If, instead of using homogeneous light, as we have been supposing, plane-polarized white light is employed, it is found that the different rays are differently deviated. The effect on the more refrangible rays is greater than on the less refrangible, and the plane of polariza tion of the blue rays will thus be turned through a greater angle than that of the red rays. It will be perceived from this that having arranged the polarizer and analyzer, and inserted a plate of quartz, as described above, on rotating the analyzer in the direction, right-handed or left handed, that corresponds to the nature of the plate of quartz, we shall not arrive at a position of total extinction, but we shall see a most beautiful play of colors changing in order from red to yellow, then to orange, green and blue. These phenomena are among the most beautiful and most striking of all the marvelous phe nomena of light.
It has been remarked above that certain or ganic liquids and solutions have this rotatory power. Among these may be mentioned tur pentine and the essential oil of anise as in stances of left-handed rotatory substances, oil of lemon and oil of caraway and solutions of sugar, as right-handed rotatory substances, and solutions of tartaric acid as showing both ro tations, as explained below. This fact is taken advantage of in Soleil's saccharometer, an in strument for determining the value of cane sugar in a liquid.
We have spoken above of the right-handed and left-handed properties of quartz; a dis covery of Haiiy leads us here to the very threshold of the molecular structure of crystals. We may yet hope for discoveries in this direc tion. On comparing crystals of quartz that give us right-handed and left-handed polarization, it is found that a very remarkable property con nects their forms. The crystals that give r:ght handed and left-handed polarization are of an unsymmetrical construction, such that either viewed in a looking-glass gives an image of the same form as the other. Pasteur, examining the crystals of the two varieties of tartaric acid whose solutions have opposite rotational powers, but whose chemical properties are very nearly the same, showed that the same law holds for them; and, having crystallized what is known as neutral tartaric acid, was able, by picking out the crystals by hand, to separate it into equal Portions of lmvo -tartaric acid and dextro tartaric acid. But we must refer the reader to the special articles on the chemistry of this substance.
One of Faraday's most brilliant discoveries was the rotatory power of glass under the action of a powerful magnet. The reader is referred for an account of it to the article POLARIZATION