800 



THE SENSE OF SIGHT 



different kinds of lenses are obtained, namely (a) plano-convex, (6) biconvex, 

 (c) concavo-convex, (rf) plano-concave, (e) biconcave and (/) concavo-convex 

 (Fig. 411). The lens in our eye is a double convex or biconvex lens, but we shall 

 have occasion to refer to the other types of lenses, as well as to prisms and planes 

 when discussing errors in refraction and their correction. 



Refraction by a Biconvex Lens. — A biconvex lens is essentially 

 the segment formed at the intersection of two spheres drawn upon the 

 same line with either the same or different radii (Fig. 412). Hence, 



Fig. 412. 

 Fig. 412. — Diagram Illustrating the Formation of 

 FiQ. 413. — Structure of Biconvex Lens. (From Draper 



B C 

 Fig. 413. 

 Biconvex Lens., 

 ' Medical Physics.") 



SL line prolonged through the centers of curvature of the two surfaces 

 of this lens (AB), must form the principal axis of this system. Be- 

 tween these two centers lies a point C which possesses the property of 

 permitting rays to pass without refraction, so "that the emergent ray 

 is parallel to the incident ray. This point constitutes the optical 

 center of the lens. Any other line passing through this center is a 

 secondary axis. 



The action of a biconvex lens upon the entering rays of light is easily understood 

 if the lens is imagined to be composed of a number of prisms arranged in the manner 



Fig. 414. — Convex Lens Dissected. (From Draper "Medical Physics.") 



indicated in Fig. 413. It will be remembered that a prism deflects or deviates the 

 ray toward its base ; hence, a biconvex lens deflects the entering rays in accordance 

 with the refractive power of its prismatic constituents. Inasmuch as the central 

 prisms d, e, etc., have a smaller refracting angle than the outer one / and g, they 

 must give rise to a lesser deviation. The same holds true of the prismatic elements 

 situated above the principal axis, and whether in the vertical, horizontal or oblique 

 meridian of this lens. Their tips are of course directed outward and their bases 

 inward; and furthermore, the central ray following the line of the principal axis, 

 is not deflected at all It will be seen, therefore, that a biconvex lens possessing 



