OUR APPROACH 11 



nomic, hygienic, and theoretical matters connected with human 

 life, and at the same time they are of vast importance in aiding 

 us to understand the principles of biological science. 



Growth of Knowledge of Microscopic Organisms 



Our knowledge concerning these minute creatures has grown 

 step by step with the improvements made in the compound micro- 

 scope. Toward the end of the seventeenth century the Dutch 

 naturalist, Antonius van Leeuwenhoek, fastened a simple but 



Fig. 4— MODERN MICROSCOPES IN USE BY THE AUTHOR 

 Right to left: (1) a low-power binocular microscope, useful in searching over a large field; 

 (2) a monocular microscope for general work; (3) a microscope with special binocular attach- 

 ment, used for high-power work; (4) a monocular microscope with special micro-dissection 



apparatus 



powerful lens on a standard and thus made a simple micro- 

 scope. Credit for the invention of the microscope is usually 

 given to Galileo, but it was Leeuwenhoek who was the first 

 to use a microscope in biological investigation and credit must 

 be given to him for his discovery, in 1675, of the world of 

 microscopic life. Modern microscopes have evolved from these 

 primitive instruments into more and more powerful tools for 

 seeing into the invisible. The advances have been in the tech- 

 nical perfection of lens-making, and for giving precision and 

 clearness of vision as well as magnification (Fig. 4). It is 

 amazing to see what biologists were able to do with the imper- 

 fect microscopes of one hundred years ago, and still more amaz- 

 ing to see what Leeuwenhoek did with the simple lens system of 



