c=R 3 a;—R 3 S 
= 26° 15'31".. . (7) 
and 
\ = $x ' a 
where a refers to the face P. 
The position of the wave normal is also given by 
where 
\'=Sa2 — at! 
a' being the x direction angle of the face Q. 
From these equations we get the following table of values to determine 6 the angle 
between the optic axis and any wave normal. 
