The correction of the chromatic aberration of lenses is commonly effected by combining two, or more, made of materials possessing different ‘dispersive’ powers. Thus, the spectrum formed by flint glass is longer than that formed by crown glass, for the same deviation. When the two are combined, so as to form a compound lens, the one tends to correct the ‘dispersion’ of the other. On this principle ACHROMATIC GLASSES are generally formed in this country. A convex lens of crown glass is combined with a weaker concave lens of flint glass, the latter counteracting the dispersion of the former, without materially interfering with its refractive power. The resulting combination is not absolutely achromatic, but is sufficiently so for all ordinary purposes. According to Dr Blair, a compound lens perfectly achromatic for the intermediate, as well as for the extreme rays, may be made by confining certain fluids, as hydrochloric acid, between two lenses of crown glass. In order to produce nearly perfect achromatism in the object-glasses of telescopes, microscopes, cameras, &c., a concave lens of flint glass is commonly placed between two convex lenses of crown or plate glass, the adjacent surfaces being cemented with the purest Canada balsam, to prevent the loss of light by reflection from so many surfaces. _Obs._ The production of perfect achromatism in lenses is a subject not less fraught with difficulty than with practical importance to the astronomer, the mariner, the microscopist, and the photographer; and it has hence engaged the attention of the leading mathematicians and artists of Europe up to the present time. All the larger object-glasses lately manufactured are said to consist of only two lenses; the resulting achromatism proving sufficiently exact for all useful purposes. Those of recent production have come chiefly from the workshops of Dollond, of London, and the opticians of Bavaria and Switzerland. The achromatism of prisms depends upon the same principles, and it is effected in the same way as that of lenses. =ACIC′ULAR.= Needle-shaped; slender or sharp pointed; spicular; in _botany_, applied to leaves, and in _chemistry_, to crystals. The last are also sometimes termed ACIC′ULÆ. =ACID=, _Syn._ ACIDUM, L.; ACIDE, Fr.; ACIDO, Ital.; SÄURE, G. In familiar language, any substance possessing a sour taste. In _chemistry_, substances are said to be acid, or to have an acid reaction, when they are capable of turning blue litmus red. In _chemistry_, also, the term acid is applied to a very large class of compounds containing hydrogen (hydrogen salts), and in which one or more atoms of that element may be replaced by an equivalent quantity of a metal or other basic radical; _e.g._—— 1. The one atom of hydrogen in hydrochloric acid (HCl) may be replaced by sodium, producing the salt sodium chloride (NaCl). 2. The one atom of hydrogen in nitric acid (HNO_{3}) may be replaced by silver, producing the salt silver nitrate (AgNO_{3}).