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 niicroscope 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 ^ polarization tube. Fig. 104. The tube 
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 
Fig. 104. as follows. Remove all condensing lenses from above 
Tubr^BaTes polarizer. Remove the objective of the micro- 
Type. 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. 
' Made by the Bausch & Lomb Optical Co., Rochester, N. Y. 
