O P A 



519 



OPH 



so called from its resemblance to the stone 

 called onyx, and tmguis, from its resem- 

 blance to the nail of the finger. 



O'OLITE, from 5K, an egg, and Xidof , 

 stone ; eggstone. A. species of limestone 

 composed of globules clustered together, 

 commonly without ^iny visible cement ; 

 hence called also roestone, from its resem- 

 blance to the roe of a fish. The oolite in 

 England forms a considerable formation, 

 commencing with the Portland beds 

 above, and terminating in the inferior 

 oolite resting upon the lias. These beds 

 comprehend sandstones, marls, and clays ; 

 and are exceedingly rich in fossil remains 

 of ammonites and belemnites. Many 

 limestones in other groups are oolite. 



OOST, ) A stove in which the picked 



OAST. ) hops are dried. 



OOZ'E. 1. Soft mud or slime. 2. The 



liquor of a tan-vat. 



OOZO'A, uov, egg, faiv, animal. A pri- 

 mary division of the animal kingdom, in 

 which the nervous and sanguiferous sys- 

 tems are completely developed, corres- 

 ponding to the Acrita. 



O'PAL. An ornamental stone of mo- 

 derate value, a sub-species of indivisi- 

 ble quartz, of which there are several 

 varieties, found in different parts of 

 Europe and the East Indies. The princi- 

 pal of these are (1.) The noble opal, a 

 milk-white or pearly-grey variety, with 

 a beautiful play of very various and rich 

 colours ; (2.) Fire opal, a variety which by 

 reflected light exhibits, as its position is 

 varied, elegant and most beautiful irides- 

 cent colours, particularly emerald-green, 

 golden-yellow, flame and fire-red, violet 

 and purple, and celestial blue, beautifully 

 blended ; (3.) When the colour is arranged 

 in small spangles, it takes the name of 

 the Harlequin opal ; and (4.) "When the 

 stone is a monochromatic yellow, it is 

 named Golden opal ; (5.) The semi-opal is 

 a feebly translucent variety, of a white, 

 grey, or brown colour ; (6.) Menilite is a 

 variety occurring in small irregular 

 roundish masses, often tuberose, often 

 bluish or striped, and generally translu- 

 cent. These varieties consist of silex in 

 various proportions, from 85 to 95 per 

 cent., combined with oxide of iron and 

 water. The semi-opal contains a little 

 alumina. Prof. Ehrenberg states that 

 some specimens of semi-opal which he 

 has examined consist almost exclusively 

 of the shells of infusoria, some partially 

 dissolved, and others unaltered. He has 

 also found indications of animalcular 

 shells in the noble opal. 



OPALES'CENCE. A coloured shining 

 lustre, reflected from a single spot in a 

 mineral when held in some particular 

 position : iridescence. 



OPAL'IZED WOOD. Wood petrified by 

 tilica, resembling opal. 



Or'ERA (Lat., Ital., Sp., Fr., En.). A 

 dramatic composition, sot to music, and 

 sung on the stage, accompanied with 

 musical instruments, and enriched with 

 magnificent dresses, dancing, &c. Ac- 

 cording as the serious or the comic cha- 

 racter prevails, the opera is termed opera 

 seria or opera buffa (the opera comiquf of 

 the French). The grand opera is confined 

 to music and song. 



OP'ERA-GLASS. An optical instrument, 

 so called from its use in theatres, and 

 sometimes termed a diagonal perspective, 

 from its construction. It consists of a 

 tube about four inches long, in each side 

 of which there is a hole, exactly against 

 the middle of a plane mirror, which re- 

 flects the rays falling upon it to the con- 

 vex lens, through which they are re- 

 fracted to the concave eye-glass, whence 

 they emerge parallel to the eye at the 

 hole in the tube. The instrument is not 

 intended to magnify objects more than 

 two or three times. The peculiar artifice 

 is, that a person at a small distance may 

 be observed without exciting suspicion, 

 for the instrument points to a different 

 object from that viewed, and as there is a 

 hole in each side of the tube, it is not 

 even known on which hand of the obser- 

 ver the object is situated. 



OPERAM'ETER. Work-measurer. A ma- 

 chine consisting of a train of toothed 

 wheels, working in a box, having indexes 

 attached like the hand of a clock, and a 

 dial-plate, whereby the number of rota- 

 tions of a shaft projecting from the pos- 

 terior part of the box is shown. This shaft 

 is to be attached, in any convenient way, 

 to the working parts of rotatory machi- 

 nery, to show the revolutions, &c. made 

 in a given time. It is particularly useful 

 in the machinery for dressing cloths. 



OPER'CCLUM (Latin) a cover. 1. The 

 lid or cover of the peristomium, or fringe 



of mosses. 2. The flap which covers the 



gill or organ of respiration in fishes. 3. 



A lid by means of which many of the 

 mollusca close the aperture of their shells. 

 It is testaceous in some, and in others 

 horny or cartilaginous. 



OPERET'TA, diminutive of opera. Ashort 

 musical drama of a light character. The 

 French vaudeville belongs to this species 

 of composition. 



OPHI'ASIS, from ofi<s, a serpent. A form 

 of porrigo which commences at the occi- 

 put, and winds to each ear, and sometimes 

 to the forehead, with a sort of serpentine 

 course. 



OPHICEPH'ALUS. A genus of acantho- 

 pterygious fishes, resembling the Anabas, 

 Cuv.,in most of its characters, and also 

 in the habit of creeping out of the water. 

 The Indian jugglers often exhibit it out 

 of the water; and in the markets of China, 

 tte larger species are cut up alive for 



