AND DISPERSION IN ICELAND SPAR, 
431 
cos (3= 
cos # 3 —cos 
2 sin /x 
. 0 O + 6 1 . 0i — 0o 
sin — 1 sm 9 • 
Sill /x 
( 2 ) 
These formula give us the values of a and (3. 
2jx or the angle between the normals to the rhombic faces was observed in three 
pieces of the crystal used. The values were 
74° 55' 37" Mean of four measures. Maximum difference, 10". 
74° 55' 34" Mean of five measures. 
74° 55' 35" Mean of ten measures. Maximum difference, 25". 
We may therefore put with great accuracy 
2/x=74° 55' 35" 
The temperature indicated by a thermometer placed almost in contact with the 
crystal, and shaded from the direct radiation of the light used to read the vernier, 
was from 14° C. to 13° C. Each of the angles 6 1 d. 2 was observed fen times for each 
face and the mean taken, the temperature being kept as nearly as possible at 13° C. 
The greatest variation between any two observations never exceeded 40". 
Table VII.—The position of the normals to the faces of the prisms. 
Pace and direction 
of normal. 
*i 
tX 
fi 
Prism I. 
P outwards. . . 
Q inwards . 
65 48 35 65 41 32 
39 50 28 j 39 59 43 
1 
58 50 25 
14 55 34 
o , » 
89 54 43 
90 4 53 
Prism II. 
P inwards . 
Q outwards . . 
85 48 20 
53 31 35 
85 39 20 
52 53 20 
84 37 5-5 
41 0 55 
o / // 
89 52 37-7 
89 34 49 
Prism III. 
P inwards ... 70 14 35 
Q outwards . . | 104 40 25 
70 10 25 
104 43 25 
64 44 56 
108 38 34 
89 56 47 
90 2 23 
Prism IY. 
P inwards . 
Q outwards 
96 40 55 
128 58 25 
96 38 5 
128 45 20 
98 23 59 
142 14 8 
89 57 41 
89 51 37-5 
3 k 2 
