166 



ELEMENTARY CHEMICAL MICROSCOPY 



natant liquid. The apparatus is especially useful in cases 

 where fractional separations through variable rates of subsidence 

 can be practiced. 



The Bates Polarization Tube. — It sometimes happens that 

 an approximate determination is wanted of the specific rotatory 

 power of a substance, but no polarimeter is at hand although 

 a chemical microscope with polarizer and analyzer is available. 

 By introducing a tube of a solution of the substance to be studied 

 into the tube of the microscope, we can convert this instrument 

 into a polarimeter. A convenient form of observation tube for 

 this purpose is the Bates 1 polarization tube, Fig. 104. The tube 



Kf 1 '; ;;;'•» 



rn 



is filled with a solution of the substance and placed 

 within the draw-tube of the chemical microscope, 

 thus converting the instrument into a Mitscherlich 

 polarimeter of simplest possible construction. 



The results obtained are approximate only, since 

 the graduated 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 

 microscope with inserted tube is not to be regarded 

 as a substitute for a polarimeter, but as a device use- 

 ful 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 micro- 

 scope. Rack the body tube down as far as it will go. 

 Insert the empty tube in the tube of the instrument; 

 cross the nicols and note that their zero points are correctly 

 placed. Fill the tube with the solution to be examined and illu- 

 minate 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 filters giving yellow, 

 approximately monochromatic light. 



1 Made by the Bausch & Lomb Optical Co., Rochester, N. Y. 



Fig. 104. 



Polarization 



Tube. Bates 



Type. 



