374 
MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
We can, without much difficulty, calculate the amount of the change. 
Fig. 15. 
We may veiy approximately treat the rotation round 0 L as if it were about 0 C, 
C L being very small. 
The effect of a rotation of 1° about 0 C will be to bring A and B to A, B, respec¬ 
tively in the same plane, where 
AA=BB=1° 
A / B 7 being to the right of A, and B. 
The increment of 17' in L P may be effected by turning the axes through an angle 
of 17' about a normal to the plane L P passing through 0, let O S T be this normal, 
let A' B' C' be the final positions of ABC. 
a, a; b, b' are arcs of small circles whose centres lie on 0 S T at T and S respec¬ 
tively, so that 
A / TA / =B / SB , = 17' 
C C' is an arc of a great circle, and is —17'. 
B / B'=SB / x B'SB, 
806, = 60° approximately 
.-. SB'= sin 60°=-4“ 
.*. B B'= 
17V3 
BB'= ^/fBBy+B' By) 
60 'H 
3x17 2 
: 1° 2' approximately 
A,A'=ATA' sin 30 
= 8' 30" 
AA'~= ^60' 3 -t-~- ) approximately 
= 1° L approximately. 
