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. 



PHILADELPHIA, May, 1897. 



268442 



