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POLARISED LIGHT 375 polariser and the stage s (fig. 190), the latter being drawn forward sufficiently to allow of this. Oil of lemons, which will require about one pound (cost 10s.) to fill such a tube, will exhibit a nearly complete rotational spectrum, like the quartz plate ; and saturated syrup, or oil of turpentine, a very visible change of colours, in the fluid itself. By placing a bi-quartz in the stage, rotation may be shown in plain cells containing only one to two inches of fluid. Tne operation of the saccharometer is easily shown by introducing the bi-quartz or any other apparatus (such as Laurent's semi-circular half-wave plate) and arranging (as can easily be done) that an index shall rotate with the analyser round a divided glass plate in the stage, which is focussed on the screen. The index is first adjusted to zero, and then the degrees of rotation necessary to bring back the bi-quartz to equilibrium after a tube of strong sugar syrup is introduced, can be seen. Also, by having a second tube of half the length, it can be demonstrated that the rotation is proportional to the length of the column of fluid ; and by filling one of two tubes with a one-half dilution, that it is proportional to the strength. 219. Electro-magnetic Rotation.—For lecture purposes this is generally shown with a bar of Faraday's heavy glass, having rectangular polished ends ; other heavy glass has somewhat less effect, and common glass, and many fluids and gases which show the same phenomena, do not do so with sufficient prominence for public demonstration. A bar of Faraday glass three to six inches long, and from half an inch square, will suffice. This should be placed in a circular tube of annealed iron, and if the bar be square, all the vacant space be filled with annealed iron wire, and the whole sur- rounded by an insulated coil of wire. This beinp introduced between the polariser and the stage, and the analyser crossed for dark field, on passing a pretty good current through the coil, light will appear on the screen, It will depend upon the length of glass and the current, whether any perceptible