246 ROTATORY POLARIZATION. 



of rock-crystal of a suitable thickness, and of the opposite 

 action. 



(250) The quantity to be measured, in all such experi- 

 ments, being the angle through which the plane of polarization 

 has been caused to rotate, it is necessary to determine with 

 precision the position of that plane before, and after, the 

 interposition of the rotating liquid. Now this determination 

 cannot be made with the required accuracy by means of Ni- 

 col's prism, or any of the other analyzers heretofore in use ; 

 for, in all of these, the position in which the intensity of the 

 light is evanescent, or a minimum, can only be known by 

 the comparison of two lights which are not simultaneously 

 observed. To remedy this defect, Mr. Jellett has devised an 

 analyzer, in which the two lights to be compared are seen 

 at the same time, and in juxtaposition. A rhomboid of 

 Iceland spar, about two inches in length, is cut by two planes 

 perpendicular to the longitudinal edges, so as to form a right 

 prism. This prism is then divided by a plane parallel to 

 the edges, and making a small angle with the longer dia- 

 gonal of the base. One of the two halves is then reversed, 

 and the two parts are cemented together with the surfaces 

 of section in contact. Finally the two ends are ground and 

 polished. 



Now let a cylindrical pencil of plane-polarized light tra- 

 verse such a prism in a direction parallel to the edges, so as 

 to be equally divided by the plane of section, each half of the 

 cylinder traversing one-half of the compound prism. Each of 

 these semi-cylindrical pencils will be doubly-refracted, and 

 the two extraordinary pencils will be made to deviate nearly 

 in opposite directions. These two pencils are to be stopped 

 by diaphragms suitably placed. On the other hand, the two 

 ordinary pencils are undeviated, and will therefore compose 

 a cylindrical pencil which is equally divided by the plane of 



