MICROSCOPIC EXAMINATION OF PREPARATIONS 209 



lightly by its end, it may be moved about under the objec- 

 tive. At the same time the screw of the fine adjustment 

 must be turned back and forth, so that the different planes 

 of the preparation may be brought into focus one after the 

 other. In this way the whole section or preparation may 

 be inspected. When the examination is finished raise the 

 objective from the preparation by turning the screw of the 

 coarse adjustment toward you. Remove the preparation 

 from the stage, and, with a fine silk cloth or handkerchief, 

 wipe very gently and carefully the oil from the face of the lens. 



During work, of course, the lens need not be cleaned and 

 put away after each examination; but when the work for 

 the day is over an immersion lens is best protected in this 

 way. Under no circumstances should it be allowed to remain 

 in the immersion oil or v exposed to dust for any length of 

 time. 



Examination of Unstained Preparations. "Hanging drops." 

 It frequently becomes necessary to examine bacteria in the 

 unstained condition. The circumstances calling for this 

 arise while studying the multiplication of cells, the germina- 

 tion of spores, and the absence or presence of motility. 



In this method the organisms to be studied are suspended 

 in a drop of physiological salt solution or of bouillon, or a 

 tiny drop of either agar-agar or gelatin, inoculated with 

 the organism, may be employed. The drop is placed in the 

 center of a clean cover-slip which has been sterilized in the 

 flame and which is then inverted over the depression in 

 a sterilized so-called "hollow-ground" slide to which it is 

 sealed with vaseline. A convenient and quick method of 

 making the preparation is, after placing the drop in the 

 center of the cover-slip, to invert over it the slide, around 

 the depression in which a ring of vaseline has been painted. 

 14 



