PREFACE, vil 
No apology will be made for the numerous repetitions 
which will be found in this book. They are essential 
to its scope, which is to give clear accounts of the 
processes with as little reference to other chapters as 
possible. In the majority of cases each section is 
complete in itself. 
These instructions are followed by information as to 
the interpretation of the results which may be obtained ; 
and this information applies equally whether the medi- 
cal man has made the examination for himself or has 
obtained it ready made from a public laboratory. It too 
commonly happens that practitioners feel themselves 
aggrieved because they get a negative report (as to the 
presence or absence of Widal’s reaction) on blood taken 
during the first few days of an illness which turns out to 
be typhoid fever, or are inclined to discredit bacterio- 
logical examinations because diphtheria bacilli are found 
in throats which exhibit no membrane and clear up in a 
few days without serious symptoms. 
In the second place, there are a good many cases in 
which the investigation had better be made in a public 
laboratory. In these the questions, of what to send, 
and how to send it, are fully explained. This a most 
important point. A bacteriologist is not a magician 
who is able to weave a spell if he has a small portion of 
his victim’s anatomy to work upon; and the materials 
must be taken in the proper way if his results are not to 
be useless or even misleading. This is well seen in the 
examination of the blood for bacteria. In many cases the 
blood is drawn in such a manner that it must necessarily 
be contaminated from the skin during the operation 
and is transmitted in vaccine tubes which were almost 
certainly not sterile before being filled. Under such 
circumstances the bacteriologist will probably report 
the presence of streptococci or staphylococci, and the 
practitioner who does not understand the fallacies of 
the examination may be led to make a diagnosis which 
will be disastrous to his own reputation and may be 
injurious to the patient. 
Lastly, it need scarcely be said that this is not 
intended to be a substitute for any one of the numerous 
