106 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, typhoid 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. 
