442 
MR. R. T. GLAZEBROOK ON DOUBLE REFRACTION 
Thus this variation has tended to decrease the differences between observation and 
theory, and has reduced them to almost the same magnitude as those given by the 
face Q of the prism. They now agree more nearly with the results of prisms I., III., 
and IV., though even yet the differences observed are greater than in any of the other 
prisms. 
Prism II., however, was at first cut wrongly from the crystal, and when recut it 
was so small that I formed the intention of not using it at all, and leaving a gap in 
my series of observations between the values 8— 27° and 0=41°. I found, however, 
on a second and more careful trial, that the images formed by it were clearer and 
brighter than I had thought, and so determined to take a series of observations with 
it. When I observed a second time the angle of prism II., I took a series of measure¬ 
ments of deviation, &c., which lead to results in agreement with Tables X. (a), X. (b), 
so that on the whole the results given by this prism are in accordance with those 
already arrived at in prisms I., III., and IV. 
Our next step is to consider the theory for the rays F and g. 
The position of the plane containing the two normals to the faces of the prism is 
of course the same, and therefore so also are the formulae which give the relations 
between 9 and <£', 8 and r//. 
The values of the axes of spheroid on Hctyghen’s theory are, however, different. 
Let us take the green line, F, first. 
p, is, as before, the value of the ordinary refractive index. 
We have as for the line C the four values 
^=1-66780 Prism I. 
1-667761 
> Prism III. 
1 -66778 J 
1-66783 Prism IV. 
We take pj=l‘66779. p e is the value of p when d=90 c in Table XI. 
Now for d=89° 49' 6" experiment gives 
p=l‘49074 
we take this as the value of p c . 
Hence for F we have 
p, = 1-66779 
p.,= 1-49074 
Mascart and PiUdberg give respectively 
and 
1-66802, 1-49075 
1-66793, 1-49084 
