332 
MR. R, T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL'. 
The face m / was cut so as to be inclined at about 35° 20' to P, the line of inter 
section of this face (Q, fig. 2) and P being also nearly parallel to that of P and P. 
So that the principal planes of the prisms formed by P and P, P and Q respectively, 
were nearly coincident, and as my observations extended almost from perpendicular 
incidence on Q to perpendicular incidence on P : they embraced an arc of over 70°. 
The instrument used and the methods adopted for levelling, reading, &c., have been 
fully described in the previous part of the paper. 
The results of the experiments are contained in Tables I., II., III., and IV. 
Tables I. and II. refer to the prism R P. 
r/j is the angle made by the wave normal in air with the normal to P. 
c/f the corresponding angle in the crystal. 
D is the observed deviation. 
i the angle of prism. 
D -}- i is given, for it occurred in the calculation, and was just as easily found as D. 
The value taken for i is 
i=2,7° 2' 56" 
This value is the mean of twelve observations, none of which differed from the mean 
by 10". 
(f)' is found from the formula; <f>'= i 
, d>' — \fr' i (A + V <A—ip 
and tan ■ 0 = tan - cot 0 tan —-— 
which has already been proved (Part I., Section I.). 
In Table I. d> is the same for both outer and inner sheet, being the angle of 
incidence. 
In Table II. (f, has different values for the two sheets, being the angle of emergence, 
for as already explained, on passing through the position of minimum deviation, the 
prism was reversed so that the face of incidence became that of emergence. 
Throughout the work /X, refers to the inner, /x 3 to the outer sheet. 
The values of 2(f)', S/x are calculated as before on the assumption of errors of 10" in 
2(f) and D-fff, the observed quantities taken; these errors being combined so as to 
produce the greatest possible effect in the result. 
Tables III. and IV. refer to P Q, for which t = 35° 18' 50". 
The symbols have the same meaning, but while in I. and II. the wave normal falls 
on the same side of the normal to P as the axis O C, in III. and IV. it is on the 
other side of 0 P. 
In III. (f) is the angle of emergence, and therefore different for the inner and outer 
sheets; in IV. it is the angle of incidence, and the same for both. 
Each observation was repeated twice on different occasions; the results of two 
measurements rarely differed by 20'. 
