65 



CHAPTEE IV 



EXAMINATION OF MICRO-ORGANISMS UNDER THE MICROSCOPE, 

 AND BY EXPERIMENTS ON LIVING ANIMALS 



Examination in the fresh state. In making microscopic 

 examinations we begin with the simplest mode of procedure, 

 which consists in taking minute samples from the individual 

 colonies on the plate with the help of the platinum needle, 

 floating them in water, and subjecting them to observation. 

 It must be seen to that too much fluid is not taken, but only 

 enough to fill the interspace between cover-glass and slide. 

 The former ought not to float about loosely, nor should 

 the fluid extend beyond its edges. When examining with 

 high powers it must be noted which form the particular 

 micro-organism takes that is, whether rods or cocci are to 

 be dealt with, whether they are connected with one another 

 in chains, whether, if so, the chains run straight or spirally, 

 and, in the case of cocci, whether they lie scattered or in 

 rows. Size is measured in micromillimeters (micra, com- 

 monly written p, = the thousandth part of a millimeter), 

 or by comparison with other similar forms, especially red 

 corpuscles. 



Examination in the hanging drop. A very useful method 

 of observing freshly -obtained micro-organisms is the 

 examination in the ' hanging drop.' For this purpose the 

 end of a platinum wire is bent with the aid of a pliers into 

 a little loop. When this is dipped into a liquid containing 

 bacteria, enough of the liquid remains adhering to it to 



