XViii INTRODUCTION 
The experience in this laboratory has been that the best 
results are obtained by teaching the more fundamental prin- 
ciples and methods as such before attempting to apply them 
in the study of the various species of bacteria or in practical 
diagnostic work. It has happened, even when the number 
of exercises is very limited, that a preliminary drill in the 
methods is greatly to the advantage of the student. From 
the nature of the subject, its application can be made and 
benefit derived therefrom only by those who know how to do 
the things that the exigencies of the moment demand. This 
means efficiency in knowing how. In following these direc- 
tions, therefore, the student must understand that the purpose 
of the first twenty exercises is to teach him how to do the 
things called for in the later exercises in the study of species 
and in some of the practical applications of bacteriology. 
Another feature of these directions is that they aim to teach 
the student how to study and observe bacteria in their cul- 
tures rather than to tell him what he is to observe. It is not 
intended that they should take the place of lectures and text- 
books in bacteriology. Their mission is to aid the student in 
finding out for himself what the text-books relate concerning 
certain species, and to guide him in the elementary steps in 
the more important diagnostic procedures, tests, and analyses. 
The fact must also be recognized that in a short elementary 
course it is not possible to try several methods for doing the 
same thing. This restricts us to the use of a single pro- 
cedure. The one is given that seems to us best adapted to 
the limited time and facilities of the student. It may happen, 
however, that other methods would be preferable under other 
