140 ELEMENTARY CHEMICAL MICROSCOPY 



draw-tube of the chemical microscope, thus converting the instru- 

 ment into a Mitscherlich polarimeter of simplest possible con- 

 struction. 



The results obtained are approximate only, since the gradu- 

 ated circles usually attached to the analyzer (or polarizer) are of 

 such small circumference that the readings are rarely accurate 

 to even a degree; moreover, the end point is generally far from 

 being sharp. It is therefore evident that the polarizing micro- 

 scope with inserted tube is not to be regarded as a substitute for 

 a polarimeter, but as a device useful in qualitative analysis, and 

 offering a means of obtaining rough quantitative results. 



To employ the microscope as a polarimeter, proceed as follows. 

 Remove all condensing lenses from above the polarizer. Remove 

 the objective of the microscope. Rack the body tube down as 

 far as it will go. Insert the empty tube in the tube of the instru- 

 ment; cross the nicols and note that their zero points are cor- 

 rectly placed. Fill the tube with the solution to be examined 

 and illuminate with parallel light. Between radiant and plane 

 mirror place a plano-convex lens to assure parallel rays. It will 

 also generally be found essential to employ ray niters giving 

 yellow, approximately monochromatic light. 



It is even better to incline the body of the microscope until 

 the tube is in a horizontal position, swing the mirror to one side 

 and project the beam of parallel light upon the polarizer. A 

 dark cloth thrown over the instrument and the head of the ob- 

 server prevents light from entering between the polarizer and the 

 tube of the microscope and any side light from entering the eye. 



Upon looking into the microscope, the field will no longer be 

 dark gray or black. Turn the analyzer until the field again 

 acquires its maximum darkness and read the scale. The amount 

 of displacement to the right or left, as the case may be, is the 

 rotation of the solution. Dextrorotatory substances give a 

 smaller angle when the nicol is turned to the right, to obtain 

 maximum darkness, than when turned toward the left; while 

 laevorotatory substances will give the smaller angle when the 

 displacement from zero is to the left than when to the right. In 

 all cases a series of angle measurements should be made and the 



