PHYSICS. 471 



monochromatic piano polarized light, iiumei aus alternations of bright 

 and dark elliptical bands are seen, most distinctly when the plane con- 

 taining the optic axes makes an angle of 45^ with the plane of polar- 

 ization of the light. Since in Iceland spar the natural ftices of the 

 rhombohedrou make an angle of nearly 45° with the optic axis, Madau 

 suggests the use of a simple cleavage plate, 2 by l-"-" in size and 2™"' 

 thick, broken in halves, and one half turned round ISQo, superposed on 

 the other, and cemented with Canada balsam. On placing this on the 

 eye-lens of a microscope, with the analyzer just above it, the ellipses will 

 be well seen when sodium light is used. (Xature, November, 1884, xxxi, 

 83.) 



Mace de Lepinay has shown that the graphic method of Cornu may 

 be applied with great facility to the study of the diffraction fringes pro- 

 duced by an opaque rod, by means of an auxiliary curve, which is the 

 same in all cases, since it is only th e primitive spiral displaced par- 

 allel to itself by a fixed quantity. {J. Phys., January, 1884, II, iii, 11.) 



Hartley has described a simple method of observing faint lines with 

 diffraction spectroscopes. The operation is conducted in a darkened 

 room, the goniometer of the spectroscope being illuminated by a shaded 

 lamp placed on the right of the telescope. The grating is movable, 

 the collimator and telescope being fixed so as to include as small an 

 angle between them as possible. The telescope being to the right of 

 the collimator, a small gas-jet is placed on the left, the rays of which 

 are reflected into the telescope from the grating. By the adjustment of 

 this light the field may be illuminated with any color of the spectrum, 

 and by selecting that tint which is complementary to tiie color of the 

 lines to be measured they are sure to stand out, apparently, in relief on 

 a bright ground. {Nature, March, 1884, xxix, 470.) 



Mertchifjg, under Egoroff's direction, has determined experimentally 

 the focal distances for monochromatic light of a reflecting grating of 

 Rutherfnrd having 17,296 lines in an inch, for all incidences. In the 

 spectra of the first and the second order the variations of the focal dis- 

 tances of the images situated to the right of the reflected image of the 

 slit were inconsiderable, while those on the left were much greater and 

 of contrary sign. The results, represented graphically, give a curve in 

 which the focal distance is a function of the angle of deviation, and 

 which the author regards as a variety of the hyperbola of the second 

 degree. (J.- Phys., October, 1884, II, iii, 459.) 



Bucking has submitted crystals to i^ressure, with the view of deter- 

 mining the influence thus exerted on their double refraction. The crystal 

 plate is i^laced on the stage of a polarizing microscope, pressure l)eing 

 exerted by a screw and measured by a dynamometer. In the case of 

 apatite, for example, the angle between the axes, in the plane parallel 

 to the direction of the pressure, but before the pressure was applied, 

 was 3°. The plate used was 5"""' square and 4""" thick, and the press- 

 ure varied from to 100 pounds. The angle between the axes became 



