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ABERFOYLE. It and the neighboring Lake of Menteith are the scenes of incidents in Scott's Rob Roy.

ABERGAVENNY, ab'er-gan'ni, or ab'er-ga- ven'ni (the Roman Gobannium). A market town of Monmouthshire, England, 13 miles west of Monmouth, beautifully situated in the valley of the Usk (Map: England, D 5). The town is regularly and compactly built, and many im- provements have of late years been made. It was incorporated in 1899. St. Mary's Church, which was once a fine cruciform structure, and contains many interesting monuments, has been spoiled by restorations. The castle, built by Hammeline de Baladun, soon after the Conquest, is now a ruin. There are collieries and iron works in the neigh- borhood. Pop., 1891, 7700; 1901, 7800.

AB'ERNE'THY. A village in Perthshire, Scotland, on the Tay, about six miles southeast of Perth (Map: Scotland, E 3). It is believed to have been the capital of the Picts, and for many years in the ninth century was the seat of the only bishopric in Scotland. It is chiefly notable, however, for its ancient round tower, like which there is only one other in Scotland. Pop., 1901, police burgh, 623, civil parish, 1270.

ABERNETHY, (1815-96). A Scotch civil engineer. He was born at Aberdeen. In 1841 he was resident engineer of the Aberdeen harbor works, and from 1842 to 1852 was surveying officer for the Admiralty. He was the first to apply hydraulic power to the working of lock-gates, and constructed such important works as the Birkenhead docks, the Hull docks, and the Turin and Savona Railway (Italy), He was also the director of the works for the draining of Lake Abukir, Egypt, by which twenty thousand acres were reclaimed. In 1881 he was elected President of the Institute of Civil Engineers.

ABERNETHY, ( 1680-1740). An Irish dissenting minister. He was born at Colerain, Ireland, the son of a dissenting Presbyterian min- ister; was educated at Glasgow and Edinburgh, and was licensed to preach before he was twenty-one years old. He was ordained at Antrim in 1703; in 1717 he was invited to a congregation in Dublin and another in Bel- fast, while Antrim desired him to remain. The synod was appealed to and decided that he should go to Dublin, but he declined and remained at Antrim. This refusal to obey the synod was unheard of and was considered ecclesi- astical rebellion, and a fierce controversy en- sued, the parties dividing into "subscribers" and "non-subscribers." Though himself strictly evangelical, Abernethy and his associates were remotely the occasion of the contest which ended in eliminating Arian and Socinian elements from the Irish Presbyterian Church. In 1726, Aber- nethy and all the "non-subscribers" were turned out with due ban and solemnity, but only four years afterward he was called to a "regular" congregation in Dublin. In 1731, in the con- troversy regarding the test act, Abernethy took broad ground "against all laws that, upon ac- count of mere differences of religious opinions and forms of worship, excluded men of integrity and ability from serving their country." He was a century ahead of the time, and had to argue against those who denied that a Roman Catholic or a dissenter could be a "man of in- tegrity and ability." Abernethy was foremost where unpopular truth and right were to be

maintained, and his Tracts, collected after his death, did good service for generations. He died in Dublin, December, 1740. Consult Drechal, Sermons of John Abernethy, with his Life (London, 1748-51).

ABERNETHY, (1764-1831). An eminent English surgeon. He was born in London. He was a pupil of John Hunter; in 1787 was appointed assistant-surgeon of St. Bartholomew's Hospital, and in 1815 chief surgeon. Soon after his appointment he began to lecture in the hospital on anatomy and surgery, and may be said to have laid the foundation of its character as a school of surgery. His clear, simple, and positive style, illustrated by an inexhaustible variety of apt anecdotes, made him the most popular medical teacher of his day. In 1813 he was appointed surgeon to Christ's Hospital, and in 1814 professor of anatomy and surgery to the College of Surgeons. His practice increased with his celebrity, which the singular eccentricity and occasional rudeness of his manners contributed to heighten. Of his works, the most important are his Observations on the Constitutional Origin and Treatment of Local Diseases (1806), and his Lectures on the Theory and Practice of Surgery (1830).

AB'ERRA'TION, (from Lat. ab, away + errare, to wander, and Gk. χρῶμα, chrōma, color, literally colored deviation). A phenomenon observed when images of an object, emitting white light are formed by a lens or a prism, it being observed that there is then not one white image, but many colored ones, which do not occupy the same position, and which are of different sizes, thus producing a blurred image with a colored border. It is explained in the article that the sensations of different colors are due to waves in the ether of different wave-number or wave-length, and that these waves, in passing through portions of transparent matter, such as glass, travel with different velocities, depending upon their wave-number. As a consequence of this, in passing through lenses or prisms, waves of different wave-number have different paths. White light is shown to be due to the reception by the eye of waves of different wave-number; or, in other words, from a "white object," or an object "emitting white light," waves of different wave-numbers proceed outward. These waves are such that each train of waves of a definite wave-number would produce in the eye a definite color-sensation, e.g., blue, green, etc. In this sense we may speak of "blue-waves," "green-waves," etc.; and in general white light is due to the reception by the eye of waves which correspond to the "colors of the spectrum" — violet, blue, green, yellow, orange, red, and all the intermediate shades. Therefore, owing to this difference in path in a lens or prism of waves of different color, if an image of a white object is formed there will be a series of images corresponding to the different colors, these images differing in position and size, as well as in color. This result is said to he due to the "chromatic aberration" of the lens or prism. (There are, of course, ether-waves which do not affect the sense of sight; and any prism or lens which is transparent to them will in general deviate waves of different wave-number differently, and so have this same kind of aberration, as ordinary glass lenses have for visible waves.) Mirrors do not