ACHROMATIC without color, the name applied to lenses and telescopes through which objects are seen without false colors, or, in other words, free from that colored fringe which, in the old telescopes, surrounded the object, and diminished its distinctness. The white, or rather colorless ray of light is of several colored rays which have various degrees of refrangibility. See REFRACTION, LIGHT, COLOR. When the direct ray is refracted, it divides itself into colored rays, deviating in various degrees from the right line of the primitive ray. The rays thus refracted by the convex object-glass do not meet exactly in one point, the focus of the glass, but rather at several points, so as to produce the various colors, red, blue and yellow, which surround the object. Newton, misled by imperfect experiments, believed it impossible to find any remedy for this defect; i but Euler. in 1747, expressed his conviction that the desired A. improvement was prac ticable, and this belief was confirmed by the researches of the Swedish mathematician Klingenstierna. The practical solution of the difficulty was reserved for John Dollond; though, when lie obtained a 'patent for his A. telescope, a priority of invention was
claimed for a gentleman of the name of Hall. Dollond succeeded in forming an A. object glass by a combination of crown-glass and flint-glass, which follow one law as to their relative refractive powers, and another as to their powers of dispersing the colors. By uniting a convex lens of crown-glass with a concave one of flint-glass, in certain relative dimensions, a reunion of the colored rays may be effected, and the object will be seen without false colors. In the construction of A. telescopes, Dollond was followed by his son Peter, and also by the optician Ramsden. A further improvement was made by Fraunhofer of Munich, who succeeded in producing perfectly pure glass—a very diffi cult achievement in the case of flint-glass. We owe an important improvement of the A. telescope to the Viennese optician Pldssl, who has lately invented what he calls the dialytic telescope, in which the several kinds of glass composing the compound object glass are not placed close together, but at regulated distances apart. This arrangement allows a shortening of the tube. See TELESCOPE.