Appendix. 147 



mirror is most important. Successful observation frequently 

 depends on the origin and intensity of the light and also upon 

 the direction from which the mirror throws light upon the 

 object. The arm of the microscope, reaching out from the 

 pillar, supports more or less directly the tube, which in turn 

 carries, partly within itself, the draw-tube. The tube may 

 be moved up and down either by a rack and pinion 

 " coarse adjustment" or by sliding easily through a collar 

 supported by the arm. More delicate motion is secured by 

 the use of the "fine adjustment" a small milled wheel, gen- 

 erally placed at the apex of the pillar. 



The parts of the microscope thus far mentioned are gen- 

 erally included in the term " microscope stand." The opti- 

 cal parts consist of a series of eye-pieces or oculars, which fit 

 into the upper part of the draw- tube, and a series of much 

 more expensive objectives which screw into the lower end of 

 the tube. A very convenient, though not essential, appara- 

 tus is the triple nose-piece, by means of which three objec- 

 tives, revolving around a common centre, may be attached 

 to the lower end of the tube, permitting an instantaneous 

 change of magnifying powers. A little careful experiment- 

 ing with the eye-pieces and objectives, under the direction 

 of the instructor, will soon show their relative values as 

 magnifying agents. One rule for adjusting the focus has no 

 exceptions. "Always place the front of the objective near 

 the object before looking in the ocular, and then move the 

 tube upward in focussing." By observing this rule the objec- 

 tive will not be ruined by being forced into the object. 



Objects may be examined as they float in watch-crystals, 

 or they may be mounted on slides and covered with cover- 

 glasses. 



In case objectives or oculars become soiled, it is well to 



