IOO CHOICE OF A MICROSCOPE [Ch. Ill 



rapid movement with rack and pinion, and a fine adjustment by means 

 of a micrometer screw. Both adjustments should move the entire 

 tube of the microscope. The body or tube should be short enough 

 for objectives corrected for the short or 160 millimeter tube-length. 

 It is an advantage to have the draw-tube graduated in centimeters 

 and millimeters. The lower end of the draw-tube and of the tube 

 should each possess a standard screw for objectives (fig. 64). The 

 stage should be quite large for the examination of slides with serial 

 sections and other large objects. The substage fittings should be 

 so arranged as to enable one to use the condenser or to dispense 

 entirely with it. The condenser mounting should allow up and 

 down motion (fig. 25). 



§ 171. Quality and cost. — Laboratory microscopes which will 

 answer nearly all the requirements for work in Biology, including His- 

 tology, Embryology, Pathology, and Bacteriology, are listed in the 

 makers' catalogues at about $65— $75. The less expensive microscopes 

 are listed at $25-145. Fortunately in the State of New York the 

 State pays half for high school apparatus, so that there is no reason 

 why each high school should not be properly equipped with micro- 

 scopes of good grade. To avoid misunderstanding it should be 

 added that the quality of the oculars and objectives on the cheaper 

 microscopes is the same as for the best laboratory microscopes. 

 The mechanical work also is of excellent quality. 



During the last few years great vigor has been shown in the micro- 

 scopical world. This has been stimulated largely by the activity in 

 biological and chemico-physical science and the widespread appre- 

 ciation of the microscope, not only as a desirable, but as a necessary 

 instrument for study and research. The production of the new kinds 

 of glass and the apochromatic objectives has been a no less potent 

 factor in promoting progress. 



Markers and Mechanical Stages 



Markers are devices to facilitate the finding of some object or part 



which it is especially desired to refer to again or to demonstrate to 



a class. The mechanical stage makes it much easier to follow out a 



series of objects to move the slide when using high powers, and for 



