INDUSTRIAL PROGRESS DURING THE YEAR 1875. J x ix 



stant of a compound may be tolerably well calculated from 

 those of its constituents. 



Descloizeaux has published an elaborate paper on the dou- 

 bly refractive properties of the triclinic feldspars albite, oli- 

 goclase, labradorite, and anorthite; in which he shows that, 

 though so difficult of exact determination by present phys- 

 ical or chemical means, these feldspars may very readily be 

 distinguished from each other by their optical characters. 



Noclot proposes to use either sugar, potassium bichromate, 

 or tartaric-acid crystals in place of aragonite for exhibiting 

 the phenomena of conical refraction. The two former crys- 

 tals arc cut without difficulty, since a natural face of the first 

 and a cleavage face of the second are normal to one of the 

 optic axes. But with tartaric acid it is necessary to get 

 such a face by trial. One is compensated for the labor, 

 however, by the result, a cone being obtained which, for the 

 same thickness of plate, has twice the angle of aragonite. 



Mach has devised a new and convenient optical apparatus 

 for certain polarization phenomena. Above the Nicol eye- 

 piece of an ordinary polariscope is placed a small achromatic 

 prism. The field of view will appear, of course, uncolored, 

 but displaced toward the base of the prism. If now the 

 whole eye-piece thus arranged be made to rotate about a 

 vertical axis, each point of the field becomes a circle. If 

 now in the field a quartz plate be placed, cut perpendicular 

 to the axis, and which is covered with a screen having a 

 small hole in it, a circle of colored light will be seen by 

 persistence of vision, the order of colors being in the direc- 

 tion of rotation or the reverse as the quartz is right or left 

 handed. The same apparatus may be used to project the 

 phenomena on a screen. If an unachromatized prism be 

 used, the spectra of the polarization colors are obtained. 



Adams has devised a new polariscope for examining the 

 rings of crystals, the objects had in view being (1) to obtain 

 a large field, (2) to secure the means of measuring both the 

 rings and the axial angles, and (3) to be able to immerse the 

 crystal in liquid. The peculiarity of the optical arrange- 

 ment is that the crystal section is placed at the common cen- 

 tre of curvature of two nearly hemispherical lenses, so that 

 its relation to these is unchanged when the crystal and 

 lenses are rotated about any axis parallel to its surfaces and 

 passing through this centre. 



