. Polarisation of light . eincident beam, it will be found that, by bringing thevibrations alternately into and perpendicular to theplane of incidence, we shall render the intensity ofthe reflected and refracted rays alternately a much for the verification of the law. But notonly so ; if we take different fluids, and for each ofthem in succession adjust the incident beam in thesame manner, we shall only have to read off the angle I.] POLARISATION OF LIGHT. II of incidence in order to ascertain the polarising angleof the fluid under examination. The polarising angle for glass is 54
. Polarisation of light . eincident beam, it will be found that, by bringing thevibrations alternately into and perpendicular to theplane of incidence, we shall render the intensity ofthe reflected and refracted rays alternately a much for the verification of the law. But notonly so ; if we take different fluids, and for each ofthem in succession adjust the incident beam in thesame manner, we shall only have to read off the angle I.] POLARISATION OF LIGHT. II of incidence in order to ascertain the polarising angleof the fluid under examination. The polarising angle for glass is 54° 35. Returning to the general question of polarisationby reflexion, and calling to mind a principle statedabove, viz. that any process which will serve forpolarising will serve also for analysing, it follows thatwe may replace the analysing tourmalin by a secondplate of glass (or whatever substance has been usedfor the first reflexion), placed parallel to the first, andin such a position as to receive the reflected Thus, in Fig. 7, let a hh^ the incident, and b c thereflected ray at the first plate ; b c the incident, andc d the reflected ray, at the second plate ; then theray b c will be polarised more or less, according tothe angle of incidence, at b, and will be analysed atc. If the second plate be then turned round the ray 12 POLARISATION OF LIGHT. [chap. b c reflected from the first plate as an axis, it will befound that at two positions of rotation (first, when theplates are parallel, and, secondly, when one of themhas been turned through i8o°) the light reflectedfrom the second plate is brightest, and at two posi-tions at right angles to the former the reflected rayis least bright. The degree of dimness at the twopositions last mentioned will depend upon the accu-racy with which the reflecting plates have beenadjusted to the polarising angle ; and when this hasbeen completely effected, the light will be altogetherextinguished. Suppose, now, that the reflecting s
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