IV PREFACE. 



subject-matter be presented in as direct a manner as 

 possible. 



Presuming the reader to be unfamiliar with the sub- 

 ject, the author has restricted himself to those funda- 

 mental features that are essential to its understanding. 

 The object has been to present the important ideas and 

 methods as concisely as is compatible with clearness, 

 and at the same time to accentuate throughout the 

 underlying principles which govern the work. 



With the view of inducing independent thought on 

 the part of the student, and of diminishing the fre- 

 quency of that oft-heard query, , " What shall I do 

 next?" experiments have been suggested wherever it 

 is possible. These have been arranged to illustrate the 

 salient points of the work and to attract attention to the 

 minute details, upon the observation of which so much 

 in bacteriology depends. 



A. C. A. 



Philadelphia, December, 1891. 



