18 INTRODUCTION. 



Focus of a Lens. For a lens to give a perfect image of an 

 object, all the rays of light coming from that object and pass- 

 ing through the lens must meet at the same point on the 

 other side of the lens. This point is known as the focal point 

 or focus of the lens. 



The Spherical Aberration of Lenses. 



Definition. It is difficult, however, so to construct a con- 

 vex lens that all the rays of light that pass through it shall 

 come to the same focus. As a rule, the rays which traverse 

 its peripheral or marginal portion come to a shorter focus 

 than those which pass through its more central portion. 



Correction. Distortion of the image is thus caused, and is 

 known as spherical aberration. Theoretically, spherical aber- 

 ration might be corrected by making the curvature of the 

 periphery of the lens less than that of its more central por- 

 tion ; but the difficulties in the mechanical construction of 

 such a lens would be very great, and opticians have found it 

 more practical to correct this defect by coupling with a con- 

 vex lens a concave one of less curvature, but which is subject 

 to exactly the opposite error of refraction. 



Doublets. These combinations of convex and concave 

 lenses, or doublets, act as a single convex lens. 



Triplets. Sometimes two convex and one concave lens are 

 used in combination, and are called triplets. 



The Chromatic Aberration of Lenses. 



Definition. When light traverses a convex lens, the differ- 

 ent colors which compose it do not all come to the same focus 

 that is, the colors are of unequal refrangibility ; and the 

 image then is seen chiefly in that color which chances to be 

 in focus. This color-distortion is especially noticeable with 

 the marginal rays, and is known as chromatic aberration. 



Correction. Though exclusion of the marginal rays can, 

 as with spherical aberration, partly correct this defect, yet 

 this is not sufficient, and chromatic aberration is remedied 

 best by constructing the convex lenses of the combination 



