CHAPTER Ix. 
THE MICROSCOPE IN MEDICINE AND SANITATION. 
1. The Microscope in Biology.—The microscope bears 
to biology much the same relation that the balance 
bears to quantitative chemistry. It is the fundamental 
instrument without which a true science of living things 
would be almost impossible. 
The invention of the compound microscope and the 
construction of simple microscopes of high power at 
the middle of the seventeenth century stimulated a dozen 
observers to the study of the finer structures of plants 
and animals. Leeuwenhoek discovered in water and 
in decomposing organic matter a teeming world of minute 
forms of life invisible to the naked eye. Hooke, the 
English botanist, made out the cellular structure of 
plants in his examination of cork. Malpighi and Swam- 
merdam figured the microscopic details of the insect 
body with marvellous patience and success. 
It was only, however, after the perfection of the 
achromatic objective about 1835 that these early labors 
could bear abundant fruit. In 1838-39 Schleiden and 
Schwann developed that great generalization which bears 
their name, the doctrine that all plants and animals are 
104. 
