192 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



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 amovmt 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- 



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 

 lino 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 0° position the two halves of the field appear equally 

 illuminated : in any other position, or if rotation has been produced by the 

 solution when the nicols have been set at zero, the two halves appear un- 



