826 APPENDIX. 



are, need not be an expensive 6ne. T'drnierly microscopes 

 of any value cost at least fifty or sixty dollars ; now very 

 satisfactory ones maybe had for balf that money. It is not 

 the high magnifying power that should determine the choice 

 of microscopes or lenses, but mainly their penetrating and 

 defining power, the flatness of the field, etc. The system of 

 combination of lenses at the lower end of the tube is called 

 the objective. Objectives are generally named or desig- 

 nated by their focal distance : thuS, two inch, one inch, one- 

 half inch, one-quarter inch, one-eighth inch, etc. The first 

 being of very low maghifyihg power, the next higher, the 

 third still higher, and so on. The coinbination of lenses 

 and the frame which holds them at the upper end of 

 the tube of the microscope is fcalled the eye-piece. The 

 instrument without eye-pieces and objectives is called the 

 stand. The plate below the objective, on which the object 

 to be examined is placed, is called the atage. In the 

 middle of it is an -aperture, a short distance below which 

 is placed a small, round mirr&r, generally reversible, the 

 one side having a plane and the other side a concave 

 reflecting surface. It is movable, and is to be so adjusted 

 that it will reflect the light up through the object which is 

 placed on the stage to be examined. The tube holding the 

 eye-piece and objective is movable either by smoothly 

 sliding through a supporting tube or by means of a rack- 

 and-pinion. This is called the coarse adjiistment, and 

 by it the objective is brought to a certain point near the 

 object, at which the latter can be most cleariy seen ; or, 

 as it is said, brought into focus. With only very low- 

 powered objectives, however. Can this point be readily 

 attained without a fine adjustment. This is secured by 

 means of a fine screw, or othei-wisei ■ The object when 



