PHYSICS. 463 



osents appear to show that the jjenerally received formulas for metal- 

 lic reflection are approximately correct, but that the actual iutensity of 

 the reflected light is always less than the theoretical intensity. Hence, 

 unless the surfaces be defective, these formulas do not express the laws 

 of metallic reflection. If, as appears to be the case, .a change in the re- 

 flective power of a plate can occur without any change in the values of 

 the principal incidence and azimuth, the formulas must be regarded 

 as only approximately true, and there is additional reason for thinking, 

 with Stokes, that three constants are required to define a metal optic- 

 ally. {Wature, February, 1884, xxix, 398.) 



Gouy has studied the light diffused from depolished glass and metal 

 surfaces. The apparatus consisted of a mirror and attached polarizer, 

 by which a beam of light polarized in any plane could be thrown at 

 any incidence upon the roughened plate, placed horizontally. The dif- 

 fused light was observed by means of a j)olariscope. Taking a plate 

 of ordinary ground glass, the incident ray being polarized in a plane 

 perpendicularly to the plane of incidence, and the angle of incidence 

 being 60°, he finds that there are two ne.utral directions, making an 

 angle of 47° with the normal to the i^late and symmetrically situated 

 with reference to plane of incidence, so that that plane which contains 

 either of them and the normal makes an angle of 22° with the plane of 

 incidence. On the one side of the incident i^lane the light is circularly 

 or elliptically polarized in the right handed direction, and on the other 

 side left-handed, while in the plane of incidence it is plane polarized. 

 If, however, the incident light is polarized in the plane of incidence, 

 the two neutral directions still exist, and are symmetrical with refer- 

 ence to this plane, but are otherwise quite diflerent from those just de- 

 scribed. The two angles are 77° and 95°, respectively, and the gyration 

 of the rays is inverted, so that the right and left handed rays have 

 changed sides. (C. B., April, 1884, xcviii, 978.) 



Dufet has examined the influence exerted by temperature on the re- 

 fractive index of quartz, studying, first, the variation of the double refrac- 

 tion, and, second, the variation of the ordinary and extraordinary indices. 

 The quartz used was a rectangular parallelopipedon, 14.07""" in the 

 direction of the axis and 14.614'"™ in the perpendicular direction. The 

 refractive indices taken were those of Mascart, and the expansion co- 

 efficients those of Benoit. The variation of the double refraction was 

 measured by the displacement of Fizeau's and Foucault's fringes, and 

 that of the two indices by the displacement of Talbot's bauds. He finds 

 that the indices of both rays diminish with the temperature by values 

 considerably above those of Fizeau. (0. B., May. 1884, xcviii, 1265 ; 

 J. Phys., June, 1884, II, iii, 251.) 



Bauerwald has determined the refractive indices of rutite, the speci- 

 men being a remarkably transparent crystal from Syssert, in the Ural, 

 cut into a prism of 25°. The ordinary index for the lithium line was 

 2.5671, for the sodium line 2.6158, and for the thallium line 2.6725. The 



