Page:The American Cyclopædia (1879) Volume X.djvu/455

 LIGHT 449 last described, and a cone of converging or diverging rays is passed through it, or if the analyzer is brought so near to the eye that the visual rays converge toward its optic centre, brilliant colored rings are produced, differing in form according as the plate is uniaxial or FIG. 24. Tourmaline Pincetto. biaxial. A simple mode of viewing the phe- nomena is by employing the tourmaline pincet- to, fig. 24, a small instrument made by placing two tourmalines cut parallel to their axes in two metallic disks, a 5, so that they may be turned in parallel planes and their axes given any angle to each other. A spring presses them together, by which means the substance to be examined may be held in position. When the plate of Iceland spar or other uniaxial crystal cut perpendicular to its optic axis is E laced between the crossed tourmalines, and eld near to the eye and toward the light, the rings above mentioned, intersected by a black cross, fig. 25, will be observed. If the tour- malines are placed with their axes parallel, the FIG. 25. FIG. 26. cross will be white, as shown in fig. 26, while the order of colors will be complementary. If homogeneous light is used, as for instance red, the rings will be simply red and black. If the light is violet, the rings will be violet and black, and they will be smaller, their size vary- ing with the increase of ref rangibility from red to violet. To understand the formation of these rings, it must be remembered that rays of light which travel through the axis of a uniaxial crystal are alike in velocity, and there- fore no chromatic effects will be produced in the centre ; but the converging polarized rays, being inclined to the axis, will be divided into ordinary and extraordinary rays, with sufficient difference in phase to produce fringes by in- terference when reduced again to the same plane by the polarizer. The thickness of plate which the rays traverse increases with the divergence, so that at equal distances from the centre they will alternately conspire together or destroy each other, producing bright and dark rings. The explanation of the cause of the appearance of the crosses requires abstruse mathematical reasoning. The conclusions ar- rived at, however, are that in the two planes passing through the axis of the interposed plate, which are parallel and perpendicular to the axis of the polarizing tourmaline, the po- larized ray is not resolved into two components, and consequently there are in those directions no conditions for interference. Therefore, when the tourmalines have their axes at right angles the cross will be black, and when they are parallel it will be white. In biaxial crys- tals colored rings are produced having more complicated forms, the colored bands having the form of curves with two centres corre- sponding to the two optic axes of the crystal. If a plate of a biaxial crystal which has its optic axes inclined at a small angle, not ex- ceeding 5 or 6, is cut at right angles to the FIG. 27. FIG. 28. medial line and held between the tourmalines in such a manner that the plane of the optic axes is parallel to the axis of one of the tour- malines, an appearance represented in fig. 2V will be presented when the tourmalines are crossed. When the double-refracting plate is turned around in its plane, the rings turn in the same direction, while the cross separates into two branches, as seen in fig. 28; and when the plane of the axes makes an angle of 45 with the axes of the tourmalines, the ap- pearance seen in fig. 29 results, the branches of the cross having the form of hyperbolas. When the axes of the tourmalines are parallel, the colors are complementary and the cross is white. When a biaxial crystal is cut per- pendicularly to one of its optic axes, it will present the appearance shown in fig. 30. FIG. 29. FIG. 30. Rotation of Plane of Polarization. In the cases which have been considered the changes of plane which take place when a polarized ray is reflected or refracted are definite and independent of the distances through which it passes in either medium; but there are sub- stances which change the plane of polariza-