OPTICAL ELASTICITY IN OBLIQUE-PRISMATIC CRYSTALS. 437 



(4). In Felspar (Fig. 5.) the optic axes lie in the plane of the 

 most perfect cleavage, and make with a normal to M, angles of about 

 57" or 58°, (58^^ according to Sir David Brewster) which increase when 

 the crystal is heated. Hence, ^^' is the axis of the zone PM. 



(5). The optic axes of Pyroxene (Fig. 6.) seen in air through a 

 slice cut perpendicular to MM are in the plane Pr, and make angles 

 of 16" with the axis of the zone MM. Hence, ^^' is the axis of the 

 zone MM', a, /3 approach ^ when the crystal is heated. At ordinary 

 temperatures a/3 is probably about 19"^. The best measurements of 

 Pyroxene shew that Pr, tr are nearly but not exactly equal, and 

 therefore, that its faces cannot be referred to ^^', YY', X,'C as crystallo- 

 graphic axes. In all the crystals of Pyroxene which I have examined, 

 the rings surrounding ad are brighter than the rings surrounding /3/3'. 



(6). The form of Borax (Fig. 7.) closely resembles that of Py- 

 roxene ; its optic axes however are very differently situated. It was 

 observed by Sir John Herschel and also by Professor Nbrrenberg, that 

 the optic axes for different colours do not lie in the same plane. This 

 being the case, we cannot expect to find any simple connexion between 

 the form and the directions of the axes of elasticity. 



The mean directions of the axes seen in air through the faces 7'T" 

 make angles of aO"^, with a normal to the faces TT', and a perpendi- 

 cular to them makes an angle of 55° with MM'. The rings sur- 

 rounding ad, /3/3' are indistinct on the sides towards M'P and MP' 

 respectively, the extremities a, /3 of the axes being next to the eye of 

 the observer. This shews that the positions of ^f', ^^' vary slightly 

 with the colour of the light employed. 



(7). In Chromate of Oxide of Lead, as I have been informed by 

 Professor Norrenberg of Tiibingen, the axis of the zone MM (see the 

 figure in Phillips or Naumann) bisects the angle between the optic 

 axes, and is therefore one of the axes of elasticity. The other two 

 axes of elasticity are, without doubt, the lines which bisect the angles 

 formed by normals to MM'. 



