402 REFRACTION OF LIGHT. 



faces of the lens. Any straight line, other than the prin- 

 cipal axis, passing through the optical centre is a second- 

 ary axis. 



(a.) If a ray of light passing through the optical centre be re- 

 fracted at all, the two refractions will be equal and opposite in direc- 

 tion. The slight lateral aberration thus produced may be disregarded. 



624. Principal Focus. All rays parallel to 

 the principal axis will, after two refractions, con- 

 verge at a point called the principal focus. This 

 point may lie on either side of the lens, according to the 

 direction in which the light moves; it is a real focus. The 

 greater the refracting power of the substance of which the 



FIG. 305. 



(ens is made, the nearer the principal focus will be to the 

 lens. In a double-convex lens of crown glass, the principal 

 focal distance is equal to the radius of curvature; in a 

 plano-convex lens of the same material, it is twice as great. 



(a.) The position of the principal focus of a lens is easily deter- 

 mined. Hold the lens facing the sun. The parallel solar rays 

 incident upon the lens will converge at the principal focus. Find 

 this point by moving a sheet of paper back and forth behind the 

 lens until the bright spot formed upon the paper is brightest and 

 smallest. 



(&.) It is also true that rays diverging from a point at twice the 

 principal focal distance from the lens will converge at a point just 

 as far distant on the other side of the lens. Rays diverging from 

 / will converge at /', these two points being at twice the focal dis- 

 tance from the lens. By experimenting with a lens and candle- 

 flame until the flame and its image are at equal distances from the 

 lens, we are able, in a second way, to determine the principal focal 

 distance of the lens. The conjugate foci situated at twice the prin- 

 cipal focal distance are called secondary foci. 



