REFRACTION OF LIGHT. 321 



the incident and emergent rays are parallel. Such a point 

 is called the optical center of a lens. 



What is the The axis of a lens is a straight line passing 

 axis of a lens? through the Center perpendicular to the sur- 

 face of the lens. 



,„ . . On this line ■will be situated the geometrical centers of the 



When 18 a lens „ ,,, ,o, , „,., 



considered ex- two surfaces of the lens, or rather of the spheres of which 



actly centered 1 ^j^g^ ^j.^^ portions. 



A lens is said to be truly or exactly centered when its optical center is sit- 

 uated at a point on the axis equally distant from corresponding parts of tho 

 surface in every direction ; as then objects seen through the lens will not ap- 

 pear altered in position when it is turned round perpendicularly to its axis. 



In what man- 679. Parallel rays of light falling upon a 

 rfys"^ affected double-convcx Icns are converged to a focus 

 uaa,] """"^""^ ^^ ^ distance varj'ing with the curvature of 

 its sides. 



Fig. 260. " The double-convex lens may be regarded aa 



two prisms, with curved surfaces, united at 

 their bases, as is represented in Fig. 2G0 ; 

 and as in a prism the ray of light refracted 

 by it is always turned toward its back, or 

 thicker part (whether that be turned upward, 

 downward, or to either side), it follows that 

 when parallel rays fall upon a double-convex 

 lens, or two prisms united at their bases, they 

 •will converge to a point. 



What is the The point where parallel rays of light fall- 

 principawocas jjjg upou oue sldc of a couvcx leos unite by 

 lens? refraction upon the opposite side, is called the 



principal focus of a lens. 



The distance from the middle of a lens to 



What is the 

 focal disi 

 of a lens ? 



focal distance j|.g px-jncipal focus, is called the focal distance 



of a lens. 



Tliis in a single convex lens is equal to the diameter of the sphere of which 

 the lens is a portion ; in a double-convex lens it is equal to the radius, or 

 Bemi-diameter of the sphere of which the lens is a portion. 



The focal distance of parallel rays falling upon a convex lens is repre- 

 sented at A, Fig. 261. If the rays are converging, as at B, -they will come 

 to a focus sooner, and if diverging, as at C, the focus will be further from tho 

 lens than for parallel rays. 



The focus of a convex lens may be easily found by allowing the rays of 

 the sun to fall perpendicularly upon one side of it, while a sheet of paper is 



14* 



