. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 134 CEYSTALS CUT ACROSS THE AXIS laminte whose thicknesses are the differences of those employed in the experi- ment. This supposes that the hamina; are of the same kind. If they are not, the actual thicknesses arc not to be employed in the calculations, but what may be called the hi-refr'mgent equivalents of thickness—that is to say, their measured thicknesses divided by the numbers expressing their chromatic relation to the plates of air
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 134 CEYSTALS CUT ACROSS THE AXIS laminte whose thicknesses are the differences of those employed in the experi- ment. This supposes that the hamina; are of the same kind. If they are not, the actual thicknesses arc not to be employed in the calculations, but what may be called the hi-refr'mgent equivalents of thickness—that is to say, their measured thicknesses divided by the numbers expressing their chromatic relation to the plates of air which give Newton's rings—which latter numbers may be called their chromatic equivalents. If, then, the difference of these quotients, multi- plied by th-G chromatic equivalent corresponding to the greater quotient, is ^vithin the limits at which the crystal to which the greater quotient belongs gives colors, the combination will give the color belonging to the value of that product. If the lamina? belong to crystals of which one is positive and the other negative, they are not to be crossed in this experiment, but their principal sec- tions must be parallel. This furnishes an easy test for determining whether a given crystal'is positive or negative. Having prepared a lamina of the crystal to be examined, (which may be of any convenient thickness,) apply it upon lamina? of Iceland spar of different thicknesses, with the principal sections suc- cessively parallel and crossed. If the colors appear v>^hen the planes are parallel, the signs are opposite, since, either plate being too thick to produce color alone, the sum of their effects cannot, of course, do so. If the crystals are of similar sign, the colors will appear when the planes are crossed. Another class of chromatic effects produced by crystalline plates viewed in polarized light was first observed by Dr. WoUaston in Iceland spar, in vrhich the display is, perhaps, the most brilliant. In these cases, the crystal i
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