OPTICAL IMAGES. 



length. In the figure, the convex lens is next to the object. 

 This is neither necessary nor usual. They are commonly placed 

 in the contrary position. 



The artist has therefore a wide latitude in the construction of 

 achromatic lenses, of which the most eminent opticians have 

 availed themselves with consummate skill and address, so as to 

 efface by the happy combination of curves, not only the spherical 

 aberration, but also the chromatic aberration of 

 the eye-glass, and the spherical distortion of the 

 final image in the compound microscope, as we shall 

 show in our Tract on that instrument. 



One of the forms of compound lens, which calcu- 

 lation shows to be most free from aberration, is a com- 

 bination of a double-convex lens of crown-glass, with 

 equal convexities, and a double- concave of flint-glass ; 

 the concavity of one face corresponding with the 

 convexity of the crown lens, the radius of the con- 

 cavity of the other face being 23| times that of the 

 crown lens. But since such a concavity within the 

 limits of the face of the lens would (fig. 30) be 

 practically undistinguishable from a plane surface, 

 opticians have combined a plano-concave of flint 

 with the double-convex of crown-glass, which gives 

 all the achromatism that can be desired. 



An achromatic lens of this kind is shown in section in fig. 38, 

 where c c is the double-convex crown, and F F the plano-convex 

 flint lens. 



The discovery of the method of constructing achromatic object- 

 glasses for telescopes and microscopes, constitutes a most important 

 epoch in the history of the progress of physical science. The re- 

 fraction of light without the production of coloured fringes, which 

 was regarded^ by Newton, his contemporaries, and his immediate 

 successors, as incompatible with the established properties of light, 

 was first shown to be possible, and, as it appears, even experi- 

 mentally proved by Mr. Hall, a country gentleman of Worcester- 

 shire, about the year 1730. Three years later, he caused an 

 achromatic telescope to be constructed by one of the London makers. 

 Nevertheless, from some cause not known, this discovery proved 

 fruitless, and the matter was neglected and forgotten. 



The practical realisation of achromatism in telescope lenses is 

 undoubtedly due to John Dollond, who arrived at their construc- 

 tion through a long course of skilful and systematical experiments 

 undertaken for the express purpose. The possibility of solving 

 the problem had been proved theoretically previous to this by 

 Euler, upon reasoning based upon the structure of the eye. 

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