450 LIGHT tion in proportion to their thickness. If a ray of polarized light is sent through a plate of Iceland spar or other uniaxial crystal in the direction of its axis, its plane is not changed ; but when a plate of rock crystal is cut in the same direction and a polarized ray of homoge- neous light is passed through its axis, its plane will be found changed on emergence. It will have rotated on its axis, and this rotation may be either to the right or to the left, and the amount of rotation will depend upon the thick- ness of the plate. The direction of the rota- tion serves to classify such crystals into right- and left-handed. If the prisms of the polar- izing apparatus are crossed so as to produce extinction of light, and the substance to be ex- amined then introduced, there will be, if it pos- sesses the properties above mentioned, a partial restoration of light. If now the analyzer is turned through a certain number of degrees, the light will again disappear, and the angle through which the analyzer has been turned will be the measure of the rotating power of the substance. The principal laws of rotatory polarization, the discovery of which is due to Biot, embrace the following facts: 1. With the same sub- stance the rotation of the plane of polariza- tion is in proportion to the thickness of the substance traversed. 2. When two plates are placed together, the effect is nearly the same as that of a single plate whose thickness is equal to the sum or difference of the two plates, accord- ing as they rotate the ray in the same or op- posite directions. 3. The degrees of rotation of the plane of polarization vary with the dif- ferent rays of the spectrum, and increase with their refrangibility. For a given plate the an- gle of rotation is inversely as the square of the length of the wave. Therefore, as the rays of different colors emerge polarized in differ- ent planes, if a beam of white light is sent through a rotating crystal or substance and then received by the analyzing prism or plate, as this is turned the different colored rays of polarized light will make their appearance in succession. Sir David Brewster discovered that amethyst or violet quartz is made up of al- ternate layers of right- and left-handed quartz; and the structure may be distinguished in the fracture of the crystal, which presents a pecu- liar undulating appearance. Biot and Seebeck discovered that many liquids and vapors have, like quartz, the power of rotating the plane of polarization. Oil of lemon, solution of sugar in water, and solution of camphor in alcohol rotate the polarized ray to the left ; oil of tur- pentine rotates it to the right. The power of these liquids is, however, much feebler than that of quartz, and therefore greater thick- nesses are required to be employed. When liquids having this property are mixed, the ro- tation produced by the mixture is equal to the sum or difference of that of the ingredients, and Biot made an application of this principle to the analysis of compounds containing a sub- Vance having rotatory power, combined with others which are neutral. Polarized light, therefore, has a practical application as a test for a variety of substances; many cases of doubtful identity in chemical analysis being alone decided by its use. The saccharometer of M. Soleil is constructed in accordance with these properties of light. It is used in the arts for ascertaining the percentage of sugar in so- lutions, and in medicine for testing its pres- ence, as well as determining its quantity in the fluids of the body. It may also be applied in detecting albumen and other organic bodies. Arago employed polarization by double refrac- tion in the construction of a photometer ; and he has also shown how rocks beneath the surface of water may be discovered by using a Nicol's prism to extinguish the reflected rays by which the submerged rocks are pre- vented from being seen. Chromatic polariza- tion may be employed with advantage in crys- tallography, to indicate whether a crystal has one or two axes of symmetry, and also the positions of these axes. By the use of the polariscope we may ascertain whether the light which comes to us from the heavenly bodies is reflected from their surfaces, as from the moon and planets, or whether the bodies are self-luminous. Faraday made the discov- ery that the plane of polarization can be ro- tated by the action of magnetism. If a cylin- drical or rectangular bar of "heavy" or "Far- aday's glass " (silico-borate of lead) is placed longitudinally between the poles of a powerful electro-magnet which is hollow in its axis (to admit of observation), and a Nicol's prism is placed in one end of the magnet as polarizer, and another in the other end as analyzer, and they be so turned that no light passes through both, then, as long as no current passes around the temporary magnet, the interposition of the glass bar will have no effect ; but when a cur- rent is passed around the magnet, rotation of the plane of polarization takes place, and in the direction of the current. The degree of rotation is in proportion to the length of the bar and the strength of the current. Flint glass is acted on with about half as great effect as heavy glass, and all transparent solids and liquids are more or less affected in the same manner. Faraday thought that the magnet- ism had a direct action on the light, but others have since believed that the rotation is pro- duced by a molecular change induced in the glass by the magnet. It has been stated that when glass is subjected to strain or unequal pressure its homogeneous texture is altered, and the particles are so disposed that it acquires the property of double refraction. It conse- quently has the power of producing polariza- tion of light transmitted through it, analogous to that possessed by natural double-refracting crystals. The compression or strain may be produced by rapid cooling of fused glass, or the glass may be compressed in a vice. From the fact that polarization always takes place when rays of light are reflected from surfaces
