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 



