26 THE ELEMENTS OF VEGETABLE HISTOLOGY 



concave or divergent meniscus. The six types of 

 lenses are illustrated in Fig. 9. By reference to Fig. 

 10, it will be seen that light rays passing through the 

 central part of a lens are not refracted. A line passing 

 through the thickest part of a convex lens and the 

 thinnest part of a concave lens is termed the optic 

 axis or principal axis. Light rays passing through 

 a convex lens are refracted toward the optic axis, 

 whereas in concave lenses they are refracted away 

 from the optic axis (Fig. 10). The extent of refrac- 



Fig. 9. — Types of Lenses. 



1. Double convex. 2. Plano-convex. 3. Convexo-convex, 

 cave. 5. Plano-concave. 6. Concavo-concave. 



4. Double con- 



tion or bending of light rays differs in different parts 

 of the lens. Rays entering the edges of a lens are 

 refracted to a greater extent than those entering 

 nearer the optic axis; therefore light rays entering 

 a biconcave lens are bent so that they meet at a 

 point along the optic axis. The point at which these 

 rays cross the optic axis is termed the principal focus. 

 The distance between lens and principal focus is termed 

 the focal distance, or focal length, of the lens. A bi- 

 convex lens held in the sun will illustrate the fore- 

 going, as it will so concentrate the light rays at a 



