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 



