Light; a course of experimental optics, chiefly with the lantern . mens, prepared by Mr. Fox, whicli are as perfect inrings, and brushes, and Airys spirals, as any quartzes could possibly be. 332 LIGHT. [CH. XV. such slices singly show h)perbolas; but if two such slicesare crossed at right angles, these become in convergentlight straight lines, called Savarts bands, and such a com-bination is a very sensitive test of polarised light, the leastdegree of polarisation making the bands visible. Slicesof calc spar split naturally and crossed give the sameresults (E, Plate VI.). All these phenomena
Light; a course of experimental optics, chiefly with the lantern . mens, prepared by Mr. Fox, whicli are as perfect inrings, and brushes, and Airys spirals, as any quartzes could possibly be. 332 LIGHT. [CH. XV. such slices singly show h)perbolas; but if two such slicesare crossed at right angles, these become in convergentlight straight lines, called Savarts bands, and such a com-bination is a very sensitive test of polarised light, the leastdegree of polarisation making the bands visible. Slicesof calc spar split naturally and crossed give the sameresults (E, Plate VI.). All these phenomena of quartz have been worked outmathematically from the theory, most of them by Mr. Airy. It may be mentioned here, that in these phenomena, asin all others, if the thin glass analyser. Fig. 155, be usedinstead of the Nicol, the complementary image to that on thescreen will always appear on the Complicated asthe phenomena may be, such an analyser never fails thus tosort them out rigidly into two complementary sets. ROTARY AND CIRCULAR A QfjuirtM,TiriBt,aiuilyaer crotsed,. D Caloite,poUtri*ecL£ analysed^ aircidarbr. B „ andfyseir paralleL. C JKtre, DMo Ditto. C Calioite,j>olaz%xeA circvSUwIy. F Qtuirtz in, cownrergent circularh^-pdlarizei, lyht. CHAPTER XVI. OPTICAL PHENOMENA OF CRYSTALS IN CIRCULARLY-POLARISED LIGHT. Modifications in Crystal Figures Produced by one Quarter-Wave Plate—Explanation of the Phenomena—Results of Polarising and AnalysingCircularly—Quartz in Circularly-Polarised Light—Spiral Figuresshowing the Relation of Uni-axial and Bi-axial Axes. Having traced the general phenomena of crystals whenexamined in convergent plane-polarised light, we nextproceed to examine the appearances they present incircularly-polarised light; being prepared by our previousexperiments with this description of light for some inter-esting variations of the phenomena. 180. Effects of a Single Quarter-Wave Plateon the Rings of Crystals.—We pl
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