AND DISPERSION IN ICELAND SPAR, 
429 
Table VI.—Results of the Experiments. 
Ray F. 
i 
Angle of 
incidence. 
Ray g. 
Prism III. 
i=43° 53' 57". 
/*• 
!'■ 
D + i. 
D + i. 
yjs / . 
/'• 
1-57014 
1-55876 
1-54914 
1-53312 
1-52241 
36 47 24 
33 0 11 
29 41 15 
23 44 11 
19 14 15 
O 1 II 
81 18 40 
75 14 35 
72 27 35 
70 0 25 
69 32 35 
O 1 ll 
70 6 24 
58 6 24 
50 6 24 
38 6 24 
30 6 24 
81 33 30 
75 28 30 
72 41 10 
70 13 40 
69 45 50 
36 39 26 
32 53 30 
29 35 29 
23 39 56 
19 11 5 
1-57502 
1-56345 
1-55371 
1-53745 
1-52644 
+ 
!’■ 
D + i. 
.0. 
D + i. 
0’. 
+ 
1-52573 
1-51571 
1-50475 
1-50005 
1-49610 
23 10 8 
27 45 2 
33 49 30 
37 7 53 
40 37 1 
69 34 39 
69 49 24 
72 9 4 
75 4 19 
81 48 14 
36 53 26 
44 53 26 
56 53 26 
64 53 26 
76 53 26 
69 48 14 
70 1 34 
72 20 49 
75 15 59 
82 0 4 
23 5 54 
27 40 10 
33 43 28 
37 1 15 
40 29 39 
1-53014 
1-51981 
1-50870 
1-50389 
1-49982 
Prism IV. 
■i= 43° 51'. 
Prism IV. 
n- 
0'- 
D + i. 
0. 
D + i. 
0'. 
+ 
1-49507 
1-49114 
27 37 28 
20 44 52 
o / n 
59 7 35 
60 0 35 
o , „ 
43 53 9 
31 53 9 
58 57 15 
59 49 40 
27 33 16 
20 41 49 
1-49856 
1-49460 
/t. 
!'■ 
D + i. 
D + i. 
V- 
/'• 
1-49074 
0 , „ 
26 10 22 
68 1 36 
41 6 43 
68 12 11 
O / // 
26 6 21 
1-49430 
Section III. 
I. Determination of the position of the principal plane of the prism. 
II. Proposition proved.—The principal plane of prisms I., HI., and IV. map he 
treated as if it passed through the optic axis. 
III. Theoretical calculations for the reciprocal of the wave velocity. 
Our next step will be the determination of the position of the faces of the prisms 
with reference to the optic axis. 
This was accurately determined for each prism by measuring the angles between 
them and two of the rhombic faces of the crystal. 
The angle between these faces and also the angle between the cut faces of each of 
the prisms were accurately observed. 
Let us take point 0 within the crystal as origin, arid from it draw normals to the 
MDCCCLXXX. 3 K 
