204 BACTERIOLOGY 



By using the agglutination test we now know that the 

 organisms causing pneumonia may be definitely subdivided 

 into at least four groups; that the pathogenic streptococci, 

 fall into at least two, possibly four groups; that several 

 strains of dysenteric bacilli and at least two strains of 

 meningococci have been demonstrated. And throughout we 

 find a constant specific relationship between strains and 

 their homologous antibodies. To repeat: This is a matter 

 of the greatest practical moment, for the serum from an 

 animal that is immune from the members of one group, while 

 it may possess high potency if used against infections caused 

 by members of that group, may be of but little or no value 

 if used in the treatment of infections caused by members of 

 another closely related group. (See " Pneumococcus " and 

 "Streptococcus" paragraphs on variations.) 



MICROSCOPIC EXAMINATION OF PREPARATIONS. 



The Different Parts of the Microscope. Before describing 

 the method of examining preparations microscopically, a 

 few definitions of the terms used in connection with the 

 microscope may not be out of place. (The different parts 

 of the microscope referred to below are indicated by letters 

 in Fig. 34.) 



The ocular or eye-piece (A) is the lens at which the eye is 

 placed when looking through the instrument. It serves to 

 magnify the image projected through the objective. 



The objective (B) is the lens which is at the distal end of 

 the barrel of the instrument, and which serves to magnify 

 the object to be examined. 



The stage (c) is the shelf or platform of the microscope on 

 which the object to be examined rests. 



