14 VEGETABLE HISTOLOGY. 



converging light rays, the last three are thinner in the middle 

 than on the edges and diverge light rays; the first are called 

 converging lenses, the latter diverging lenses. 



For our purpose it will suffice to consider the properties of 

 the two simplest lenses of the series, namely, the biconvex and 

 the biconcave, as the general behavior of the others of the same 

 class is the same. 



BICONVEX LENS. Fig. 9 represents a section through the 

 center of a biconvex lens. The faces ADB and ANB are spher- 

 ical and their centers of curvature are C and C' respectively, 



AB 



Fig. 9. 



which may or may not be equally distant from the center O of 

 the lens. Usually they are equally distant. The line joining 

 C and C' is called the optical or principal axis. 



If a beam of parallel rays fall upon the lens parallel to the 

 principal axis the rays will be converged to a point called the 

 principal focus, which, for a lens of crown glass having surfaces 

 of equal or nearly equal curvatures, coincides very nearly with 

 the center of curvature C'. Of course, there are two focal 

 points, one on each side of the lens, equally distant from the 

 center. This distance C'O is called the focal length of the lens, 

 and it varies with the index of refraction of the glass and the 

 radius of curvature of the faces. The shorter the radius of 

 curvature, i. e., the thicker the lens, the shorter is the focal 

 length. 



Conversely to the above case, if a divergent beam of rays, 

 emanating from a source placed at the principal focus, fall 

 upon the lens, the rays will emerge parallel to the axis. Fig. 9 

 illustrates this condition. 



If divergent rays emanate from a point on the axis anywhere 

 between the principal focus and a distant point, they will be 



converged after passing through the lens to a point on the axis, 

 as shown in Fig. 10. 



