. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 136 AirA S f-TIRALS—FEESKEL S PRISM. %/ ^-^^ixiJii^ It will be easily conceived tliat a right handed and a left-lianded crystal of equal thickness?, superposed upon each other, will produce a resultant rotation equal to zero. But two such plates so super- posed, examined in polarized light, ex- hibit a remarkable spiral cross, such as is seen in the figure annexed. These spi- rals were first observed by Mr. Airy, and are commonly known as A
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 136 AirA S f-TIRALS—FEESKEL S PRISM. %/ ^-^^ixiJii^ It will be easily conceived tliat a right handed and a left-lianded crystal of equal thickness?, superposed upon each other, will produce a resultant rotation equal to zero. But two such plates so super- posed, examined in polarized light, ex- hibit a remarkable spiral cross, such as is seen in the figure annexed. These spi- rals were first observed by Mr. Airy, and are commonly known as Airy's spirals. Two contrary plates of unequal thick- ness, superposed as above, produce an amount of rotation proportional to their difference of thickness. The power of rotation of the same crystal is different for the different colors, being, on the undulatory theory of light, an inverse function of the length of the undulations. By employing the successive colors of the spectrum sepa- rately, Mr. Biot determined the absolute rotatory power of a crystalline plate of quartz one twenty-fifth of an inch in thickness, for each, as follows:. Extreme red Limit, red and orange Limit, orange and yellow— . Limit, yellow and green Limit, green and blue Limit, bhie and indigo Limit, indigo and violet Extreme- violet 'J'his property of rotatory polarization does not exist in plates of quartz cut parallel to the axis. In such plates ordinary double refraction exists ; but it is the extraordinary instead of the ordinary ray whose velocity is least, or the crystal is a positive one. The physical cause of rotatory polarization is unknown. Mr. Biot supposed it to belong to the ultimate molecules of the substance; but this hypothesis Sir David Brewster believed to be disproved by the fact that the property ceases to appear in quartz Vi^hose crystalline structure has been destroyed by fusion. This argument seems, nevertheless, n
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