192 BACTERIOLOGY 



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 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 

 centre 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 

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

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

 The slip adheres and the preparation may then be handled 

 without fear of disturbing the drop or the position of the 

 slip over the depression. When completed it has the appear- 

 ance shown in Fig. 35. The drop hangs in an air-tight 

 chamber so that both evaporation and contamination are 

 prevented. 



This is known as the "hanging-drop" method of exami- 

 nation or cultivation. It is indispensable for the purposes 

 mentioned, and at the same time requires considerable care 

 in its manipulation. The fluid is so transparent that the 

 cover-slip may be broken by the objective being brought 



