Page:The American Cyclopædia (1879) Volume I.djvu/86

66 ACID ACLAND tion will bring the rays to a focus without sep- arating the luminous rays into their colored con- stituents ; see fig. 6. Such a lens is said to be corrected for chromatic aberration. Sometimes the concave correcting lens of flint glass does not quite accomplish the purpose, and then the combination is said to be under-corrected ; but sometimes the opposite is the case, when the combination is said to be over-corrected. In this case the chromatic aberration will be the reverse of what it is with a single convex lens. As the different parts of the colored spectra produced by different media have not an exact proportionality toward one another, an abso- lute achromatism is impossible ; but successful attempts have been made to cure it in some degree by the addition of a third lens of plate glass. Attempts to make achromatic lenses by enclosing fluids of different diffractive powers between glass lenses have all failed, by reason of the variability in such fluids ; in the course of time portions of higher refractive power will accumulate at the lower sides, and by changes of temperature currents will be set up which disturb the images seen. As the manufacture of flint glass for large achromatic lenses is a very difficult and uncertain operation, and therefore very expensive, their size has been re- duced by placing an over-corrected combination of half the size in the middle of the telescope ; such an instrument is called a dialitic tele- scope. Recently the plan of the elder Her- schel has been revived, namely, to use no large achromatic objective lenses at all, but reflec- tors, which of course can have no chromatic aberration, which is the result of refraction. ACID, a compound of hydrogen, in which that element is united to an electro-negative radical. In common language the term is equivalent to the Latin word acidus, meaning anything sour. Oxygen was formerly con- sidered to be the element upon which the existence of the acid character mainly de- pended, as its name (signifying generator of acids) implies; but later researches have brought to light a number of compounds containing hydrogen possessed of acid proper- ties in which oxygen is not present. Hence hydrogen is now regarded as more truly the generator of acids than oxygen. The usual test for the presence of an acid is its prop- erty of changing blue vegetable colors to red. We are already acquainted with several hun- dred acids, most of them belonging to the organic kingdom, and new ones are con- stantly discovered by chemists. The juices of plants and the constituents of animal bod- ies furnish their peculiar acids ; and with the changes these undergo new acids are gen- erated by different modes of combination, which processes are now imitated by art so as to reproduce by synthesis a number of organic acids. Some acids, when uncom- bined, are gaseous, others fluid, and others solid. Their properties also are as various as the conditions in which they exist. ACILIUS 6LABRIO, Manias, a Roman general, who became consul in 191 B. 0. He was of plebeian origin, but rose by regular gradation. He supported Cornelius Scipio; commanded as consul against Antiochus the Great of Syria, and defeated him at Thermopylae ; and sub- sequently carried on the war against the ^Etolians with equal success. On his return he had a triumph. But this elevation and success of a. plebeian gave offence to the pa- tricians of Rome, who stirred up annoyances and accused him of keeping back the public spoils ; but he was not condemned. He was the first to whom a statue of gold was erected in Italy. He wrote the annals of Rome in Greek, a narrative full of fables. ACI REALE, a seaport town on the E. coast of Sicily, in the province of Catania, cel- ebrated for its mineral waters; pop. in 1871, 35,787. It is situated on a hill of lava with a precipice over 650 feet high facing the sea, in the highly picturesque region between Mount Etna and Catania, 11 m. N. N. E. of the latter, at the mouth of the small river Aci ; is well built, principally of lava, and has many churches, convents, and towers. Great quan- tities of diaper are made. Near the town are the famous cave of Polyphemus and the grotto of Galatea. ACIS, in Ovid, son of Faunus and Symeethis, beloved by the nereid Galatea, and through jealousy crushed to death under a huge rock by Polyphemus. Galatea changed his blood into the river Acis, on which now stands the town of Aci Reale, where the scenes of the legend are still shown. ACKERMAim. I. H.mrad Ernst, a German comedian, regarded as one of the founders of the German stage, born in Schwerin in 1710, died in Hamburg, Nov. 13, 1771. In 1740 he made his debut as an actor under the auspices of SchSnemann, and afterward organized a travelling company, with which he performed in many places. He is celebrated as the founder of the Hamburg theatre (1765), whose performances inspired Lessing's famous com- ments on dramatic art. II. Sophie Charlotte, wife of the preceding (1749), previously widow of the organist Schroder, born in Berlin in 1714, died Oct. 14, 1792. She was not only distinguished as an accomplished actress both in tragedy and comedy, and teacher of the histrionic art, but also as the mother by her first marriage of Friedrich Ludwig Schroder (see SCHBODEB), and of two daughters by her second marriage, also very distinguished : Do- BOTHEA, who retired from the stage in 1778 on marrying Prof. Unzer, and CHABLOTTE, whose death in 1775, in her 18th year, was generally deplored at Hamburg. ACLAND. I. John Dyke, a British major, son of a baronet, commander of the grenadiers in the battle of Stillwater in the American revo- lution, Oct. 7, 1777, died in 1778. When overpowered by numbers the British retreated to their camp, which was furiously stormed by