Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/194

 phenomenon was studied shortly afterwards by Biot, who showed that the alteration consists in a rotation of the plane of polarization about the direction of propagation: the angle of rotation is proportional to the thickness of the plate and inversely proportional to the square of the wave-length.

In some specimens of quartz the rotation is from left to right, in others from right to left. This distinction was shown by Sir John Herschel (b. 1792, d. 1871) in 1820 to be associated with differences in the crystalline forn of the specimens, the two types bearing the same relation to each other as a right-handed and left-handed helix respectively. Fresnel and W. Thomson proposed the term helical to denoto the property of rotating the plane of polarization, exhibited by such bodies as quartz: the less appropriato term natural rotatory polarization is, however, generally used.

Biot showed that many liquid organic bodies, e.g. turpentine and sugar solutions, possess the natural rotatory property: we might be led to infer the presence of a helical structure in the molecules of such substances; and this inference is supported by the study of their chemical constitution; for they are invariably of the mirror-image' or "enantiomorphous" type, in which one of the atoms (generally carbon) is asymmetrically linked to other atoms.

The next advance in the subject was due to Fresnel, who showed that in naturally active bodies the velocity of propagation of circularly polarized light is different according as the polarization is right-handed or left-handed. From this property the rotation of the plane of polarization of a plane. polarized ray may be immediately deduced; for the plane-polarized ray may be resolved into two rays circularly polarized in opposite senses, and these advance in phase by different