THE MICROSCOPE 43 



the modern microscope represents optical and mechan- 

 ical ingenuity of the highest order. 



The microscope rests upon a horse-shoe shaped 

 casting termed the base. This base, of cast iron or 

 brass, may be hollowed out, and the cavity filled with 

 lead, to insure greater stability. Arising from the 

 base is the pillar, which is usually jointed so that the 

 instrument may be inclined toward the worker. At 

 a point above the inclination joint, the stage is attached. 

 The stage serves as a rigid support for the specimen, 

 and is perforated to accommodate the condenser. 

 Branching from the upper part of the pillar is the 

 arm, to which is attached the tube bearing the ocular 

 and objectives. The tube is usually fitted with an 

 inner tube or draw-tube, which may be used to vary 

 the distance between objectives and ocular. The 

 draw tube is usually graduated in millimeters and 

 should be drawn out to the figure 160 (or 16) as this 

 is the tube length for which microscope lenses are 

 ordinarily corrected. The ocular slips into the upper 

 end of the tube; but the lower end is provided with 

 a circular metal plate, the nosepiece, with threaded 

 collars in which the objectives fit. Nosepieces accom- 

 modate two, three or four objectives, and turn on a 

 pinion so that the different objectives may be brought 

 into place at the lower end of the tube. The con- 

 denser mounting, with iris diaphragm and mirror 

 support, is usually attached to the under surface of 

 the stage, and these devices are collectively known 

 as the substage. 



A microscope is fitted with two adjustments for 

 focusing or changing the position of the tube, with 

 relation to the specimen. The coarse adjustment con- 



