LESSONS IN PHYSICS. 



79 



foci of the lens, as light diverging from one point is converged 

 to the other. 



Suppose that a ray of light from H parallel with the principal 

 axis, falls upon the lens at the point A, it will be bent toward 

 the perpendicular C' A to the point of G. At G it will be bent 

 from the perpendicular CG to F, which is called the principal 

 focus, while rays diverging from the principal focus become par- 

 allel on the other side of the lens. Thus the general effect of the 

 double convex lens is to collect the rays of light. Diverging rays 

 are made to diverge less, parallel rays are made to converge, and 



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F.G 13 



the converging rays will be made more convergent. Experiments 

 with the other convex lenses show the same effect. The point S 

 in the center of the lens on the principal axis is called the optical 

 center of the lens. Any line, as LSO, through the optical center 

 is called the secondary axis. 



The effect of the concave lenses on rays of light may be shown 

 as follows: (See Fig. 14.) Let MN represent a double concave 

 lens, C and C' being the centers of curvature. Suppose the ray 

 of light S should fall upon the lens at A it will be bent toward 

 the perpendicular CB to D, on emerging from the lens it will be 

 bent from the perpendicular C' D to F, showing that the effect of 



