Elements of natural philosophy (Volume 2-3) . ptical axis ; it is only necessary,therefore, to convert the crystal, by grinding, into aprism whose refracting faces shall be parallel to theaxis, when both the ordinary and extraordinary indexmay be ascertained by the method explained in § (25).To distinguish between the rays, it will, in general, besufficient to move the prism so as to give the plane ofincidence a slight inclination to its length, as in that casethe extraordinary ray will be thrown out of this plane,and thus become Iceland spar, These values forIceland spar j. m0 = 1,65
Elements of natural philosophy (Volume 2-3) . ptical axis ; it is only necessary,therefore, to convert the crystal, by grinding, into aprism whose refracting faces shall be parallel to theaxis, when both the ordinary and extraordinary indexmay be ascertained by the method explained in § (25).To distinguish between the rays, it will, in general, besufficient to move the prism so as to give the plane ofincidence a slight inclination to its length, as in that casethe extraordinary ray will be thrown out of this plane,and thus become Iceland spar, These values forIceland spar j. m0 = 1,6543,me = 1,4833; hence, Its naturededuced fromformula (112); A = 0,60449,B = 0,67417; the ellipsoid is, therefore, oblate ; and the coefficient ofsin2 d, negative. Tourmaline, beryl, emerald, apatite, &c,also belong to this class. Quartz, ice, zercon, oxide oftin, &c, give the coefficient of sin2 d positive ; they are,other substances therefore, of the positive class, and the ellipsoid is pro- classifitd. llte ELEMENTS OF OPTICS. 343. § 148. If a plane waveW W\ of common light beincident on the upper sur-face of a crystal of Icelandspar to which it is parallel,this wave will be resolvedinto two components, one ofwhich will take the directionof and be normal to an ob-lique line P e, and will berefracted according to theextraordinary law ; the otherwill preserve its original course and pass through without deviation. These waveswill both leave the crystal normal to that plane of prin-cipal section which is perpendicular to its upper face,the waves themselves becoming parallel; each will beplane polarized, the plane of polarization of the ordinarywave coinciding with the plane of principal section justnamed, and that of the extraordinary wave being atright angles to it. If these component waves be received upon the uppernirface of a second crystal of the same kind, and whoseoptical axis is parallel to that of the first, they will takethe directions e e and of o\ parallel, respect
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