194 Botanical Microtechnique 



such a source is superimposed on the object fickl. uniloi ni illumination 

 is obtained. For elementary class use, and for many roiuine tasks in 

 research, the nearest approach to optimum ilhmiination seems to be 

 an opal glass disk in a suitable lamp housing, with a 50 to 60 watt 

 frosted blue mazda bulb. Place the lamp 8-12 inches from the 

 microscope and manipidate the mirror until the field of view, with 

 an object in focus, is imiformly illuminated. If the microscope has a 

 condenser, place the point of a pencil against the lamp disk and 

 adjust the height of the condenser initil the pencil point is in focus. 

 If finely ground glass, or a plano-convex lens with a ground fiat 

 surface is used instead of opal glass, the condenser must be moved 

 out of ojjtimum position to eliminate the granularity in the field of 

 \ iew. Ihe use of a lamp that has a condensing lens system and a 

 diaj^hragm is discussed in the chapter on photomicrography, and the 

 Avorker who wishes to do critical visual work should consult that 

 chapter. 



Mechanical Operation 



A microscope usually has a set of two to foin- objectives per- 

 manently installed on a revolving nosepeice. The objective are 

 centered and parfocalized, each screwed into its designated opening 

 in the nosepiece. The older nosepieces ha\'e adjustable stops for lateral 

 centering of individual objectives. Improvements in manidacturing 

 methods have made possible the quantity jjroduction of nosepieces of 

 such precision that no adjustments for centering are recjuired on the 

 nosepiece. The removal of objectives should be strictly forbidden in 

 the classroom. 



The bod\ tube of the microscope on \vhi(h ihe objectives and 

 ocidar are mounted is ni()\cc! up and down b\ two mechanisms, a 

 coarse adjustment whidi produces lapid tlisplacemeiu. and a luu' 

 adjustiiuni whidi movis the i)od\ tid)e very slowly. Ihe coarse 

 adjustment is actuated i)y a lack and pinion. This de\ice is j)ractically 

 identical in the se\eral leading makes. The liiihlness ol the action can 

 be adjusted easily by tightening or loosening the sj)lit bearing block 

 against the pinion shaft b\ means of the readily accessil)le scre^vs. In 

 the Zeiss instnunent the action is tightened by grasping the pinicju 

 heads firmly and screwing them lowaid each other. 



I'he (ine-adjustmc nt mechanism dillers radically in the dilferent 

 makes. One tyj)e employs a gear-and-sec tor device in which onh a 

 few teetii are in contact. This action, liiough \ ci \ smooth and 

 responsive, is rather delicate and easily damaged. The most rugged 



