. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. CRYSTALS OF TWO AXES. 139 /. The rings seen ii\ ciystals cut across tlie axis, wlien examined in cii'cularly polarized light, exhibit some singular peculiarities. They are divided into quadrants by a cross which is neither very daik nor very biig^t, and which does not change in intensity witli the revolution of the analyzer, but turns with it. The rings in the alternate quadrants are vncovformahle, those in one opposite pair being nearer to
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. CRYSTALS OF TWO AXES. 139 /. The rings seen ii\ ciystals cut across tlie axis, wlien examined in cii'cularly polarized light, exhibit some singular peculiarities. They are divided into quadrants by a cross which is neither very daik nor very biig^t, and which does not change in intensity witli the revolution of the analyzer, but turns with it. The rings in the alternate quadrants are vncovformahle, those in one opposite pair being nearer to the centre, and those in the other more distant from the centre, by a quarter of an interval, than the corresponding rings in plane polarized light. This singular arrangement is shown in Fig. 22. Mr. Airy found that light may be circu- larly polarized by refraction, in passing through laminte of crystals which doubly refract; provided the thickness of the laminai used is such as, ou the undulatory theory of light, is just sufficient to effect a. retardation of one of the rays produced by the double refraction, one- quarter of an undulation behind the other, or to advance it one-quarter of an undulation before the (ither. The mineral employed by him for this purpose, and which is more conveniently prepared of suitable thickness than most others, is mica; of which the laminae are easily separable, and cleave in large sheets without breaking. A lamina reduced to a thickness proper to produce circular polarization is commonly called a " quarter-wave ; For some time after the discoveries had been made of which a brief account has here been given, it was supposed that all doubly refracting crystals have but a single optic axis. In the year 1817, however. Sir David Brewster announced the remarkable fact that most crystals have two optic axes instead of one. The rings seen in crystals of two axes are elliptical, when the axes are so far apart that only one can be obse
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