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Updated: May 24, 2025
The very best optician will always find that on a first trial his glass is not perfect. He will find that he has not given exactly the proper curves to secure achromatism. He must then change the figure of one or both the glasses by polishing it upon a tool of slightly different curvature. He may also find that there is some spherical aberration outstanding.
True achromatism cannot be obtained with ordinary flint and crown-glass; and although in lenses of "Jena glass," outstanding colour is reduced to about one-sixth its usual amount, their term of service is fatally abridged by rapid deterioration.
The perfect achromatism of a reflector is, of course, a great advantage, but the chromatic aberration of refractors is now so well corrected that their inferiority in that respect may be disregarded.
And the mere fact that the problem of achromatism was solved by "the mind of a sagacious optician inquiring how this matter was managed in the eye," no more proves that "this could not be in the eye without purpose, which suggested to the optician the only effectual means of attaining that purpose," than would the fact, say, of the winnowing of corn having suggested the fanning-machine prove that air currents were designed for the purpose of eliminating chaff from grain.
A long list of indictments might indeed be brought against the eye its opacity, its want of symmetry, its lack of achromatism, its partial blindness. All these taken together caused Helmholt to say that, if any optician sent him an instrument so defective, he would be justified in sending it back with the severest censure. But the eye is not to be judged from the standpoint of theory.
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