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 
Fic. 20.—REFRACTION By AIR-BUBBLES AND O1L-props. (After Gage.) 
the inclusjon 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 lighter center, 
