Light; a course of experimental optics, chiefly with the lantern . r half-wave retardations of the calciteare therefore in two quadrants augmented by an additionalquarter-wave retardation from the mica; and in the inter-mediate quadrants counteracted by a quarter-wave retardationof the opposite polarising plane. The result is thereforea dislocation by two quarters, or half a wave; whichbrings the bright rings of one against the dark rings of itsneighbouring quarter. 336 LIGHT. [CHAP. 182, Result of Polarising and Analysing —This having been demonstrated, place boihquarter-wave pla
Light; a course of experimental optics, chiefly with the lantern . r half-wave retardations of the calciteare therefore in two quadrants augmented by an additionalquarter-wave retardation from the mica; and in the inter-mediate quadrants counteracted by a quarter-wave retardationof the opposite polarising plane. The result is thereforea dislocation by two quarters, or half a wave; whichbrings the bright rings of one against the dark rings of itsneighbouring quarter. 336 LIGHT. [CHAP. 182, Result of Polarising and Analysing —This having been demonstrated, place boihquarter-wave plates in their usual positions, and the calcitein the crystal stage between them. Another very beautifulmodification follows. With analyser crossed or parallel, allcross lines or dislocations have vanished, leaving only perfectcircular coloured rings (D, Plate VII.). But as the analyseris rotated, alternate quadrants expand and contract in abeautiful manner, till at 45° we have the dislocated quadrants,and at 90° the complementdry perfect rings to those in the. Fig. 188. -Uni-axial Crystal Circularly Polarised. first position ; the successive appearances being representedin Fig. 188, A, B, c. J The appearances in a bi-axial, such as nitre, are analogousand beautiful, all brushes having vanished, and each axisbeing surrounded by unbroken rings (E, Plate VII.). Andlastly, at the point when the rings are perfect and unbroken,if either the crystal stage, with all it bears—crystal, quarter-wave plate, and analyser—or even only the quarter-wave plateand analyser, be rotated, no change whatever occurs; the ringsremain unbroken, and all trace of polarising planes, as theory : SPIRAL FIGURES.
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Keywords: ., bookcentury1800, bookdecade1880, bookidcu3192403121, bookyear1882
