phenomenon was studied shortly afterwards by Biot,[1] 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[2] (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[3] and W. Thomson[4] 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.[5]
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,[6] 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
- ↑ Mém. de l'Institut, 1812, Part I, p. 218, 899.; Annales de Chin., ix (1818), p. 372; < (1819), p. 63.
- ↑ Camb. Phil. Soc. Trans. i, p. 43.
- ↑ Mém. de l'Inst. vii, p. 73.
- ↑ Baltimore Lectures (ed. 1904), p. 31.
- ↑ The term rotatory may be applied with propriety to the property discovered by Faraday, which will be discussed later.
- ↑ Annales de Chim, xxviii (1825), p. 147.
