34 THE NATURE AND NEEDS OF LIVING MATTER 



a. What fractional numbers do you find on the case of the high and 

 the low power objectives, respectively ? 



7. Note. The objectives are attached to a revolving device, the 

 nose piece. 



a. What are the advantages of a revolving nose piece ? 



8. Note. To obtain a clear image of the object under examination, 

 we must be able to vary the distance between the lenses and the object ; 

 that is, to focus the instrument. The microscope is brought into focus 

 by slightly turning either of the large wheels placed at the top of the arm 

 near the tube. . 



a. Why are these wheels called the coarse adjustment ? (Turn one 

 of them gently.) 



6. What movement results ? 



9. Note. The milled head of the fine adjustment is found at the top 

 of the pillar. 



a. Carefully turn the 'fine adjustment back and forth. ( No more than 

 half a turn in either direction.} Why is this adjustment called ' fine ' ? 



CAUTIONS AND HINTS 



1. Do not touch either lenses or mirrors. 



2. Do not rest either your arm or your head on the instrument. 



3. Never make more than half a turn either way with the fine adjust- 

 ment. 



4. Get a fairly clear image with the coarse adjustment : use the fine 

 adjustment only to complete the operation of focusing. 



5. An object to be viewed with the compound microscope should be 

 very thin and transparent. 



PRELIMINARY PRACTICE WITH THE MICROSCOPE 



Materials. Slides, cover glasses, free-hand sections of potato tuber 

 stained with iodine solution, compound microscope. 



Directions for Mounting Sections and Adjusting Microscopes. 



1. Place the given section on a slide in a drop of water. 



2. Clean a cover glass and lower it gently over the object. (Avoid 

 air bubbles !) 



3. Put the slide on the stage with the object over the stage perforation. 



4. Tilt the mirror so that the object may be well lighted. 



5. By aid of the coarse adjustment, bring the low power near to the 

 object. 



6. With your eye at the ocular, raise the tube from the stage till the 

 image appears rather clearly. 



