1894 THE MICROSCOPE. 135 



the drop adhering to this applied to the lens. One leg 

 of a rubber hair-pin forced through the cork is useful 

 for the purpose, as none of the chemical liquids used for 

 immersion purposes will act on it, as some of them will 

 act on a metal wire. When the drop has been applied 

 to the front of the lens, the objective is attached to the 

 body-tube and racked down until the fluid touches the 

 cover; and as the microscopist looks across the slide, be- 

 tween the cover and the lens, the objective is still fur- 

 ther lowered while the lessening distance and the expan- 

 sion of the fluid are watched, the expansion being con- 

 tinued until the objective is supposed to be approximately 

 focussed, when the fine-adjustment focusses it upward 

 or downward to the proper point. These movements 

 demand exceedingly great deliberation and caution ; de- 

 liberation so that the object may not be disarranged by 

 the slow expansion of the thick immersion-fluid, if it is 

 not permanently mounted, and caution that the objective 

 be not injured, for immersion lenses are easily disor- 

 dered. Their lenses are larger than those of smaller 

 angled dry objectives, their construction is more delicate, 

 and they must be treated with more care. 



When a water-immersion is to be used, that is, an ob- 

 jective with which water is the immersion liquid, I am 

 accustomed to focus it as a dry objective, as may easily 

 be done, although the definition will probably be abomi- 

 nable and the field dimly lighted, yet enough may be 

 seen to show that the desired object is in view. With 

 a camel's-hair brush a drop of water is then added to 

 the cover near the edge of the objective, under which 

 it will run by capillary attraction. Here all danger of 

 forcing the object out of position or of injuring the ob- 

 jective or the cover, is with ordinary caution, reduced 

 to nothing. 



To clean the front of a water-immersion, after using 

 it, the careful employment of the Japanese filter-paper 



