MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
345 
we may take therefore 
/a= 1*68550 
This then must be the length of the radius vector of an ellipse, axes fx a and fi C) which 
passes through the point L— i.e., which is inclined at an angle CL=1° 21' 42" to the 
axis fi a [ vide Section II. (15)]. 
Now if 0 be the angle which a radius vector r makes with the axis /x a 
in the case considered 
cos 3 0 sin 3 6 
2 * 2 
Ha He 
0=1° 21' 42" 
sin 3 0— -000565 
and the error made in giving to /i c an approximate value will be inappreciable. 
Let us take, therefore, for /i c the value found in the previous part which agrees with 
Rudberg’s value 
Now 
whence 
{jl c = 1-53013 
sin 3 0 
He 
= •000241 
r= L68550 
4= -351919 
r 3 
_1 /I sin 3 0 
/.i a ~ cos 3 #\r 3 c 3 
p, —1-68560 
It now remains to determine /r,. accurately. 
The natural faces a t m' (fig. 1) formed a prism whose edge was parallel to the axis O C. 
By passing light through these faces and observing the incidence and deviation as 
before, I was enabled to calculate /v 
To pass light through these faces it was necessary, however, to polish them, and this 
operation altered their position appreciably. The values obtained by direct measure¬ 
ment required a small correction, and to calculate this it was necessary to determine 
the position of these new faces. 
Let the faces approximately coincident with a / and m' be called X Y respectively. 
On levelling the faces of reference m m', I found that Y remained very nearly in the 
same zone as before, viz. : that of m m m / m' ; wdiile X fell rather below this zone. 
Observations on the angles between X and Y and the faces m m respectively gave 
MDCCCLXXIX. 2 Y 
