METHODS OF STUDYING BACTERIA 45 



ocular. When using high-power objective use weak ocular. 

 A revolving nose-piece will be found very useful, since it is 

 sometimes necessary to change the objective on the same 

 field, and this insures a great steadiness of the object. 



Great cleanliness is needed in all bacteriologic methods, 

 but nowhere more so than in the microscopic examination. 



The cover-glass should be very carefully washed in alcohol, 

 and dried with a soft linen rag. To remove the stains on the 

 cover-glasses that have been used they should be soaked in 

 hydrochloric acid or placed in a 6 per cent, aqueous solution 

 of potassium dichromate with 6 per cent, of strong sulphuric 

 acid, washed in water, and kept in absolute alcohol. 



Examination of Unstained Bacteria. As the coloring of 

 bacteria kills them and changes their shape to some extent, it 



Fig. ii. Platinum needles for transferring bacteria, made from No. 27 

 platinum wire inserted in glass rods: a, Looped needle; b, straight- 

 pointed needle (McFarland). 



is preferable to examine bacteria, when possible, in their 

 natural state. 



We obtain the bacteria for examination either from liquid 

 or solid media. 



From Liquids. With a long platinum needle the end of 

 which is bent into a loop (Fig. n, a) obtain a small drop from 

 the liquid containing the bacteria, and place it on a cover- 

 glass or slide, careful that no bubbles remain. 



Sterilize Instruments. Right here we might say that 

 it is best to accustom one's self to pass all instruments, 

 needles, etc., through the flame before and after each proce- 

 dure; it insures safety; and once in the habit, it will be done 

 automatically. 



From Solid Media. With a straight-pointed platinum 

 needle (Fig. n, b) a small speck of the medium is taken and 



