28 THE ELEMENTS OF VEGETABLE HISTOLOGY 



to the distance of the screen from the lens. Objects 

 placed at a point within the focal distance of a lens 

 will also give rise to enlarged images; but these can- 

 not be projected upon a screen and are termed virtual 

 images. The changes in direction which light rays 

 undergo in the production of a real image are entirely- 

 due to the refractive effects of the lens (Fig. 11). 



B' 



Fig. 11. — Formation of Real Image by Convex Lens. 



1. Optic axis. 2. Focal points. A-B. Object beyond focal point (2). A'-B' 

 Real Image of object A-B. 



The apparent enlargement in the production of a 

 virtual image is due to the action of the lens supple- 

 mented by the tendency of the eye to follow light 

 rays through a lens disregarding the refractive effects 

 of the latter (Fig. 12). Real images always appear 

 inverted whereas virtual images appear erect. 



Spherical and chromatic aberration are properties of 

 lenses which formerly caused considerable difficulty. 

 Spherical aberration is due to the fact that light rays 



