Certain substances, as quartz parallel to the optic axis, solutions of sugar, tartaric acid, etc., possess the peculiar property of rotating the plane of polarization to an extent depending upon the wave length of the light and the thickness of the layer traversed. That is to say, if the polar•iscope is set 'crossed' and a plate of quartz cut perpendicular to the axis introduced. the eye-piece field will appear lighter if monoehro: matic light is used, and it will be necessary to rotate the analyzer in order to again obtain the dark field. This rotation may be clockwise as viewed from the polarizer, 'right rotating' or 'dextro-rotary,' or it may be the reverse. 'left rotating' or 'levulo-rotary.' If white tight is used, colors will appear in the field of view, and the tint 'crossed' will always be complementary to that seen in the 'parallel' position. These colors are in general due to the fact that, as the rotation for different colors is different, certain ones will be passed through the analyzer and others cut off.
The rotation of the plane of polarization in solutions of cane sugar is made use of commer cially on a very large scale in testing raw sugar for its content of crystallizable sugar. Very many forms of apparatus have been devised for this especial purpose. One of the more com plicated. due to Soleil, is shown in Fig. 5; D is the tube in which the sugar solution is placed: C is a double quartz plate, half right and half left rotating, which affords a very sensitive means of setting the instrument to the same adjustment each time; B is the polarizer; A is a combination of a Nicol prism and a quartz plate, which by rotation enables the observer to correct for the color of the solution and obtain the most sensitive tint in the double quartz plate, C; F is the analyzer; G is a small Galilean telescope focused upon C; and E is the com pensating system which corrects the rotation due to the solution instead of following it up.
A plate of right quartz, b. is just neutralized by wedges of left quartz, a, when they arc as shown at a. If the solution rotates right, then the wedges are slid together so as to offer an excess of left rotation over b and neutralize the right rotation of the solution. If the solu tion is left rotating. the wedges are slid apart and the of right rotation of b neutralizes the rotation of the solution. The motion of the wedges is controlled by a rack and pinion, and is measured by a scale which may he made to read off percentage of sugar directly, provided a solu tion of certain standard strength and length is used. In 1845 Faraday demonstrated that a piece of glass in a strong magnetic field would rotate the plane of polarization in the direction in which the current of the electromagnet flows, and to an extent dependent upon the glass and the strength of the magnetic field. It has now been proved by Perkin and others that this is a rather common property of transparent media.
I1in4tocaArnv. Consult: Preston, The Theory of Light (London and New York, 1895) ; Spottis woode, Polarization of Light (London, 1895) ; Thompson, Light, Visible and Invisible (New York, 1897) ; Slaseart, Trade d'optiquo (Paris, 1$S9) ; Landolt, Handbook of the Polariseopc, English translation by Robb and Veley (London, 1899).