MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 347 
Solving, we get 
XY=119° 5' . . ,.(1) 
Experiment gave 
119° 4' 
Here again we have considerable confirmation. 
Pig. 9. 
Let the plane Y X L be the plane of the prism cutting C A C' in L; it will be 
necessary to find A L, X L, and the angle A L Y. 
The right angled triangle LAY gives 
sin AY = tan AL cot AYL. 
The triangle X A Y gives 
sin LYA= 
sin AX sin YAX 
sin XY 
whence 
LYA=3° 28' 30" 
Substituting this value, we get 
AL = 2° 59' 10".(2) 
From the same triangle we get 
cos YL= cosAY cos AL 
whence 
YL=120° 47' 10" 
but 
XY=119° 4' 
XL=1° 43' 10".(3) 
From the same triangle again 
cos ALT’ = tan AL cot LY 
whence 
r ALY=91° 47" 
2 Y 2 
