MODERN INVENTIONS AND THEIE RESULTS. 15 



pass through the peripheral portion of the lens 

 (or near its outer edges) are more refracted than 

 those which pass through the central portion of 

 the lens ; consequently 3 all the rays do not 

 come to the same focus, and the object of vision 

 is less distinctly seen at the focus. This is 

 called Spherical A berration. This difficulty has 

 been obviated by making use of a combination 

 of lenses of different forms (convex and con- 

 cave), by which their several aberrations correct 

 each other, and bring the rays of light to one 

 and the same point, thus placing the object to 

 be viewed in the focus of the now combined 

 rays of light. A second difficulty remained to 

 be removed. It is known that rays of different 

 colours make up ivliite, or colourless light. 

 These rays possess different degrees of refrangi- 

 bility, and this difference causes them, passing 

 through a lens, to come some to one focus, some 

 to another. This is called Chromatic A berra- 

 tion. This difficulty is overcome by a combina- 

 tion of lenses of a different density, flint-glass 

 and croiun-glass affording the materials for their 

 construction. These lenses correct each other ; 

 bring the different coloured rays to the same 



