GOG SCIENTIFIC RECORD FOR 188S. 



so that two of its faces are at right angles to the optic axis, while the 

 other four are parallel to it, its length being three times its width. Cut 

 it by a plane inclined 20° to its length, polish, and re-cement these faces 

 with Canada balsam. Such a prism has the following advantages: 

 (1) An object seen through it is not displaced laterally; (2) a conical 

 pencil whose axis passes directly through it is more nearly jdane polar- 

 ized than in other prisms, and (3) if the direction of the wave normal 

 within the prism does not quite coincide with the axis of rotation, the 

 average error in the position of the plane of polarization isless than 

 for any other method of cutting. The angular aperture of the field 

 is 10°. (Phil. Mag., May, 1833, V, xv, 352.) 



Gouy has devised an apparatus for the synthetic production of cir- 

 cular double refraction. It is formed of a collection of parallel plates 

 cut from a uniaxial crystal parallel to the axis. These all have the 

 same thickness, which corresponds to a difference of path of either one 

 semi-undulation or of an uneven number of semi undulations between 

 the ordinary and extraordinary rays for sodium light at a normal inci- 

 dence. These plates are cut into equal rectangular bands, long and 

 narrow, and then these are placed side by side, like the boards in a 

 floor, and cemented together between two parallel glass plates with 

 Canada balsam. In the first strip the principal section of the crystal 

 has an arbitrary direction ; in the following ones the principal section 

 of each strip makes with that which precedes a constant angle in mag- 

 nitude and direction, upon which depends the properties of the appa- 

 ratus. Upon the whole is superposed a half- wave plate without regard 

 to direction. (J. Phys., August, 1883, II, n, 360.) 



Some time ago Kighi demonstrated that if two rays were made to in- 

 terfere when their vibration numbers were slightly different, there ap- 

 peared on the screen, instead of the ordinary fringes, fringes which had 

 a uniform movement in a direction perpendicular to their length, with 

 a velocity such that there would pass a given point in one second a 

 number of lumiuous fringes equal to the difference of the vibration 

 numbers. The phenomenon was identical with that of beats produced 

 by sonorous air vibrations. He has now succeeded in realizing the con- 

 dition experimentally, and has described the apparatus necessary as 

 well as the conditions of success. (J. Phys., October, 1883, II, n, 437.) 



Gouy has examined the condition of diffracted light as regards its 

 polarization. He finds that if the incident light is natural, the diffracted 

 light is polarized, very strongly if the diffraction angle exceeds 50°, 

 and always in a plane parallel to the edge of the screen or perpendicular 

 to the diffraction plane. If the incident light is plane polarized, the dif- 

 fracted light is also plane polarized, but in a plane making a greater 

 angle with the plane of diffraction. With reference to the light dif- 

 fracted away from the shadow, the polarization phenomena are contrary. 

 With ordinary light the diffracted beam is now polarized in the plane 

 of diffraction; the polarization being almost complete if the diffraction 



