188 LABORATORY EXERCISES IN BACTERIOLOGY. 



dried and fixed on the slide, a drop of immersion oil may be applied directly to the 

 film without first adjusting a cover-glass if the oil-immersion lens be employed. In 

 all other cases a thin, clear cover is essential for every microscopic preparation. 



After using an oil-immersion lens the oil should be removed from the surface 

 of the lens as well as from the preparation by a bit of soft absorbent paper furnished 

 by dealers under the name of "lens paper" (dental absorbent paper). If the oil has 

 become dried upon a preparation a bit of the paper moistened in xylol will be found 

 efficient for its removal; but xylol or any solvent should be used with extreme caution 

 in cleaning an objective lest it remove the balsam settings of the lenses and thus impair 

 the whole apparatus. 



Slides and Covers. The slides made of soft white glass now usually sold by 

 dealers are on the whole the best ; but in warm, moist climates they are very liable to 

 corrosion, especially if kept packed closely, and in such localities the green glass regarded 

 as inferior is for common use preferable. Especial attention should be had that the 

 slides are perfectly flat and free from flaw. 



Covers should be of white glass, very thin (No. 1 of the manufacturer), and their 

 surfaces true. Squares, five-eighths or three-fourths of an inch in size, will be found 

 most convenient. Like the slides, they should not be kept in close packets lest they 

 become opaque from corrosion. 



New slides and covers should be unpacked, soaked for several hours in equal 

 parts of water and one of the strong mineral acids, well rinsed in several changes of 

 clean water, and kept in closed dishes in alcohol to which a little ammonia has been 

 added. Old slides and covers may be easily cleaned after being allowed to soak for 

 some hours or several days in equal parts of strong ammonia and water ; after cleansing 

 they should be treated as new slides, being soaked for several hours in a dilute acid 

 well rinsed in clean water, and placed for keeping in dishes of ammoniated alcohol. 

 It is best to keep old slides and covers in dishes separate from those in which are kept 

 the new supply. 



MICROSCOPIC EXAMINATION OF CULTURES. 



Colonies of bacteria growing in films of the nutrient on plates or in dishes or rolled 

 tubes are to be examined with the low powers of the microscope (twenty to sixty 

 diameters) in order to study their internal structure as to refraction, granulation, 

 markings, etc. (see Lesson VI). For this purpose the plate, dish, or tube is placed 

 directly on the stage of the instrument, and observation made with field darkened 

 as much as permissible, by contraction of the diaphragm ; and subsequently reflected 

 light should also be used, the light from the substage mirror being entirely cut off and 

 the rays from some white cloud in the sky concentrated on the surface of the prepara- 

 tion by mirror or bull's-eye condenser. For such examination it is best to have the 

 culture entirely exposed, and for this reason the cover of the Petri dish is removed; 

 but where such exposure is objectionable much may be distinguished through the glass 

 of the cover, or better through the bottom of the dish which has been turned over. 



A favorite device used in watching the development of a colony of bacteria and 

 noting its minute features is the "hanging-drop" preparation, also employed in high- 

 power studies of the motility, grouping, spore-formation and germination, agglutina- 

 tion, etc., of bacteria. A hanging drop (Fig. 57) requires a special slide, in the upper 

 surface of which a small concavity has been ground and polished. A clean cover-glass 

 having been procured, a drop of physiologic salt solution, bouillon, or of liquefied agar or 



