472 SCIENTIFIC RECORD FOR 18B4. 



zero for a pressure of 6 pounds, equal to 10^ for a pressure of 33 pounds, 

 and to 170 for one of 100 pounds. After the removal of the pressure 

 the angle between the axes was 5° in the primitive plane, and did not 

 vary on repeating- the experiment. Beryl gave no peruiaueut variation. 

 Vitreous orthoclase, whose axes are either in the plane of symmetry or 

 perpendicular to it, shows, under pressure perpendicular to this plane, 

 an increase of the angle in this plane and a decrease iu the perpendic- 

 ular plane. {J. Phys., February, 1884, II, iii, 106.) 



Von Fleischl has communicated to the Vienna Academy the dis- 

 covery of double refracting liquids. The apparatus used was a com- 

 pound hollow prism resembling, in general, Fresnel's quartz combination. 

 Concentrated solutions of ta^rtaric acid and of various sugars were em- 

 ployed, and also certain optically active oils. These doubly refracting 

 liquids possess no optic axis, and the wave surfaces are in every instance 

 two concentric spheres. {Nature, January, 1885, xxxi, 204.) 



Eontgen has i)ublished a valuable memoir on the variation of the 

 double refraction of quartz produced by electric force. Pressure ex-. 

 periments show that in a crystal of quartz of normal structure the sur- 

 face of a section normal to the principal axis may be divided into six 

 fields by lines intersecting at an angle of 60°. A pressure exerted in, 

 any direction passing through this point develoi)s contrary electricities 

 at its extremities. When a fragment of quartz is submitted to electric 

 induction, so that the lines of force, without being parallel to an axis oi 

 no i)ressure-electricity, are normal to the principal axis, the natural 

 polarization is modified by the action of the electric forces, and at the 

 same time the double refraction of the rays normal to the priucii)al axis 

 and to the lines of force is increased or diminished according to the di- 

 rection of the lines of force. These results are of importance in connec- 

 tion with the theory of pyroelectricity proposed by Sir. W. Thomson. 

 ( Wied. Ann., xviii, 213 ; J. Fliyn., January, 1884, II, iii, 35.) 



Two new forms of polarizing prism' have api>eared, one contrived by 

 Feussner and the other by Bertrand. Sleeman has described the former, 

 giving at the same time an illustrated account of the various polarizing 

 prisms thus far proposed, six in number. The jjrism of Feussner con- 

 •sists of a thin plate of a doubly refracting crystal cemented between 

 two wedge-shaped ])ieces of glass, the terminal faces of which are normal 

 to their length. The refractive indices of the glass and the cementing 

 medium should correspond with the greater index of the crystal; and 

 the directions of greatest and least elasticity in the latter must stand iu a 

 plane perpendicular to the direction of the section. One great advan- 

 tage of this i^rism is that other crystalline substances may be used in- 

 stead of (;alc spar, provided the difference between the ordinarj^ and the 

 extraordinary index is greater than in calc si>ar. Feussner has used 

 for this purpose a plate of sodium nitrate, whose indices are 1.587 and 

 1.330. In this, as in calc spar, cleavage plates may be used. As a 

 cementing material a mixture of gum dammar with monobrumnaphtha- 



