CHROMATIC ABERRATION. 



33 



from the sizes of the circles of dissipation, than from 

 the iridescent border, and it may still exist, although 

 the spherical aberration of the lens is quite corrected. 

 Chromatic aberration is, as before stated, corrected by 

 combining, in the construction of lenses, two media 

 of opposite forms, differing from each other in the 

 proportion in which they respectively refract and dis- 

 perse the rays of light ; so that the one medium may, 

 by equal and contrary dispersion, neutralize the disper- 

 sion caused by the other, without, at the same time, 

 wholly neutralizing its refraction. .It is a remarkable 

 fact that the media found most valuable for the purpose 

 should be a combination of pieces of crown and flint 

 glass, of crown-glass whose index of refraction is T519, 

 and dispersive power O036, and of flint-glass whose 

 index of refraction is 1*589, and dispersive power 



FIG. 19. 



The focal length of the convex crown-glass 

 lens must be 4J inches, and that of the concave flint-glass 

 lens 7f inches, the combined focal length of which is 

 10 inches. The diagram, fig. 19, shows how rays of light 

 are brought to a focus, free from colour. 



In this diagram, L L is a convex lens of crown-glass, 

 and I I a concave one of flint-glass. A convex lens will! 

 refract a ray of light (s) falling at P on it exactly in 

 the same manner as the prism A B c, whose faces touch 

 the two services of the lens at the points where the ray 

 enters, and quits. The ray S F, thus refracted by the 

 lens L L, or prism ABC, would have formed a spectrum 

 (i j T) on a screen or wall, had there been no other lens, 



