FUNCTION /iND PARTS OF THE MICROSCOPE. n 



Hans and Zacharias Janssen, and supposed to date from 

 about 1590. It is certain that a compound microscope 

 was independently invented by Galileo in 16 10, and that 

 Cornelius Drebbel in Holland was credited with the 

 introduction of the instrument about 1621. These early 



Fig. 9. — Leetjwenhoek's Microscope (Circa 1700). 

 (After Carpenter-Dallinger.) 



instruments and their illuminating apparatus were cum- 

 brous and unwieldy in the extreme (see Fig. 10). 



The compound microscope is characterized by the fact 

 that it contains two or more lenses or systems of lenses, 

 one of which forms an image of the object, while the other 

 forms a second image of the first image. The course of 

 the rays in such an instrument is shown in Fig. 11. The 

 object ^5 lies outside the principal focus of the lens system 

 marked Objective, and its real inverted image is formed 

 at A ^B^. This image is produced at a point inside the 

 principal focus of the Ocular or Eyepiece; and the eye- 



