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 have been carried on for centuries in the mountains near Eperjes, and some remarkable stones from this locality are preserved in the Imperial Natural History Museum in Vienna, including an uncut specimen weighing about 3000 carats. Precious opal is found also in Honduras, especially in trachyte near Gracias á Dios; and in Mexico, where it occurs in a porphyritic rock at Esperanza in the state of Queretaro. A remarkable kind of opal, of yellow or hyacinth-red colour, occurs in trachytic porphyry at Zimapan in Hidalgo, Mexico, and is known as “fire-opal.” This variety is not only cut en cabochon but is also faceted. Fire-opal is sometimes called “girasol.” Much precious opal is worked in Australia. In Queensland it is found lining cracks in nodules of brown ironstone in the Desert Sandstone, a rock of Upper Cretaceous age, and is distributed over a wide area near the Barcoo river. Bulla Creek is a well-known locality. The layer of opal, when too thin to be cut with a convex surface, is used for inlaid work or is carved into cameos which show to much advantage against the dark-brown matrix. The matrix penetrated by veins and spots of opal, and perhaps heightened in colour artificially, has been called “black opal”; but true black opal occurs in New South Wales. The “root of opal” consists of the mineral disseminated through the matrix. In New South Wales precious opal was accidentally discovered in 1889, and is now largely worked at White Cliffs, Yungnulgra county, where it is found in nodules and seams in a siliceous rock of the Upper Cretaceous series. It is notable that the opal sometimes replaces shells and even reptilian bones, whilst curious pseudomorphs, known as “pineapple opal,” show the opal in the form of aggregated crystals, perhaps of gypsum, gaylussite or glauberite.

“Common opal” is the name generally applied to the varieties which exhibit no beauty of colour, and may be nearly opaque. It is frequently found in the vesicular lavas of the N.E. of Ireland, the west of Scotland, the Faroe Isles and Iceland. When of milky-white colour it is known as “milk opal”; when of resinous and waxy appearance as "resin opal"; if banded it is called “agate opal”; a green variety is termed “prase opal”; a dark red, ferruginous variety “jaspar opal”; whilst “rose opal” is a beautiful pink mineral, coloured with organic matter, found at Quincy, near Méhun-sur-Yèvre, in France. A brown or grey concretionary opal from Tertiary shales at Menilmontant, near Paris, is known as menilite or “liver opal.” A dull opaque form of opal, with a fracture imperfectly conchoidal, is called “semi-opal”; whilst the opal which not infrequently forms the mineralizing substance of fossil wood passes as “wood opal.” The name hydrophane is applied to a porous opal, perhaps partially dehydrated, which is almost opaque when dry but becomes more or less transparent when immersed in water. It has been sometimes sold in America as “magic stone.” Cacholong is another kind of porous opal with a lustre rather like that of mother-of-pearl, said to have been named from the Cach river in Bokhara, but the word is probably of Tatar origin.

OPALINA (so named by J. E. Purkinjě and G. Valentin), a genus of Protozoa, without mouth or contractile vacuole, covered with nearly equal flagelliform cilia, and possessing numerous nuclei, all similar. It has been referred to Aspirotricha by Bütschli, but by M. Hartog (Cambridge Natural History, vol. ii., 1906) has been transferred to the (q.v.). All the species are parasitic in cold-blooded Vertebrates.

OPATA (“enemies,” so called by their neighbours the Pimas), a tribe of Mexican Indians of Piman stock. Their country is the mountainous district of north-eastern Sonora and north-western Chihuahua, Mexico. Though usually loyal to the Mexican government, they rebelled in 1820, but after a gallant effort were defeated. They number now about 5000, and still largely retain their ancient autonomy.

OPERA (Italian for “work”), a drama set to music, as distinguished from plays in which music is merely incidental. Music has been a resource of the drama from the earliest times, and doubtless the results of researches in the early history of this connexion have been made very interesting, but they are hardly relevant to a history of opera as an art-form. If language has meaning, an art-form can hardly be said to exist under conditions where the only real connexions between its alleged origin and its modern maturity are such universal means of expression as can equally well connect it with almost everything else. We will therefore pass over the orthodox history of opera as traceable from the music of Greek tragedy to that of miracle-plays, and will begin with its real beginning, the first dramas that were set to music in order to be produced as musical works of art, at the beginning of the 17th century.

There seems no reason to doubt the story, given by Doni, of the meetings held by a group of amateurs at the house of the Bardi in Florence in the last years of the 16th century, with the object of trying experiments in emotional musical expression by the use of instruments and solo voices. Before this time there was no real opportunity for music-drama. The only high musical art of the 16th century was unaccompanied choral music: its expression was perfect within its limits, and its limits so absolutely excluded all but what may be called static or contemplative emotion that “dramatic music” was as inconceivable as “dramatic architecture.” But the literary and musical dilettanti who met at the house of the Bardi were not mature musical artists; they therefore had no scruples, and their imaginations were fired by the dream of restoring the glories of Greek tragedy, especially on the side of its musical declamation. The first pioneer in the new “monodic” movement seems to have been Vincenzo Galilei, the father of Galileo. This enthusiastic amateur warbled the story of Ugolino to the accompaniment of the lute, much to the amusement of expert musicians; but he gained the respect and sympathy of those whose culture was literary rather than musical. His efforts must have been not unlike a wild caricature of Mr. W. B. Yeats’s method of reciting poetry to the psaltery. The first public production in the new style was Jacopo Peri’s Euridice (1600), which was followed by a less successful effort of Caccini’s on the same subject. To us it is astonishing that an art so great as the polyphony of the 16th century could ever have become forgotten in a new venture so feeble in its first steps. Sir Hubert Parry has happily characterized the general effect of the new movement on contemporary imagination as something like that of laying a foundation-stone—the suggestion of a vista of possibilities so inspiriting as to exclude all sense of the triviality of the present achievement. Meanwhile those composers who retained the mastery of polyphonic music tried to find a purely vocal and polyphonic solution of the problem of music-drama; and the Amfiparnasso of Orazio Vecchi (written in 1594, the year of Palestrina’s death, and produced three years later) is not alone, though it is by far the most remarkable, among attempts to make a music-drama out of a series of madrigals. From the woodcuts which adorn the first edition of the Amfiparnasso it has been conjectured that the actors sang one voice each, while the rest of the harmony was supplied by singers behind the stage ; and this may have been the case with other works of this kind. But the words of Vecchi’s introductory chorus contradict this idea, for they tell the audience that “the theatre of this drama is the world” and that the spectators must “hear instead of seeing.”