422 



PEACTICAL PHYSIOLOGY 



plates are dispensed with, the amount of rotation being measured by 

 rotating the next part of the instrument, namely the 



"" ! " :i " !:!: 



FIG. 267. Polarimeter of Mischerlich with Laurent's polariser. P, polariser and device 

 for obtaining half shadow ; R, fluid container ; 2', scale with vernier c attached to 

 pointer; A, compensator and analyser; F, lens. 



(5) Analyser, so as to obtain uniformity of tint in the two halves of 

 field. 



(6) A Lens. 



Fio. 268. Diagram of scale and field of vision of polarimeter. Above is represented 

 the scale for measuring the compensation necessary. In the position represented in 

 the diagram the reading is 2 -7 dextro rotation. The lower part of the diagram shows 

 the three appearances of the field of the polarimeter, the central one representing the 

 appearance at zero, i.e. when there is no rotation. 



When the tube (3) is filled with water or an optically inactive fluid, 

 and the compensator or analyser rotated until a violet colour of 

 uniform tint fills the field, the indicator will be seen to stand at zero (if 

 not so, the error must be noted). If now, an optically active fluid be 



