FUNCTION AND PARTS OF THE MICROSCOPE. 7 



the lens, though of course 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, 



Fig. 6. — Course of Rays according to the Relation of a Luminous 

 PonsTT to the Principal Focus. 



Fig. 6) v^ill actually converge afteir passage through the 

 lens and will meet at a definite point to form a real 

 image. 



6. Construction of Real and Virtual linages.— 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 is 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 



