THE POLARLMETER 



481 



Crossed spider lines arranged in the instrument aid in giving definition to the 

 critical position. This device is used in Wild's polarimeter. 



The Senarmont polariscope-^ is a composite plate of quartz made up of 

 four similar right-angled wedges of this material. The wedges are united 

 two and two along their h3'potenuse5, and are cut in such a manner that 

 light enters and leaves by surfaces perpendicular to the optic axes. Each 

 of the four wedges which make up the plate is opposed vertically and hori- 



FiG. 300 



zontally by a fellow-wedge of opposite optical activity. When mounted 

 between two nicols interference bands are seen. With parallel nicols the 

 bands take the form shown in Fig. 300, but in all other positions the lines in 

 the upper and lower parts of the field are not continuous, as in Fig. 301. 

 This device is used in an instrument of Trannin,*^ and in another by Duboscq 

 and Duboscq. 2^ 



Compensation.— By compensation in polarimetry is meant the means 

 adopted to restore the plane of polarization to that originall}' occupied at 

 zero of the scale connected with the compensation device. 



Rotation compensation. — The most direct, accurate, and simple means of 

 compensation lies in rotation of the analyser (or polarizer) through an 

 angle equal in magnitude but opposite in sign to that through which the 

 plane has been rotated bj' the active material whose rotation is being ob- 

 serv^ed. This means is used in those instruments which employ mono- 



FiG. 302 



chromatic light, but is inapplicable to those using white light unless the 

 rotation to be measured is very small. The analyser is the element usually 

 rotated, and to it is attached an alidade moving over a graduated circle, 

 whence is read off the rotation required to effect compensation. 



Introduction into Field of Opposed Rotation. — This means is used in the 

 quartz wedge compensator of Soleil and Duboscq^o. It consists of a device 

 whereby a variable thickness of active quartz may be interposed and a 



2K 



