Handbook of the polariscope and its pracitcal applications / adapted from the German editon of H. Landolt, by D.C. Robb and V.H. Veley.

Landolt, H. (Hans), 1831-1910.

Date:: 1882

Credit: Handbook of the polariscope and its pracitcal applications / adapted from the German editon of H. Landolt, by D.C. Robb and V.H. Veley. Source: Wellcome Collection.

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$equal to double the previous angle of rotation. If again brought back to the north end by a second reflection, this third trans- mission of the ray through the refractive medium will carry the analyzer placed at the north end through an angle equal to three times the original angle of rotation, and so on. The same thing happens when circular polarization is induced by an electric current the rotation always taking the direction of the induction-current from the observer’s stand-point. Magnetic rotation, not being a property of the chemical mole- cule, need not be further discussed here. § 12. The optical theory of circular polarization in quartz is due to Fresnel.1 According to him there occurs in quartz, in a direction parallel to the main axis of the crystal, a peculiar kind of double refraction, whereby a linear polarized ray on entering is decomposed into two rays, each of which pursues a helical course, the one turning to the right, the other to the left. On emerging, the two circular-polarized rays unite into a single linear-polarized ray again, but if the velocities with which they have traversed the refractive medium have been unequal, the plane of vibration of the emergent ray will have a different direction to what it had originally. It will follow the hands of a watch—that is to say, it will have rotated to the right—if the circular-polarized ray turning in that direction has had the superior velocity, and vice verm. The existence of these divided rays in rock-crystal was experimentally established by Fresnel, and subsequently by Stefan,2 and also by Dove,3 who found that in coloured quartz (amethyst) they were unequally absorbed. The theory of circular polarization has since been treated mathematically by Clebsch,4 Eisenlohr,5 6 Briot,G v. Lang7 and others. Regarding the structure requisite in a crystalline medium to produce rotation of the plane of polarization, a theory has been proposed of an unequal condensation in certain directions of the 1 Fresnel: Ann. Chim. Phys. [1] 28, 147; Wullners JMuch derPhys. 8 AufL, 2, 589. 2 Stefan: Pogg. Ann. 124, 623. 3 Dove : Pogg. Ann. 110, 284. 4 Clebsch: Crellds Joitrn. f. Math. 57, 319. 5 Eisenlohr: Pogg. Ann. 109, 241. 6 Briot: Comptes Pend. 50, 141. 7 v. Lang: Pogg. Ann. 119, 74. Erg. Bd. 8, 608.$