36 BACTEEIOLOGY 



vaseline, thus preventing evaporation and resulting currents in 

 the preparation. 



Best results with the dark-field microscope are obtained 

 only in a dark or dimly lighted room. An electric arc or a power- 

 ful gas-light may be employed as the source of light, and it is well 

 to put a flask of water between the light and the microscope to 

 eliminate the heat-rays. The substage condenser of the micro- 

 scope is replaced with the special dark-field condenser and this 

 is carefully centered. A large drop of immersion oil is placed on 

 the upper surface of the condenser. The slide is carefully placed 

 upon the stage so that the oil fills in completely the space between 

 the condenser and slide and remains free from air bubbles. 

 The preparation is then ready for examination. Objectives of 

 numerical aperture wider than i.o cannot be successfully used 

 with the ordinary dark-ground condensers and therefore it is 

 necessary to stop down the aperture of the oil-immersion objec- 

 tive before using it. A special funnel stop is furnished for this 

 purpose. When this has been "attached the preparation may be 

 studied with the oil-immersion objective in the usual way. Skill 

 in this method of studying unstained microbes is quickly acquired, 

 offering, as a rule, less difficulty than the method of central 

 illumination which is employed for the hanging- drop and hanging- 

 block. 



Smear Preparations for Staining. The examination of 

 bacteria with the microscope is carried out to a very large extent 

 by means of smears made upon thin slips of glass. Such slips 

 of glass are generally called cover-glasses. It is best to obtain the 

 kind sold by dealers as No. i, f inch squares. 



The cover-glass may be cleaned best by immersion in a mix- 

 ture of sulphuric acid and bichromate of potassium solution, and 

 afterward washed thoroughly in distilled water, and finally in 

 alcohol. A stock of clean cover-glasses may be kept in a bottle 

 of alcohol, or perhaps preferably in alcohol containing 3 per cent 

 of hydrochloric acid. 



