AND DISPERSION IN ICELAND SPAR. 
439 
If 
Sill' 
/v~/a 
/v 
cos 2 6 
(15) 
./x=/x 2 sec 8 
( 16 ) 
and we require to find /x v fx, 2 . 
jx x is the maximum radius vector of the spheroidal sheet of the surface of wave 
slowness. 
This is given by 6 = 0. 
From Table X. we have, considering at present the line C, when 
6=0° 20' 35" fi— 1 '65438 
jx v is also the refractive index of the ordinary wave. Its value was determined by 
observations on the angle of incidence and deviation of the ordinary ray in prisms I., 
III., and IV. 
The values were 
1-65438 
1-65438 
1-65433 
1-65433 
Prism I. 
Prism III. 
Prism IV. 
We take then as the value of /x l5 
1-65436 
Observations of the minimum deviation were made to determine /x, from the usual 
formula 
D + i 
sm 
2 
/x: 
D being the minimum deviation. 
The mean of these was 
sin 
1-65441 
but the error of this last method is much greater than that in the former, and as any 
error in the observed value of D would probably increase D, through the prism not 
being exactly in the position of minimum deviation, we should expect to get a value 
for fx 1 rather in excess of the true. 
The values given by Mascart and Rudberg are respectively 
1-65452 
and ’ 1-65446 
To determine [x 2 we must consider the minimum radius vector of the spheroidal 
sheet; this is given by 
6= 90' 
