FUNCTION AND PARTS OF THE MICROSCOPE. 11 
Hans and Zacharias Janssen, and supposed to date from 
about 1590. It is certain that a compound microscope 
was independently invented by Galileo in 1610, and that 
Cornelius Drebbel in Holland was credited with the 
introduction of the instrument about 1621. These early 
Fic. 9.—LEEUWENHOEK’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 A B 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- 
