MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 309 
We may note, however, that his values of /x 4 deduced from two different pieces of 
crystal differed by '0004. 
Now jJL„ is the value of the refractive index for the first prism for the wave E when 
(f>'= 20° O' 39" 
The values of p, at about 30' on either side of this are 
1-68578 Table YL, 1-68589 
1-68574 Table XII, 1-68569 
The value 
^=1*68580 .(5) 
seems to represent these fairly. Let us take it at present, and see how far it gives 
consistent results. 
PtUDBERG gives 
^=1-68589 
To verify these results let us find the angle between the optic axes. 
If 28 ' be this angle in the crystal 
tonS'=\/ \ hYf f 
! --- 1 
u s h-j 
Whence 
S'=9° 4' 58".(6) 
and if 28 be the angle in air as seen through a face normal to the least axis of the 
crystal 
sin 8=p,j sin S' 
whence 
8= 15° 23' 30" 
( 7 ) 
Kirchoff found 15° 27' as the result of experiments. 
The value given by Rudberg’s values of the indices is 
8=15° 7' 
which differs from Kirchoff’s result by 2O', whereas the value given above as the 
result of my experiments differs only by 3' 30". 
