POLARISCOPE. 164 POLARISCOPE. -nrill all enter as the ordinary in the analyzer and l)e reflected and lost, and they are said to be 'crossed' (Fig. 4) ; the eye looking in at the eye- piece sees a dark field, if a plate of transparent Fig. -. :,uu;.L MDKRG POLARISCOPE. The polarizing mirror iH Hhciwu in place, while above at^ taclied to the uprights are lenses for prnclucinK parallel, divergent, or c<»llvergent beams of light, and'a snpport on which thin plates or other olijects tn be stndied may be placed. The buniUe of thin jilates at the top used as an anal.v7.er may be replaced b,v a Nicol prism, shown to the right at the liase of the instrument, or by a mirror at the polarizing angle, shown to the left. material be introduced between the polarizer and analyzer, it may happen that the field of the crossed combination will become lifrht or colored. This "will mean that the material has in some way affected the ray emerging from the polarizt'r so that it is no longer entirely cut off by the analyzer. Such substances are in general said Polarizer Object FlO. 3. POLABISCOPE WITH POLIBIZING PBI8MS PABAI/LEL. Analyzer ■Pariller Polarizer Object L <S Analyzer "Crossed' FlO. 4. POLARISCOPE WITH POLARIZING PRISMS "CROSSED." to be 'double-refracting.' Substances which do not so afTect the polarized beam are called 'iso- tropic' or '.single-refracting.' The double-refract- ing substances are generally crystalline and fall into two broad classes, uniaxial and bia.vial. Tn certain double-refracting crystals there is one direction in which all light travels with the same velocity, and hence no double refraction occurs. This direction is called the optic axis and such substances are uniaxial. In biaxial crj'stals there are two directions, or optic axes. along which all liiiiil travels v.'ith the same velocity, and around which the optical properties group themselvcs. Sulistances are studied in the polariscope in either parallel or convergent (or divergent) light, and the (hiulile-refractiiig media in general produce two beams from the original beam coming from the ])olarizcr. and these two new beams, traveling with ditl'ereiit velocities, are made to interfere when their components are brought into the plane of the aiialyz<-r. As white light is generally emjdoyed at the polarizer, and a» the kind of interference is a function of the wave length, colors arc usually seen at the analyzer. A complete study of these color plie- nomcna. and their general behavior in the polari- scope. make it possible to determine the identity of most minerals from their optical properties. This branch of optics has developed into micro- scopic petrography, which has so supplemented the other tools of the mineralogist and geologist that these sciences have been revolutionized. Certain substances, as quartz parallel to the optic axis, solutions of sugar, tartaric acid, etc,, possess the peculiar property of rotating the plane of polarization to an extent depending upon the wave length of the light and the thickness of the layer traversed. That is to say, if the polariscope is set 'cros.sed' and a plale of quartz cut perpendicular to the axis introduced, the eye-jiieee field will ajipear lighter if monochro- matic light is used, and it will be necessary to rotate the analyzer in order to again obtain the dark field. This rotation may lie clockwise as viewed from the polarizer, "right rotating' or "dextro-rotary,' or it may be the reverse, 'left rotating' or 'levulo-rotary.' If white light is used, colors will appear in the field of view, and the tint 'cros.sed' will always be complementary to that seen in the 'parallel' position. These colors are in general due to the fact that, iis the rotation for different colors is different, certain ones will be passed through the analyzer and others cut off. The rotation of the plane of polarization in solutions of cane sugar is made use of commer- cially on a very large scale in testing raw sugar for its content of crvstallizable sugar. Very many forms of apparatus have been devised for this especial purpose. One of the more com- plicated, due to Soleil, is shown in Fig. n ; P is the tube in which the sugar solution is placed: C is a doulile quartz plate, half right and lialf left rotating, which affords a very sensitive means of setting the instrument to the same adjustment each time; B is the polarizer; A is a combination of a Nicol prism and a quartz plate, which by rotation enables the observer to correct for the color of the solution and obtain the most sensitive tint in the double quartz plate, C; F is the analyzer; G is a small (lalileaii telescope focused upon C; and E is the cmii- pensating system which corrects the rotation due to the solution instead of following it up. .■ plate of right quartz, h, is just neutralized by wedges of left quartz, n, when they are as shown at a. If the solution rotates right, then the wedges are slid together so as to offer an excess of left rotation over h and neutralize the right rotation of the solution. If the solu- tion is left rotating, the wedges are slid apart and the excess of right rotation of h neutralizes the rotation of the solution. The motion of the wedges is controlled by a rack and pinion, and
