426 
DE. A. E. H. TUTTON ON THE 
Optics. 
Nature and Orientation of the Optical Ellipsoid. —The optic axes lie in the 
symmetry plane b {010}. The sign of the double refraction is positive, the first 
median line corresponding to the refractive index y, and the second median line to a. 
Extinction determinations with two section-plates ground parallel to the symmetry 
plane afforded the following results with respect to the normal to c { 001 }. 
Extinction Direction in the Symmetry Plane. 
Plate 1 . . . 11° 2', Plate 2 . . . 10° 28'. 
Mean . . 10° 45', behind the normal to c {001}. 
This direction, between the normal to c{00l} and the vertical axis c, is the 
second median line. As the angle between the normal to c {001} and the vertical 
axis is 16° 9' (the acute axial angle ac being 73° 51'), the inclination of the second 
median line to and in front of the vertical axis c is 5° 24'. The rectangular direction 
(the other axis of the optical ellipsoid also lying in the symmetry plane) is the first 
median line, and is inclined 10° 45' to and above the inclined axis a. Both median 
avcis 
lines thus lie in the obtuse axial angle ac, and the positions will be rendered clear by 
a reference to fig. 10. 
Tops 0E and Chkistiansen ( loc. cit .) give 6° 50' and 9° 23' for the two extinction 
positions, axis c : second median line, and axis a : first median line, respectively. 
Optic Axial Angle. —Three pairs of section plates, ground perpendicular to the 
first and second median lines respectively, afforded the values given in the two 
following tables for the apparent optic axial angle in air 2E, and the apparent acute 
and obtuse optic axial angles in monobromonaphthalene 2H a and 2H 0 . The values of 
