PREFACE. Vll 



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 



