Microscope Construction, Use, and Care 195 



type is actuated by a s])lit mil which has mniicrous threads in 

 permanent contact with a worm gear, llie threads are almost 

 impossible to strip, and this action has excellent responsiveness. 

 Details ot construction of the various makes may be obtained Irom the 

 illustrations and description in the catalogues. The repair ol line- 

 adjustment actions should be entrusted only to a highly skilled 

 mechanic or to the manufacturer. 



The normal procedure in using the microscope is to locate the 

 object with a low-power objective and then turn to the next higher 

 power. Objectives of lOX or less are the most satisfactory finder lenses 

 because of their large field of view, considerable depth of focus, and 

 long working distance. Microscopes for elementary work should be 

 equipped w'ith a safety stop on the body tube which prevents contact 

 between the slide and the low-power lens. With an objective ot lOX 

 or less in position, it is safe to rack the boch tube down luitil it is 

 stopped by the safety stop. With the body ttU^e in this jjosition look 

 into the ocular and manipulate the mirror until the field of view is 

 uniformly illuminated. Move the body tube upward with the coarse 

 adjustment until the image is visible, then bring the image into sharp 

 focus with the fine adjustment. Search the section by moving the slide, 

 using the fine adjustment freely to bring into sharp focus structural 

 features at different depths in the specimen. 



\Vhen it is necessary to turn to a higher magnification, center the 

 desired structure in the field of view and bring it into sharp focus 

 with the lowest power. Without chayiging the focus, ttu-n the objective 

 of 7iext higher magnification into position. A properly parfocalized 

 objective has ample clearance. The image should now be visible, and 

 it should recjuire not more than a cjuarter turn of the fine adjustment 

 to bring the image into sharp focus. 



The safety stop provided on the barrel does not prevent pressing 

 the high-power objective upon the slide. Iherefore, the high-powder 

 objective should never be used for locating the object. If an objective 

 of 3 to 5X is used, do not change from this low magnification to 43 X' 

 btit go progressively up through the range of magnifications. Similarly, 

 go down the range progressively. The manufacturers can furnish 

 safety stops for installation on the tubes of older microscopes. 



Some teachers prefer to have the objectives adjusted so that when 

 the object is located with the low power, and the high-power objective 

 is swung into position, a slight ujnvard movement brings the object 

 into sharp focus. The objection to this arrangement is that, if the user 

 inadvertently moves the bod\ tube downward, he is mG\'ing it farther 



