PREFACE BY TRANSLATOR. 
Havine found the admirable resumé of our knowl- 
edge of the bacteria, by Dr. Magnin, of great assistance 
to me, in pursuing the investigations in which I have 
been engaged during the past year under the auspices 
of the National Board of Health, it has seemed to me 
that a translation of the work into English and its publi- 
_ cation in this country would be productive of good in 
more ways than one, and of the advancement of science. 
To the naturalist, it cannot fail to be of value, as the 
most approved classification, that of Cohn, is given, 
with a full description of species. To give additional 
value to this portion of the work, figures of many of the 
best-known forms, drawn from various foreign sources, 
and reproductions of some of my own _ photo-mico- 
graphs (by permission of the National Board of Health), 
have been introduced. 
If we are to judge from the scanty literature of the 
subject in this country, the amount of interest which 
has been aroused by the revelation of a new world of 
micro-organisms, and by the momentous questions which 
have been raised in connection with them, is far below 
that awakened in Germany, France, and England. This 
is not, however, really the case ; for, while we have but 
few active workers in the difficult fields of inquiry 
