60 ELEMENTARY CHEMICAL MICROSCOPY 
ated head, micrometric measurements of thickness are possible, 
and refractrometric determinations are simplified. 
3. The body-tube carrying the objective and eyepiece should 
be of sufficient diameter to permit the microscope being used 
for photography, and it should be provided with an inner grad¬ 
uated draw-tube whose lower end is tapped with standard or 
universal thread for the attachment of very low power objectives 
or of amplifiers. 
4. The stage should be circular, rotating and provided with 
centering screws with small milled heads. The circumference 
of the stage should be graduated in degrees and the surface 
covered with hard rubber. The stage must be constructed in 
such a manner as to be easily removed by simply loosening the 
centering screws, in order that thick objects may be examined, 
various heating devices employed, and opaque objects to be 
studied by means of vertical illuminators may be brought into 
focus on the substage without interfering with the proper ad¬ 
justment of radiant or illuminator. 
5. The substage should consist of a simple ring, raised and 
lowered by screw or rack and pinion, and must permit of being 
swung to one side from under the stage. This ring carries 
condenser, polarizer, auxiliary stage, heating devices, etc. The 
ring should be tapped at one side and fitted with thumb-screw 
or with some sort of locking device to hold firmly in place the 
accessories fitting into the substage ring. The substage ring 
should also be provided with a slot or other contrivance for 
lining up the polarizer. If the microscope is to be used chiefly 
for observations at high temperatures, polarization by reflection 
is best. 
6. It is essential that the microscope be fitted with attach¬ 
ments for study with polarized light including converging as 
well as plane. For all ordinary problems, the best system ap¬ 
pears to be rotating prisms of the type of the Nicol prism, one 
placed below the stage, the other above the objective. One or 
both of these polarizing prisms should be mounted so as to rotate 
and be provided with graduated circles. It will be found to be 
a great convenience if the construction is such that when polarizer 
