3° 



ELEMENTS OF APPLIED MICROSCOPY. 



demand. It should be noted that the excursion of the 

 fine adjustment is necessarily small; if care is not taken 

 to keep it near the middle of its range, its motion will be 

 stopped short at one end or it will be unscrewed from its 

 place at the other. 



Focusing with the higher power is more difficult. On 

 account of the short working distance it is necessary to 

 "run the objective down as close to the object as possible, 

 watching its approach from the side, and then focusing 

 up very carefully with the fine adjustment. 



Useful practice in focusing may be obtained by the 

 study of air-bubbles and oil-drops. A few drops of 

 clove-oil should be mixed with mucilage and the mixture 

 beaten up on a slide with a knife-blade so as to insure 



Fig. 20. — Refraction by Air-bubbles and Oil-drops. (After Gage.) 



the inclusion of air-bubbles; the preparation is then cov- 

 ered with a cover-slip and examined. At first glance 

 the air-bubbles and oil-bubbles appear alike as black 

 rings with lighter centers. Focusing carefully down- 

 ward, however, it will be found that the air-bubbles 

 appear first as a dark ring with a diffuse Kghter center^ 



