[ 492 ] 

 LVIII. Intelligence and Miscellaneous Articles. 



ON THE ROTATORY POLARIZATION OF QUARTZ. 

 BY J.-L. SORET AND ED. SARAZIN. 



nniXE angle which a plate of quartz, cut perpendicular to the axis. 

 -"- rotates the plane of polarization of light was determined by M. 

 Broch*, and more recently by M. Stefan f, for rays of various re- 

 frangibilities, between the limits of the solar lines B and H. We 

 have thought it would be interesting to extend those determinations 

 to the ultra-violet rays by means of the spectroscope with fluores- 

 cent ocular formerly described by one of us % We have also mea- 

 sured the rotation in the least-refrangible part of the spectrum, for 

 the wave-lengths of the lines a and A, which, to our knowledge, had 

 not previously been examined in this respect. 



We have adopted the method of Eizeau and Foucault, which had 

 been also employed by Broch and Stefan. The solar light, reflected 

 horizontally by a metallic mirror (glass silvered), was concentrated 

 by a convergent lens of 12 millims. aperture and about 1-15 metre 

 focal distance. A little before reaching the focus of the lens the 

 pencil passed through a large Nicol prism, afterwards a quartz plate 

 cut perpendicularly to the axis, then a second Mcol fixed upon a 

 graduated circle, finally entering a spectroscope, the slit of which 

 was placed at the focus of the lens. 



The spectrum observed under these conditions is crossed, as is 

 known, by black bands, the closeness of which increases in precisely 

 the same ratio with the thickness of the quartz plate. By turning 

 the analyzing Mcol, one of these bands can be brought into coinci- 

 dence with such or such a line of the solar spectrum ; and from the 

 angle through which it was necessary to turn it the rotation of the 

 light of corresponding refrangibility can be deduced. In the ultra- 

 violet part of the spectrum, observed with the fluorescent ocular, the 

 black bands manifest themselves in the same manner; and here we 

 have been able to extend the determinations as far as the line 1ST, 

 which can be distinguished with sufficient distinctness by emp^irg 

 apparatus of optical glass and a prism of white flint glass. Tor the 

 observation of the lines a and A it suffices to place before the slit 

 of the spectroscope a plate of blue cobalt glass, which permits the 

 extreme red to pass, absorbing the adjacent more refrangible radia- 

 tions. These two lines are then readily distinguished, and, at least 

 for a, the measurements can be made with as much precision as for 

 the rest of the spectrum. 



The results which we have obtained with a plate of left-handed 

 quartz of 30*085 millims. thickness are recorded in the following 

 Table :— 



* Dove's Repertorium, vol. vii. p. 115 J Annates de Chimie et de Physique, 

 ser. 3, vol. xxxiv. p. 119. 



t Sitzungsberichte der Wiener Ahad. vol. 1. p. 88. 



X Bibliotheque Universelle, Archives des Sciences Phys. etNat. vol. xlix. 

 p. 338, 1874 ; Journal de Physique, vol. iii. p. 253. 



