344 MR, R. T. GLAZEBROOK ON PLANE WAVES ENT A BIAXAL CRYSTAL. 
These can only be determined by finding the values of a , b, c. 
The results of the experiments enable us to do this with considerable exactness. 
Let [x a , fxjj, [x c be the principal refractive indices, p a being the greatest, p c the least 
1 
/L, —“ 
C 
1 
^=b 
1 
Section V.— Values of the Principal Refractive Indices. 
We are now in a position to determine from our experimental results the values of 
a, b, c the principal velocities. 
The plane P Pfi cuts the plane A C at a short distance from C. 
The section of the surface of wave slowness by the plane A 0 C consists of a circle 
of radius \x tj and an oval curve of axes p a and p c in the directions 0 C, 0 A respectively, 
which for a small distance on either side of a principal axis may be treated as an 
ellipse; as in Fresnel’s theory. 
Also, if L be the point in which P P' cuts the plane A O C 
LP=35° O' 19" 
[vide Section II. (16)j. 
The value of p corresponding to a value of f = 35° 0' 19" in Table I. for the inner 
sheet will thus be the value of p,,. 
But we have as corresponding values 
34° 28' 43" 
34° 53' 57" 
35° 16' 20" 
we may therefore take 
pi= 1*68115. 
The values of the radius vector to the outer sheet in the same neighbourhood are, 
Table I,, lines 4 and 5, 
<f>' P 
34° 47' 54" L68546 
35° 10' 1" 1-68553. 
For 
<£'=35° O' 19" 
h 
1-68115 
1-68114 
1-68115 
