PREFACE TO THE FOURTH EDITION. 



It becomes again the pleasant duty of the author 

 to express his gratification at the favorable recognition 

 that this book continues to receive, and to acknowledge 

 his indebtedness to those of his readers who have kindly 

 criticized its shortcomings and offered suggestions for 

 its betterment. 



In most cases such suggestions have been acted upon; 

 in certain others they have been of such a nature that 

 their adoption, while perhaps desirable, would have 

 increased the size of the book too greatly for its pur- 

 pose. 



In this edition an effort has been made to include the 



more important of the newer ideas bearing directly 



upon the subjects under treatment, and, when deemed 



necessary, opinions expressed in former editions have 



been made to conform to later views. In addition to 



the topics treated in the last edition there have been 



introduced illustrated descriptions of the bacillus of 



bubonic plague, of the bacillus of influenza, and of 



the micrococcus of gonorrhoea, as well as a number of 



new illustrations relating to descriptive passages in the 



text. 



A. C. A. 



Phii-adelphia, May, 1897. 



