AND DISPERSION IN ICELAND SPAR. 
423 
Section II. 
I. Description of crystal. 
II. Account of experiments with the results. 
It was my object in carrying out tlie work to secure a series of observations for 
values of 6 from 0° to 90°, differing by about 1° 30' or rather less. This I found could 
be obtained by the use of four prisms of 44° or thereabouts, each having its edge per¬ 
pendicular to the optic axis, which would therefore lie in the principal plane of each 
prism, the prisms being so cut that the optic axis made angles of —32°, 14°, 38°, and 
64°, with the outward drawn normal to one of the faces ; the angles are considered 
positive when the optic axis falls on the same side of the normal as the edge of the 
prism. Prisms cut in this manner would, I found, enable me to work over a range 
extending from about 5° on one side of the optic axis to about 100° on the other. 
Iceland spar, as is well known, cleaves readily so as to form an oblique rhombohedron. 
Eig. 1. 
Let A B C D E F G, fig. 1, represent a rhomb of spar, and let A be a solid angle, 
such that each of the three plane angles B A D, D A F, F A B is obtuse. The optic 
axis is equally inclined to each of the faces B A D, D A F, F A B, the angle of 
inclination being 26° 15' 30" about. It is, therefore, perpendicular to the interior 
bisectors of the acute angles G F A, G B A. I procured a large rhomb of spar, 
which was cut by A. Hilger, 196, Tottenham Court Road, into four prisms, the edge 
of each being nearly parallel to the interior bisectors of the acute angle of the same 
rhombic face. The angle of each prism was about 44°, and the faces were cut so as to 
be inclined to the optic axis as stated above. 
We proceed now to describe the experiments and give the results for each of these 
four prisms numbered I., II., III., and IY. In each case let P, Q denote the faces of the 
prism, i the angle between them, f xj/ the angles which the wave normal in the prism 
makes with the normals to P, Q respectively, (f) \ fj the corresponding angles in ah’; <f> is 
the angle of incidence or emergence according as the light is incident on P or Q, and 
vice versd for xfj. 
The values of the angle of incidence on one face extend from nearly grazing incidence 
3 i 2 
