Handbook of the polariscope and its pracitcal applications / adapted from the German editon of H. Landolt, by D.C. Robb and V.H. Veley.

Hans Heinrich Landolt

Date:: 1882

Credit: Handbook of the polariscope and its pracitcal applications / adapted from the German editon of H. Landolt, by D.C. Robb and V.H. Veley. Source: Wellcome Collection.

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$in the experiment agreed very nearly with their calculated densities. The polarized ray must therefore have been influenced almost entirely by individual molecules, not by groups. The rotatory power [a] was, however, manifested to its full extent, and the con- clusion is that here optical activity must be a property resident in the molecule itself, and dependent on its atomic structure. The phenomenon is thus seen to be really chemical. The optical activity of crystals and that of liquids are, therefore, wholly distinct phenomena, and to the latter Biot has given the name of moleculur rotation, indicating that it is a property resident in the individual molecule. §11. Magnetic Rotation.—A rotatory movement of the plane of vibration of a ray of polarized light can be produced in all transparent isotropic bodies, solid or liquid (as glass, water, &c.), by placing them between the poles of a magnet, or within the helix of an induction-coil. This so-called magnetic rotation differs alto- gether from rotation as seen in naturally active substances. It lasts so long only as the electric influence is continued; it varies in degree with the intensity of the latter; and it takes a right- or left-handed direction, irrespective of the medium, according to the position of the poles of the magnet or the direction of the electric current. There is also this further characteristic difference between the two. Let a polarized ray be transmitted through a naturally active substance, which, for the sake of example, we will say is right-rotating. Then the deviation of the plane of polarization will always be such that, to follow the movement of the ray, the instrument must be turned towards the right of the observer—that is to say, the direction of rotation, with reference to that of propa- gation of the ray, is invariable. If after passing through the refractive medium the ray is returned into it by reflection, and the analyzer brought round to the same side as the entering ray, it will be found that rotation is annulled. The rotation dependent on magnetism is of a quite different character. The ray transmitted, let us say, from south to north pole, in the direction of the observer, will appear deflected towards the right hand, and, trans- mitted from the opposite end of the tube, towards the left. If the ray transmitted from south to north pole be reflected back, it will appear farther deflected to the left, so that an analyzing Nicol placed to receive it must be rotated to the left through an angle$