CONSTRUCTION OF THE MICROSCOPE. 



21 



all brought to the same focus even by a lens free from spherical aber- 

 ration. It is, indeed, this difference in their refrangibility which causes 

 their complete separation by the prism into a spectrum. This is 

 termed chromatic aberration, and will be best explained by a reference 

 to fig. 15: ab are rays of white light refracted by a convex lens; 



c the focus of the violet rays, which then cross and diverge towards 

 ef; d is the focus of the red rays, which are crossed at the points 

 e e by the violet rays ; the middle point, therefore, of this line is the 

 mean focus, or focus of least aberration. 



The correction of chromatic aberration has been accomplished by 

 bringing into use the different dispersive powers of various materials 

 which bear no relation to their simple refracting power, in other words, 

 by a combination of flint and crown glass ; and by a most curious series 

 of experiments the dispersive power of flint-glass was found to be so 

 much greater than that of crown-glass, that if the lens a a (fig. 16) 

 be made of crown glass, whose index of 

 refraction is 1-519, and dispersive power 

 0-036, and the lens b b of flint-glass, 

 whose index of refraction is 1-589, and 

 dispersive power 0^0393., and if the 

 focal length of the convex crown-glass 

 lens is made 4^ inches, and that of the 

 concave flint-glass lens 7 inches, they 

 will form a lens with a focal length of 

 ten inches, and will bring rays of light, 

 c c', to a single focus, d, free of colour. 



Such a lens is called an achromatic lens; and when used in combina- 

 tion with other glasses in constructing the microscope, it is termed 

 an achromatic microscope. 



To assist us in gaining a clearer notion of the mode in which a 

 single lens serves to magnify minute objects, it is necessary to take a 



fig. 16. 



