FUNCTION AND PARTS OF THE MICROSCOPE. 7 
the lens, though of coursé less so than before (A, Fig. 6). 
No real image can therefore be formed by an object 
lying inside the principal focus. Rays from a point out- 
side the principal focus have an initial divergence so small 
as to be entirely overcome by the lens. Such rays (C, 
Fic. 6.—CoursE OF RAYS ACCORDING TO THE RELATION OF A LUMINOUS 
PoINT TO THE PRINCIPAL Focus. 
Fig. 6) will actually converge after passage through the 
lens and will meet at a definite point to form a real 
image. 
6. Construction of Real and Virtual Images.—The 
construction of the image formed by any object is very 
simple, if only its relation to the principal focus of the 
lens be determined. The course of two rays from any 
point ig known with certainty. The ray parallel to the 
principal axis will be so bent as to pass through the prin- 
cipal focus on the opposite side of the lens. The ray 
passing through the optical center of the lens will not be 
