PHYSICS. 275 



of polarized light contains one or more dark bands, corre8])ondin,i2; to 

 the colors extinguished by the interference. If the depolarizing ])late 

 be of quartz, the bands are moved along the spectrum to the right or 

 left by rotating the analyzer. Govi's device consists in rotating the 

 analyzer and the direct-vision [)rism together at the same velo(;ity. A 

 circular spectrum i.^ thus produced, red at the center and violet on the 

 edges, or the reverse. As the dark quartz baud moves along the spec- 

 trum by an amount proportional to the rotation angle, interference will 

 take place along this circular spectrum along points which form, geomet- 

 rically, a spiral of Archimedes, which persistence of vision makes per- 

 manent. Thicke* quartz gives two, three, or more interlacing spirals, 

 forming -d very striking optical experiment. — {Nature, xxii, 595, October, 

 1880.) " 



Klocke has discovered t^iat in its action on polarized light, hyposul- 

 phite of lead is anomalous. It ordinarily polarizes circularly, but when 

 plates cut ])erpendicularly to the optic axis, are viewed in a parallel beam 

 of polarized light they appear unequally bright, being divided by dark 

 bands into six sectors, opposite pairs of which are equally bright. Even 

 iu convergent light the usual ring system of uniaxial crystals is not 

 seen, but in each sector there appears a figure characteristic of biaxial 

 crystals, the plane of the optic axes being perpendicular to the neighbor- 

 ing edge of the crystal. It would thus appear that the six portions are 

 compressed equally, each in a direction perpendicular to the neighboring 

 face of the prism. — {Nature, xxiii, 209, December, 1880.) 



Becquerel has published a memoir on the polarization of the sky. 

 He finds that, contrarj- to the general opinion, the plane of polarization; 

 for a given point does not generally i^ass through the sun, but that the 

 angle which this plane makes with the plane of the sun (a plane jess- 

 ing through the point, the sun, and the eye of the observer) varien from 

 minute to minute so that the plane of polarization passes a little below 

 the sun, between it and the horizon. If the point be in the northern or 

 southern sky this angle is very small in the morning, increases to a max- 

 imum from nine to ten o'clock, disappears at noon, increases again to a 

 maximum about 2 or 3 p. m., again diminishing and becoming zero as 

 the sun sets. If the point be east or west, coincidence of the two planes 

 is not seen, though a mininmm is observed about noon. At morning 

 and evening the angle attains maxima, reaching even GP. From these 

 phenomena the appearance is as if the plane of iK)larization suffered a 

 direct rotation, viewed by an observer whose head was toward the north 

 and his feet toward the south. Since in a region perpendicular to the 

 dipping needle, the rotation is sensibly zero, the author believes that it 

 is due to the influence of the earth's magnetism — an opinion strengthened 

 by the fact that he has just measured the amount of rotation of a i)olar- 

 ized-light ray traversing a layer of carbon disulphide, under the inHu- 

 ence of the earth's magnetism. — {Ann. Chim. Phys.^ V, xix, 90, January^ 

 1880; J. Phys., ix, 51, February, 1880.) 



