CH. II '] LIGHTING AND FOCUSING 39 



Light well, and employ the proper opening in the diaphragm, etc. 

 (§ 6S). Look between the front of the objective and the object as 

 before (§72), and lower the tube with the coarse adjustment till the 

 objective almost touches the cover-glass over the object. Look into 

 the microscope, and with the coarse adjustment, raise the tube very 

 slowly until the image begins to appear, then turn the milled head of 

 the fine adjustment (frontispiece), first one way and then the other, if 

 necessary, until the image is sharply defined. 



In practice it is found of great advantage to move the preparation 

 slightly while focusing. This enables one to determine the approach 

 to the focal point either from the shadow or the color, if the object is col- 

 ored. With high powers and scattered objects there might be no object 

 in the small field (see § 50, Fig. 37 for size of field). By moving the 

 preparation an object will be moved across the field and its shadow 

 gives one the hint that the objective is approaching the focal point. It 

 is sometimes desirable to focus on the edge of the cement ring or on 

 the little ring made by the marker (see Figs. 61-66). 



Xote that this high objective must be brought nearer the object 

 than the low one, and that by changing to a higher ocular (if the ocu- 

 lars are not par-focal) or lengthening the tube of the microscope it 

 will be found necessary to bring the objective still nearer the object, as 

 with the low objective. (For reason see Fig. 58.) 



§ 75. Always Focus Up, as directed above. If one lowers the 

 tube only when looking at the end of the objective as directed above, 

 there will be no danger of bringing the objective in contact with the 

 object, as may be done if one looks into the microscope and focuses 

 down. 



When the instrument is well focused, move the object around in 

 order to bring different parts into the field. It may be necessary to 

 re-focus with the fine adjustment every time a different part is brought 

 into the field. In practical work one hand is kept on the fine adjust- 

 ment constantly, and the focus is continually varied. 



§ 76. Determination of Working Distance. — As stated in § 61, 

 this is the distance between the front lens of the objective and the 

 object when the objective is in focus. It is always less than the equiv- 

 lent focal length of the objective. 



Make a wooden wedge 10 cm. long which shall be exceedingly thin 

 at one end and about 20 mm. thick at the other. Place a slide on the 

 stage and some dust on the slide. Do not use a cover-glass. Focus the 

 dust carefully first with the low then with the high objective. When 



