314 MR, R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
The Tables XXV. and XXYI. give the results of theory for the first prism. 
Let us take the outer sheet first. 
In lines 1-12, Table XXY., and lines 1-10, Table XXVI., the agreement between 
theory and experiment is fairly close, but these series of experiments overlap and are 
in the immediate neighbourhood of an axis of the section. 
From this point onwards, we see that the experimental curve lies entirely inside 
that given by Fresnel’s theory, and the difference between the two increases as we 
approach the neighbourhood of the optic axes, line 16, Table XXY., 14 and 15 
Table XXYI., while for the rest of Table XXV. the differences diminish. 
The tables extend from about 8° on one side of the axis C, to nearly 16° on the 
other. This is obvious, for between the optic axes 6^9', beyond them 6-)r6' is very 
approximately equal to twdce N C. 
Now let us consider the inner sheet, as before in lines 1-14, Table XXV., lines 1-11, 
Table XXYI., the agreement is fairly close ; but these observations extend from about 
6° on one side, to 7° on the other of the maximum radius vector of the section. As 
in the case of the outer sheet, the differences here increase as we approach the neigh¬ 
bourhood of the optic axes, reaching a maximum of ’00044, line 15, Table XXYI. 
After passing the optic axes, the differences decrease, and for the rest of Table XXY. 
average about '00024, and in this case the curve given by experiment lies outside that 
given by theory. 
On examining the differences given in Table XXY. for the outer sheet, I noticed that 
theory and experiment agree closely up to the immediate neighbourhood of the optic 
axis, line 16 ; while in the neighbourhood of the optic axis, Table XXYI., line 15, the 
increase in the differences is much more gradual. 
In fact, if we compare the experimental values of /r 1 in Table XXY., lines 11-14, 
and Tattle XXYI., 12-15, for about the same values of 9-\-9' we find considerable 
differences. 
This led me to calculate the results of other sets of experiments covering the same 
ground, which had been already made. 
The following table gives the results of one such set, taken about two months 
previously ; during the interval the goniometer had been taken partially to pieces, and 
afterwards refocussed and set again. 
It seems worth while to give the whole set of observations which cover nearly the 
same ground as Table II., and form therefore a test of the degree of accuracy of the 
results. 
The measurements were made at angles of incidence, nearly, but not quite, coincident 
with those of Table II. 
In order to compare results, so far as the wave E (inner sheet) is concerned, a 
correction by interpolation is necessary therefore. 
The values of <£', it will be observed, differ by about 5' from those in Table II. 
In the table, the first column gives the values of <// ; the second the values of y; 
