. Polarisation of light . lined that itsplane is perpendicular to the polar axis of the the lower half of this disc is a graduated semicircle,divided into twelve parts (each of which is again sub-divided into five or ten parts), and against the divi-sions the hours of the day are marked, commencingand terminating with VI. Within the fixed brassring, containing the glass dial-plate, the broad end ofa conical tube is so fitted that it freely moves roundits own axis ; this broad end is closed by anotherglass disc, in the centre of which is a small star orother figure, formed of thin film
. Polarisation of light . lined that itsplane is perpendicular to the polar axis of the the lower half of this disc is a graduated semicircle,divided into twelve parts (each of which is again sub-divided into five or ten parts), and against the divi-sions the hours of the day are marked, commencingand terminating with VI. Within the fixed brassring, containing the glass dial-plate, the broad end ofa conical tube is so fitted that it freely moves roundits own axis ; this broad end is closed by anotherglass disc, in the centre of which is a small star orother figure, formed of thin films of selenite, exhibit-ing, when examined with polarised light, stronglycontrasted colours ; and a hand is painted in such a 86 POLARISATION OF LIGHT. [chap. position as to be a prolongation of one of the principalsections of the crystalline films. At the smaller endof the conical tube a Nicols prism is fixed so thateither of its diagonals shall be 45° from the principalsection of the selenite films. The instrument being. Fig IS.—Wheatstones PolarClock. 19. -Wheatstones PolarClock. SO fixed that the axis of the conical tube shall coin-cide with the polar axis of the earth, and the eye ofthe observer being placed to the Nicols prism, it willbe remarked that the selenite star will in general berichly coloured ; but as the tube is turned on its axis, VII.] POLARISATION OF LIGHT. 87 the colours will vary in intensity, and in two positionswill entirely disappear. In one of these positions asmaller circular disc in the centre of the star will bea certain colour (red, for instance), while in the otherposition it will exhibit the complementary effect is obtained by placing the principal sectionof the small central disc 22^° from that of the otherfilms of selenite which form the star. The rule toascertain the time by this instrument is as follows :—The tube must be turned round by the hand of theobserver until the coloured star entirely disappears,while the disc in the
Size: 1561px × 1601px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1870, bookpublisherlondo, bookyear1874
