Elementary chemical microscopy elementarychemi00cham Year: 1921 ILLUMINATION OF OBJECTS; POLARIZED LIGHT 53 Fig. 23. Position of the Prisms with Nicols Crossed. The lower nicol placed below the object is called the polarizer; the upper nicol, above the object, the analyzer, since it serves to examine or analyze the light transmitted by the object. For the best results a nicol prism must be about two and one-half times as long as it is thick. A long prism for the analyzer is cumbersome and undersirable, therefore a calcite prism ce- mented with some resin having a different refractive inde
Elementary chemical microscopy elementarychemi00cham Year: 1921 ILLUMINATION OF OBJECTS; POLARIZED LIGHT 53 Fig. 23. Position of the Prisms with Nicols Crossed. The lower nicol placed below the object is called the polarizer; the upper nicol, above the object, the analyzer, since it serves to examine or analyze the light transmitted by the object. For the best results a nicol prism must be about two and one-half times as long as it is thick. A long prism for the analyzer is cumbersome and undersirable, therefore a calcite prism ce- mented with some resin having a different refractive index than Canada balsam is generally em- ployed ; these devices are known as Thompson, Glan, Ahrens, etc., prisms after the men invent- ing Anisotropic crystals so act upon plane polarized light passing through them as to resolve the ether vibrations into two com- ponents polarized at right angles, the planes of vibration of which are not coincident with the plane of vibration of the analyzer. If a small transparent doubly refracting crystal, or a fragment of a transparent anisotropic substance be placed upon the stage of the microscope, brought under the cross-hairs of the eyepiece and examined between crossed jiicols, it will be found that the crystal or the fragment becomes alternately bright and dark as the stage is rotated. In the bright positions it may even become brilliantly colored. The bright and dark positions with reference to the cross-hairs together with the presence or absence of polarization colors are of great assistance in identifying the material being studied. The behavior of crystals under polar- ized light is discussed in Chapter XI. In order to conveniently study the effect of the crystals upon the polarized light issuing from the polarizer, it is best that the polarizer be so mounted as to permit rotation, and in many cases it will be found a great convenience if the mount is pro- 1 For a very comprehensive description of the various types of prisms,
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