POLARISCOPE. 165 POLAR RESEARCH. is measured by a scale whioli may be made to read oir percentage of sugar directly, provided a solu- tion of certain standard strength and length is used. In 1845 Faraday demonstrated that a piece of glass in a strong magnetic field would rotate the plane of polarization in the direction in which the current of the electromagnet Hows, Fig. 5. polabiscope for liquids. and to an extent dependent upon the glass and the strength of the magnetic field. It has now been ])roved by Perkin and others that this is a rather common property of transparent media. BlULiociRAPHY. Consult: Preston, The. Theory of Lir/ht (London and New York, 1895) ; Spottis- woode. Polarization of Light {London, 1895) ; Thompson, Liyht, Tisihle and Invisible (New York, 1897); Maseart, Trait{ d'optique (Paris, 1SS9) : Landolt, Handbook of the Polariseope. English translation by Eobb and Veley (London, 1899). POLARIZATION. A term used in physics with many applications, but generally to signif.y a i)i(ipcrty of matter or of the ether which has a definite direction. A magnet is said to be polarized ; so is a dielectric under the action of electricall.v charged bodies. When, owing to the passage of an electric current through an electro- lyte, there are chemical actions at the electrodes which oppose the current, the electrolytic cell is said to lie polarized. When a definite characteT- istic is given ether-waves — e.g. if the vibrations are all in one plane, or if they are all in circles or ellipses — they are called polai'ized. See Elec- tricity; Light; Voltaic Cell ok Battery; PoLARISCOPE. POLARIZATION OF SKYLIGHT. Th*> polarization of blue skylight was known to llnygens, and is evident to any one who will look at the sky through a prism of Iceland spar. After a systematic investigation of the distribution of polarized light in difl'erent parts of the slcy, Arago (1811) discovered that a certain portion or spot in the sky shows no polarization. This point varies with the altitude of the sun. Sub- sequently similar points were discoiered by Arago (1840) and Brewster (1842). The maxi- mum polarization occurs in a narrow zone 90 degrees distant from the sun. Similar but feeble polarization occurs in moonlit skies. There is no polarization in fog, cloud, or dense haze. As there is an evident connection between the polarization of skylight and the quantity of moisture, haze, or smoke in the atmosphere, there have been numerous investigations of this subject both from an observational and a theoret- ical point of view. The simplest and most accurate po- larimeter for study- ing this phenomenon was devi.sed in 1872 by Prof. E. C. Pick- ering, director of the Harvard College Observatory. Obser- vations made w'ith this apparatus seem to show that the re- flection of the sun- light from the sur- face of the sea into the atmosphere pro- duced a decidedly perceptible disturb- ance of the polariza- tion of skylight as compared with that observed from inte- rior stations where such reflection from water surfaces was impossible. Observations of skylight during recent total eclipses of the sun seem to have shown that the amount of polarized light increases during and just before and after totalit.y. Such an eft'ect might be produced either by the increased condensation of aqueous vapor, owing to the cooling of the atmosphere within the shadow of the moon, or by the fact that in place of the ordinary sunlight we now have to do with the polarized light from the solar corona. The most acceptable theory as to the origin of the polarization is that started by the experi- ments of Tyndall on the reflection of sunlight from delicate clouds of vapor and perfected by I.iu'il Hayleigh, who showed that when light is transmitted through a cloud of particles smaller than the wave length of the light, the latter is scattered laterally and polarized in the plane of scattering. In 1899 Rayleigh submitted the additional demonstration that not merel.v the grosser particles of dust and aqueous vapor, but even the molecules of oxygen and nitrogen, and probably those of other gases, take part in the process of scattering and polarization. Consult United States Monthly ^yeather Revieir (Wash- ington) for September, 1900, where will be found a summary of recent work in this field and a bililiograpliy. POLAR RESEARCH. The polar regions comprise all lands and seas within the polar circles, extending northward from the Arctic Circle and southward from the Antarctic Circle to the respective poles of the earth. The bar- renness of the land, often ice-covered and largely devoid of animal and vegetable life, with the extended and heavy ice-fields in the sea, has made exploration slow and hazardous. Motives of Polar Eeseakcu. Exploration in the far north has been more extensive than that in the far south, chiefly because the Arctic is much nearer to the great maritime nations
