. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. CRYSTALLINE PLATES IN POLARIZED LIGHT. 205. In the fourth place, the principal plane of the lamina shonld, in order to produce the most complete interference, be at an azimuth of 4''3'- to the plane of original polarization—the two rays being, in this position of the crystal, exactly e(jual to each other. In the fifth place, we must observe the phenomena by means of an analyzer, which allows only the light polarized in a single plane to com
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. CRYSTALLINE PLATES IN POLARIZED LIGHT. 205. In the fourth place, the principal plane of the lamina shonld, in order to produce the most complete interference, be at an azimuth of 4''3'- to the plane of original polarization—the two rays being, in this position of the crystal, exactly e(jual to each other. In the fifth place, we must observe the phenomena by means of an analyzer, which allows only the light polarized in a single plane to come to the eye, or which, like a doubly refracting prism, separates the emergent light which is polarized in one plane from that Avhicli is i)olarized in tlu; trairs- verse plane; otherwise in this case again we shall have; the blended efiects of two compensatory interferences. Finally, the principal plane of the analyzer should, in order to produce the best effect, be at an azimuth of 45° from the principal plane of the lamina. The necessity of this coiidition may be readily deduced from the law of Mains. The annexed figure may illustrate the changes which take place in the passage of the ray ^Bco through the system. If the arrow, P, rep- _2?/ ^-^^^^^^M^ resent the direction of an elementary mo- ^^^f'"" lecular movement of the original polarized ray, this movement will be resolved in the lamina into two movements at right anglea ''^' "?'' to each other, and each inclined 45° to V, as shown by the arrows Ro {^^(^ ordinary ray) and Ilg (the extraordinary.) Suppose these rays to emerge, without difference of path, from the lamina, and to be received upon a crystal of Iceland sj)ar whose principal plane coincides with the direction of P. .Then R^ will be resolved into Rqo and Roe; and R^ will be resolved into Roo and Rye- Rqo and R^o will conspire, and Roo and R^.^ will conflict. The first pair, on the supposition we have made, v.'ill only bo effective. The second
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