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ESSENTIALS OF CHEMICAL PHYSIOLOGY 



and scale (E in fig. 75) worked by the screw which moves the compensator. 

 If the solution is Isevo -rotatory, the screw must be turned in the opposite 

 direction. 



Zeiss's polarimeter is in principle much the same as Soleil's ; the chief 

 difference is that the rotation produced by the solution is corrected, not by a 

 quartz compensator, but by actually rotating the analyser in the same 

 direction, the amount of rotation being directly read off in degrees of a 

 circle. 



Laurent's polarimeter is a more valuable instrument. Instead of using 

 daylight, or the light of a lamp, monochromatic light (a sodium flame pro- 



M 



FIG. 76. Laurent's polarimeter. 



duced by volatilising common salt in a colourless gas flame) is employed 

 the amount of rotation varies for different colours ; and observations are 

 recorded as having been taken with light corresponding to the D or sodium 

 line of the spectrum. The essentials of the instrument are, as before, a 

 polariser, a tube for the solution, and an analyser. The polarised light 

 before passing into the solution traverses a quartz plate, which, however, 

 covers only half the field, and retards the rays passing through it by half a 

 wave-length. In the position the two halves of the field appear equally 



