O D P E 777 engraving which have descended to us are certain vessels in onyx, elaborately ornamented with cameo -work, such as the Cup of the Ptolemies, the Farnese Tazza, and the Mantuan or Brunswick Vase. According to the Authorized Version of the Old Testament, the onyx formed the eleventh stone in the breast-plate of the high priest; but it is more probable that this stone (D!"lb>) is the beryl, and that the onyx is D7PP, Exod. xxxix. 11 (E. V. diamond). It is pointed out by the Rev. C. "W. King that the earliest men tion of the onyx as a precious stone is to be found in the inscrip tions of the Parthenon, which date from the Peloponnesian War (431-404 B.C.), where a large engraved onyx is mentioned as an offering. It is to be specially noted that ancient writers applied the term 6Vi> to two entirely different stones. While one of them was the true onyx of modern mineralogists, as described above, the other was merely a stalagmitic variety of carbonate of lime, a mineral much softer, less precious, and much more widely distri buted than the chalcedonic onyx, yet resembling it in so far as it also presents a parallel-banded structure. This mineral is still known as "onyx-marble" (see MARBLE, vol. xv. p. 529). OODEYPORE, or UDAIPTJB. See vol. v. p. 768. OPAH (Lampris luna), a pelagic fish of the family Coryphsemdx or Dolphins. It differs from the typical dolphins (Coryphxna) materially in the shape of its body and the structure of the ventral fins. Instead of being elongate, the body is compressed and deep (more so than in the bream), and destitute of distinct scales. A long dorsal fin, high, and pointed anteriorly, runs along nearly the whole length of the back ; the caudal is strong and deeply cleft. The ventral fins are broad paddles pointed at the extremity, and composed of numerous rays. This latter character is quite exceptional in the order of spiny- rayed fishes, but finds its explanation in the habits of the fish. A pelagic fish with so deep a body and so short a tail as those of the opah requires additional propelling power to be able to find or pursue its prey ; and therefore to the ordinary function of the ventral fins, which in other fishes is confined to balancing the body in the water, has been superadded that of locomotion. In its gorgeous colours the opah surpasses even the dolphins, all the fins being of a bright scarlet. It is only occasionally found near the shore; its real home is the Atlantic, especially near Madeira and the Azores, but many captures are recorded from Great Britain, Ireland, and Scandinavia ; it strays as far north as Iceland and New foundland, and probably southwards to the latitudes of the coast of Guinea. It is rare in the Mediterranean. The name opah, which is now generally used, is derived from the statement of a native of the coast of Guinea who happened to be in England when the first specimen was exhibited (1750), and who thought he recognized in it a fish well known by that name in his native country. From its habit of coming to the surface in calm weather, show ing its high dorsal fin above the water, it has also received the name of " sun-fish," which it shares with Orthagoriscus and the basking shark. It grows to a length of 4 to 5 feet and a weight exceeding 100 ft), and is highly esteemed on account of the excellent flavour of its flesh. OPAL, a natural form of hydrated silica, occurring in a "porodine" condition, i.e., it has apparently hardened from a gelatinous state. During consolidation it has suffered con traction unequally in different directions ; and thus, though amorphous, it behaves in polarized light like a doubly- refracting body. The beautiful variety known as noble or precious opal is remarkable for its play of iridescent colours. In the " harlequin opal " these rainbow-tints are flashed forth from small flakes, forming a kind of polychromatic mosaic, while in other kinds of opal the colours are disposed in broad bands or in irregular patches of comparatively large area. The tints vary with the angle at which the light is incident, and with the relative position of the stone and the observer, so that by moving the opal a brilliant succession of flashes of fire may be obtained. Careful examination of a precious opal shows that these flashes are reflected from surfaces of irregular dimensions, situated at different depths in the translucent stone, and intersecting at various angles. The colours are not due to the presence of any material pigment in the mineral, but are optical effects due to certain structural peculiarities. According to Sir David Brewster the colours of the opal may be referred to the presence of multitudes of micro scopic pores arranged in parallel lines, the differences of tint arising from differences in the magnitude of these cavities. M. Descloizeaux, a very high authority on the optical properties of minerals, has also connected the iridescence of the opal with a regular system of internal cavities, but at the same time seems disposed to think that the colours bear some relation to the presence of a small quantity of organic matter in the mineral. Mr Crookes refers the colours to delicate striae or minute fissures lying in different planes within the opal. The whole subject has been exhaustively studied by H. Behrens, who explains the colour as due not to reflexion from the walls of cavities but to the interposition of very thin lamellae possessing a different index of refraction from that of the matrix in which they are embedded. He has detected in the opal a variety of microscopic enclosures, including flecks of hydrophane and delicate laminae of ferric oxide. On the whole, therefore, it seems likely that the iridescence of the opal is simply a case of the well- known colours of thin plates. A ray of light is reflected from the anterior surface of an attenuated film ; another ray, having traversed the lamina, is reflected from the posterior surface ; and the meeting of these rays differing in phase gives rise to interference phenomena. The noble opal, in consequence of its brilliant iridescence, is valued as an ornamental stone. It is displayed to best advantage when cut en cabochon or with a convex surface. Pliny relates that the rich Roman senator Nonius was proscribed by Mark Antony for the sake of a magnificent opalus which he possessed, a stone as large as a hazel nut, and valued at 20,000 sesterces. The precious opal occurs in veins in trachytic rocks, chiefly in Hungary. The best-known localities are Czerwenitza and Kaschau, but the most brilliantly - coloured stones are said to come from Kremnitz and Dubnik. It is found also in Honduras, principally near Gracias a Dios, where it occurs in like manner, forming veins in volcanic rocks. The American opal is generally less milky and less fiery than the Hungarian. Of late years some very brilliant opal has been sent into the market from Queensland. This is found as thin films spread over the walls of fissures in ironstone nodules, occurring in the neighbourhood of the Baracorra river. It is gener ally too thin to be cut en cabochon, but is largely used for camci, the iridescent layers showing to great advantage against the iron stone matrix, which when polished assumes a dark-brown colour. The so-called "black opals," which have lately become popular, consist of this matrix penetrated in all directions by veins and spots of opal, forming a mixture sometimes known as "root of opal." Certain stones sold as black opals have evidently been modified in colour by staining or by heat. See Brewster, " On the Cause of the Colours in Precious Opal," Etlin. New Phil. Journ., xxxviii., 1845, p. 385 ; Descloizeaux, Manuel de Mineralogie, vol. i., 1862, p. 22; Crookes, "On the Spectral Phenomena of Opals," Quart. Journ. the varieties of opal see MINERALOGY, vol. xvi. p. 390. OPENSHAW, a township of Lancashire, England, is situated on the Manchester and Stockport Canal, about 3| miles east from the Manchester exchange. The church of St Barnabas dates from 1838-39, and that of St Clement from 1871. The mechanics institute (1872) comprises assembly rooms with a reading-room and library. The town possesses railway-carriage and locomotive works, engineering shops, iron-foundries, cotton-mills, dyeworks, chemical- works, and breweries. The area of the township is 579 acres: population in 1871, 11,108; in 1881, 16,153. XVII. 98
