io6 ELEMENTS OF APPLIED MICROSCOPY. 



parasitic plants and animals, the epidemic plagues, 

 fevers, and pestilences, was comprehended only after 

 their exciting causes were discovered by the microscope. 

 The bacteria, observed by Leeuwenhoek in 1680, were 

 first carefully studied by Ehrenberg in the wonderful 

 decade of progress which followed the perfection of the 

 achromatic objective. Twenty years later Pasteur estab- 

 lished the casual relation of these minute fungi to disease; 

 and in the next quarter of a century the organisms caus- 

 ing anthrax, tuberculosis, fyphoid fever, diphtheria, and 

 cholera were discovered. These bacterial parasites are 

 studied largely by culture methods; but the microscope 

 is also indispensable. The animal organism causing 

 malaria cannot be grown on artificial media, and our 

 whole knowledge of it depends on optical methods. In 

 the first few years of the twentieth century, the brilliant 

 researches which revealed the parasites associated with 

 smallpox and scarlet fever were carried out with the 

 microscope alone. 



The researches into pure science which have led, and 

 are leading, to such advances in pathology and parasi- 

 tology cannot, of course, be treated here in detail. Cer- 

 tain applications of the microscope to the study of the 

 body fluids and of food material have, however, become 

 a routine part of clinical diagnosis and the sanitary in- 

 spection service. These technical applications of medi- 

 cal microscopy come legitimately within the scope of our 

 work; and a few typical methods for the study of 

 pathologic conditions and of parasitic invaders will now 

 be briefly considered. 



