Elements of natural philosophy (Volume 2-3) . greater velocityin the first shall have the less in the second. We haveonly to place the plates with their principal sections per-pendicular or parallel, according as the crystals to whichthey belong are of the same, or of opposite denomina-tions. Thus, if both the crystals be positive, or bothnegative, they are to be placed with their principal sec-tions perpendicular; and on the other hand, these sec-tions should be parallel, when one of the crystals is po-sitive and the other negative. The reason of this isevident. whera^Sd § 167; Let us now con
Elements of natural philosophy (Volume 2-3) . greater velocityin the first shall have the less in the second. We haveonly to place the plates with their principal sections per-pendicular or parallel, according as the crystals to whichthey belong are of the same, or of opposite denomina-tions. Thus, if both the crystals be positive, or bothnegative, they are to be placed with their principal sec-tions perpendicular; and on the other hand, these sec-tions should be parallel, when one of the crystals is po-sitive and the other negative. The reason of this isevident. whera^Sd § 167; Let us now consider the effects produced when wave traverses a a polarized wave traverses a imiaxal crystal, in various crystal; directions inclined to the axis at small angles; and let us suppose, for more simplicity, that the crystalline plate is cut in a direction perpendicular to the axis. Let ABOD be theplate, and iFthe place ofthe eye. The visible por-tion of the emergent beamwill form a cone, A EB,whose vertex coincides Methodexplained. Fi£. ELEMENTS OF OPTICS. 357 with the place of the eye, and axis E 0, with the axis Ray coincidingof the crystal. The ray which traverses the crystal in wih the axis J J undergoes no the direction of the axis, P 0E, will undrgo no change change; whatever; and will consequently be reflected or not from the analyzing plate, according as the plane of reflexion there coincides with, or is perpendicular to, the plane of first reflexion. But the other rays composing the cone will be modified in their passage through the crystal, and the changes which they will undergo will depend on their inclination to the optical axis, and on the position of the principal section with respect to the plane of pri- other rays win ... -i . ,. be modified. nntive polarization. Let the circle represent the sec-tion of the emergent cone of raysmade by the surface A B of thecrystal; and let MM and JVJ^\be two lines drawn through itscentre at right angles, being theinterse
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