58 THE MICR0SC01>E. Apf., 



or bottom of the cover-glass, or top of the slide, or in 

 the mounting medium, or the imperfections of the glass 

 surfaces themselves, or in the cut edge of the cover, as 

 a clue for locating approximately the plane of the object — 

 vv^hich may be unexpectedly difficult with very minute 

 transparent objects, such as a very few small diatoms or 

 slightly stained. bacteria or, still worse, very fine micro- 

 meter rulings. 



With extreme apertures, or where the objective, suita- 

 bly corrected, may nearly or actually touch the cover, the 

 greatest skill is requtred, and some danger exists even 

 in the best hands. Even then the lens can usually be set 

 as described above, so near that a glimpse, probably vague 

 and distant, can be gained of almost anything coarser 

 than the fine rulings or bacteria ; when an upward touch 

 of the ffne adjustment will either reveal the object dis- 

 tinctly or show that it is farther below. In the later 

 case, while the fine adjustment is being lowered with ex- 

 treme care, the slide may be frequently slipped upon the 

 stage by the hand, over a distance which is a very small 

 fraction ol the field of view (with a tolerably steady hand 

 there is no diflSculty in moving it as little as 1-20 mm., 

 or 1-500 inch), in order to detect, by the increased resist- 

 ance, any pressure of the objective before it becomes 

 destructive. Or, if the hand be steady, the near side of 

 the slide may be held gently down against the stage, and, 

 the spring clips if any, being turned aside, the far edge 

 of the slide be lifted by one of the finger nails until the 

 now slanting cover-glass touches one edge of the mount- 

 ing of the objective. This can be done with ease when 

 the distance is less than the thickness of a sheet of paper ; 

 it furnishes deffinite idea as to the distance, and gives 

 ample warning as that distance decreases and disappears. 

 As the available focal distance varies with the screw-collar 

 adjustment, it is well, with suspiciously thick covers, to 

 adjust for maximum distance for finding the object, and 



