OPTICAL IMAGES. 
Yiz.ZS. 
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 prown, and r r 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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