CHAPTER IX 

 MICROSCOPICAL EXAMINATION OF BACTERIA 



History of the Microscope. The principle of the lens has 

 been understood from the earliest time. Magnifying glasses 

 of rock crystal have been found in the ruins of Nimrod. 

 Aristophanes, 400 B.C., speaks of " burning spheres " which 

 were sold in the shops of Athens; and Seneca, in the middle 

 of the first century, speaks of the magnifying power of glass 

 globes filled with water. It was not, however, until the seven- 

 teenth century that powerful magnifying lenses were used 

 for scientific purposes. Leeuwenhoek's work with the simple 

 lens has already been referred to. The first compound micro- 

 scope was probably made by Hans and Zacharias Janssen of 

 Middleburg, Holland, in 1590. In the compound microscope, 

 as is well known, we observe not the object but an image of the 

 object. Such a microscope consists of two systems of lenses, 

 one with a short and the other with a long focus. These are 

 the essential parts of the microscope. There are certain me- 

 chanical appliances necessary to keep the lenses in position, 

 to afford steadiness, proper illumination, etc. The instrument 

 without the optical parts is called the stand. The lens 

 nearest the object is known as the objective, while the lens 



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