358 
MR. R. T. GLAZBBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
Let us consider first the inner sheet. 
In Table V. we see the differences are small to begin with, and increase fairly 
regularly throughout the whole of the table, the experimental value being almost 
always less than the theoretical. 
Lines 4, 5, and 6 were those chosen to determine the value of /x /y which explains the 
coincidence of the two curves in that neighbourhood. The table covers an arc of the 
surface of nearly 20°, as is seen by referring to the values of <£' in Table I. 
Throughout this arc the theoretical section lies outside the experimental, the 
difference between the two increasing as we proceed from the axis 
This continues throughout Table YI. 
The first three lines of Table YI. just overlap the last three of Table Y. The difference 
reaches a maximum value towards the end of Table YI. at a point for which <f>'= 
1° 19' 54" (vide Tables II., 21, and YI., 21), that is at about 34° away from the axis /x^. 
After this the difference begins to decrease at first slowdy throughout Table YII. 
which refers to the other prism P Q, so that the arc of section has changed slightly 
between Tables YI. and YII., both arcs passing through the point P. 
Table VII. covers an arc of 22° (Table IIP). 
The first two lines in Table YII. refer to a wave normal lying on the same side of 
P as L. In the rest of the table the wave normal is on the side of P remote from L. 
This arc is continued throughout Table VIII. ; the theoretical values of /x are still 
greater than the experimental, but the difference diminishes as we proceed along 
the arc. 
The last three observations in Table YII. and the first three in Table VIII. overlap. 
Table VIII. covers an arc of 20°. 
Thus the observations extend over two arcs, each passing through the point P 
inclined to one another at that point at so small an angle as to be almost continuous, 
the one arc being 35° 40', and the other 40° 40' in length. 
The results of theory and experiment agree at the extremity of the first arc, differ 
most widely in the neighbourhood of the point where the two arcs meet, and tend 
towards equality again throughout the second arc. 
It is worth noticing that the experimental results at the end of Table YI., the end 
of the first arc, agree closely with those at the beginning of Table YII., the beginning 
of the second arc. 
The experimental results for the inner sheet are therefore represented by a curve 
which coincides with Fresnel’s curve at the extremity of one axis, and lies inside it 
throughout the next 75° of its length. 
For the outer sheet the differences are less than for the inner sheet. 
6 — S' varies so slowly that it seemed sufficient for the purposes of comparison to 
calculate every fifth theoretical value, with the exception of that portion of Table Y. 
for which the variations of 6— O' are sensible. 
The section cuts the principal section A O C between lines 4 and 5, Table V. Just 
