formed—the one from two parts of a rhomb of spar, the other from two portions of a Nicol's prism—the two halves of the field are analysed in slightly different planes; but these, though they have certain advantages, are now seldom employed, partly on account of a difficulty in their construction and partly because their sensitiveness cannot be adjusted. The more usual half-shade analyser is available for light of only one frequency, as it depends upon the action of a half-wave plate, in traversing which the plane of polarization is turned until it makes the same angle with the principal section as at first but on the opposite side: half the field is covered with the plate, to which is attached a Nicol's prism with its principal section inclined at a small angle to that of the plate. The eye must be focussed on the edge of the plate, and the two halves of the field will only be equally dark when the principal plane of the plate is parallel to the primitive plane of polarization. Another plan, due to J H Poynting, is before analysation to impress unequal rotations upon the plane of polarization of the two parts of the field, either by means of an active medium, or by oblique transmission through glass plates.
Elliptically polarized light is investigated by the reduction of the pencil to a state of plane polarization, and a determination of the resulting plane of polarization. One method consists in finding directly the elliptic constants of the vibration by means of a quarter wave plate and an analyser; but the more usual plan is to measure the relative retardation of two rectangular components of the stream by a Babinet's compensator. This is a plate made of two equal wedges of quartz, that can be moved over one another so as to vary its thickness, and are cut so that the faces of the plate are parallel to the optic axis, which in the first wedge is perpendicular and in the second is parallel to the refracting edge. It is clear that direct transmission through the plate at a point where the thicknesses of the prisms are d1 and d2 will introduce a relative retardation of (μe − μ0)(d1 − d2) between streams polarized in planes parallel and perpendicular to the edges of the prisms, μ0, and μe, being the ordinary and the extraordinary refractive indices; and it is hence possible by an adjustment of the thickness to reduce elliptically polarized to plane polarization at an assigned point marked off by two parallel lines A subsequent determination of the plane of polarization gives the ratio of the amplitudes of the vibrations in the component streams.
For the observation and measurement of rings in crystals polariscopes are constructed on the following principle. Light from an extended source passes after polarization through two convex systems of lenses, between which the crystalline plate is placed, and is then received in an eyepiece furnished wit an analyser. If measurements be required, the plate must have a motion round an axis perpendicular to that of the optical systems, and also about an axis normal to its faces; the polarizer and analyser must also be capable of adjustment. All the rays through a given point in the first principal focal plane of the anterior system of lenses traverse the plate as a parallel beam and reunite at the corresponding point of the second focal plane of the posterior system, each in its passage being divided into two by the plate having a given relative retardation It is on this latter plane that the eyepiece must be focussed, and here the measuring web must be placed. The actual details of the systems of lenses depend upon the object for which the polariscope is intended, and are given for some of the principal types of instruments in Th. Liebisch's Physikalische Krislallographie.
Of polarimeters for the studiy of rotary polarization there are three principal forms. In Wild's polaristrobometer, light from a soda flame, rendered parallel by a lens, is polarized by a Nicol's prism, and after traversing the space into which the active substance is to be inserted, falls on a Savart's plate placed in front of an astronomical telescope of low power, that contains in its eyepiece a Nicol's prism, which with the plate forms a Savart's analyser. A web in the focal plane of telescope marks the point in the field at which the bands are to be made to disappear; this is effected by turning the polarize by means of a rack and pinion worked by an arm from the observer's end of the instrument. The polarizing prism is fixed at the centre of a circular disk, that has a scale on its circumference, which with a fixed vernier determines the positions of the polarize, for which the bands disappear at the assigned point of the field. Laurent's polarimeter is a half-shade instrument. Soda light, first sifted by passage through a plate of potassium bichromate, traverses in succession a lens, a Nicol's prism, and a glass plate half covered with a half-wave plate of quartz, that is cut parallel to the optic axis and has its principal section inclined at a small angle to that of the prism. This combination forms a half-shade polarizer, the sensitiveness of which can be varied by a slight adjustment that can be given to the Nicol. The light is finally received in a Galilean telescope, containing an analyser and carried at the centre of a circular plate, that is graduated on its rim and can be turned in front of a vernier by means of a rack and pinion. The telescope must be focussed on the edge of the quartz plate, and in order that all points of the field may be illuminated by the same part of the source, the flame must be so placed that its image is thrown by the lens on the diaphragm of the object glass of the telescope. The chief features of Soleil's saccharimeter are the biquartz and the compensator. The former consists of two semicircular plates of quartz, perpendicular to the optic axis and of opposite rotations, placed so as to have a common diameter and having such a thickness that each rotates the plane of polarization of mean yellow light through the same multiple of 90°. If then a stream of polarized white light traverse the biquartz, it is possible by an analyser to cut off the mean yellow light from each half of the field, and the whole will then have the sensitive tint; but a small change in the plane of analysation will give the one half a red and the other half a blue tone. A rotation of the plane of polarization is not, however, measured by an adjustment of the analyser, but by annulling the rotation with a compensator. This is made of two plates of quartz, cut normally to the optic axis and of opposite rotations, placed the one in front of the other: the thickness of the one plate is fixed, while that of the other can be varied, as it is formed of two equal prisms that can be moved over one another along their common face. When the plates are of equal thickness, their combined effect is nil, but by adjusting the second, a rotation in the one or the other direction may be introduced, a scale attached to one prism and a vernier to the other giving the thickness of the resultant quartz late. At one end of the instrument is placed a polarizer and the biquartz, and at the other a Galilean telescope, that must be focused on the edge of biquartz having in front of its object-glass the compensator and an analyser that is regulated for producing the sensitive tint, when the plates of the compensator have the same thickness. The sensitiveness of the instrument depends upon the exactness of the sensitive tint, when the colour of the two halves of the field are the same, and this is liable to be upset by absorption in the substance under investigation. In order to correct this, the light after analysation is passed through another plate of quartz and then the sensitive tint may be more or less restored by cutting off some colour, the same for the whole field, by a Nicol's prism placed in the eyepiece of the telescope. Sole1l's saccharimeter, as its name implies, is designed for the study of solutions of sugar, and it is clear that it will only work satisfactorily with active media that have nearly the same rotary dispersion as quartz.
Bibliography.—A bibliography of the subjects treated in this article will be found at the end of the corresponding chapters of E. Verdet's Leçons d'optique physique (1869); this work has been brought to a later date in the German translation by Karl Exner (Braunschweig, 1881); references to later papers will be found in J. Walker's The Analytical Theory of Light (1904). In addition to the above the reader may consult for the general subject of polarization the following treatises: Th. Preston (3rd. ed. by C. J. Joly), The Theory of Light (1901); A. Schuster, An Introduction to the Theory of Optics (1904); R. W. Wood, Physical Optics (1905); E Mascart, Traité d'optique (1889); and for the phenomena exhibited by crystals F. Pockel, Lehrbuch der Kristalloptik (1906); Th. Liebisch, Physikalische Kristallographie (1891). (J. Wal.*)
POLAR REGIONS, a general term for the regions about the North or South Pole, otherwise called the Arctic or Antarctic regions. The ancients had no actual knowledge of the Polar regions. They had probably heard rumours of the light summer History of Arctic Expeditions.nights and the dark winter nights in the north, as is shown by Homer's description of the Laestrygons having the short nights and the Cimmerians living in perpetual darkness. By astronomical speculations the Greeks had come to the conclusion that north of the Arctic Circle there must be midnight sun at midsummer and no sun at midwinter. The general view was that the Polar regions, north and south, belonged to the uninhabitable frozen zones; while according to a less scientific notion there was a happy region north of the north Wind (Boreas), where the sun was always shining and the Hyperboreans led a peaceful life. The first traveller of history who probably approached the Arctic Circle and reached the land of the midnight Pytheas sun was the Greek Pytheas (q.v.), from Massalia (Marseilles), who about 325 B.C. made a voyage of discovery northwards along the west coast of Europe, which is one of the most remarkable in history. He visited England, Scotland, the Scottish isles, and probably also northern Norway, which he called Thule. He moved the limits of the known world from the south coast of England northward to the Arctic Circle. It seems probable that he made two or perhaps several voyages. He also discovered the northern coasts of Germany as far east as Jutland.
We hear of no other voyages towards the Arctic regions before the Irish monk Dicuil, writing about 825, mentions the discovery by Irish monks of a group of small islands (the Faeroes), and a greater island (Iceland), which he calls Thule, where there Irish Discovery of Iceland.was hardly any night at midsummer. It is possible that Iceland and the Faeroes were inhabited by a small Celtic population before the Irish monks
