376 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
THE MICROSCOPE, THE ANIMAL CELL, AND THE GERM THEORY 
Thus, although medicine gained an impetus in the Renaissance, 
progress was slow. The new advances had to await the discoveries in 
other fields of science—physics, botany, and chemistry. The great 
physicians had developed their powers of observation to a high degree, 
but this did not suffice. It was necessary to amplify the powers of the 
sense organs, and particularly those of sight. 
The invention of the microscope has been attributed to Roger Bacon, 
who lived in the thirteenth century, but Galileo, in the seventeenth cen- 
tury was perhaps the first scientific user of this instrument. To the 
spectacle makers, however, we owe the development of the microscope, 
and particularly to Antoni van Leeuwenhoek, a man who had never 
attended a university and was entirely self-taught. He constructed a 
number of microscopes and studied all types of matter, even including 
the red blood corpuscles whose diameter he measured. 
A decisive factor in medical progress was the discovery of the animal 
cell and the establishment of the fact that the living body is a vast 
organization consisting of innumerable individual cells so small that 
they can be seen only with the aid of a microscope. Schwann, and 
later Rudolph Virchow, applied the discoveries of the botanists, who 
had demonstrated the cellular constitution of the vegetable kingdom, 
to the intimate study of disease. This was the foundation of the 
science of pathology. 
There is no doubt that one of the most important contributions of the 
nineteenth century to medicine was the demonstration of the fact that 
invisible organisms, so small that they can only be seen with the aid of 
a microscope, may cause disease. Such organisms had been observed 
in the seventeenth century but two centuries had passed before their 
significance was appreciated. In the nineteenth century, thanks to the 
work of Pasteur, Koch, and others, a revolution in the structure of 
medical thought occurred. It was finally recognized that infinitely 
small organisms, endowed with special pathogenic qualities, may play 
a preeminent role in producing disease. 
At the same time that Virchow was laying the foundations of 
pathology in Germany, Pasteur developed the germ theory in France. 
Pasteur was a chemist and in that field his earlier studies led him to 
the discovery that putrefaction is a kind of fermentation and that both 
these processes are due to micro-organisms. He showed that the dis- 
ease of wine which plagued the wine industry in France was due to 
the action of microbes and that by heating the wine for a short time to 
a temperature between 50° and 60° C., the fermenting agent could be 
destroyed while the wine would remain unaltered and would keep 
indefinitely. This process is now well known under the name “pas- 
teurization.” 
