OPERA-GLASS

them was Voltaire, to whose apology for the opera, in the preface to his tragedy of (Edipe, we take leave to refer the reader.,.. OPERA-GLASS. A small telescope used m theatres, whence it has its name. The common opera-gla is nothing else than the Galilean telescope, invented by Galilao in 1609, which was the first ever employed for the purposa of exploring the heavens. A section of the instrument is represented in the annexed figure. It consists of an object gla AB, the focal length of which is usually from 4 to 8 inches. The eyegla , C D, is a double concave or plano-concave lens, from half an inch to 2 inches focus; the distance between the two lenses is equal to the difference of their focal lengths; and the magnifying power is in the ratio of those lengths. Rays of light, R R, falling on the object gla , are refracted towards the axis X Y, and proceed to meet in the focus of the lens at F. But before they reach that point they are intercepted by the concave lens C D, the focus of which is also at F, and by which they are restored to their parallelism. The rays, therefore, proceeding from the object enter the eye, which is applied to the lens C D, without cro ing each other, or forming an image; and hence the distinctne of the Galilean exceeds that of all other telescopes; hence, also, there is no inversion of the image, and, as there are only two gla es, the lo of light is inconsiderable. The lens A B may be either single or compound. If a single lens of plate-gla is used, it should be nearly convexo-plane; if a convex of plate and a concave of flint be combined to form the compound lens, the former should be nearly convexo-plane, and the latter concavoplane. As to the eye-gla , it is always single; and the plano-concave form is perhaps better than the double concave, the eye being, in fact, never placed in actual contact with it. For any magnifying power above 3 or 4, it ought to be limited to an aperture rather le than that of the pupil of the eye; and where distinct vision is desired, the eye should be placed as accurately as po ible on the axis of the instrument. (Coddtngton's Optics.) The area or field of view of this instrument is very limited, and for this reason it cannot be used with any high magnifying power. See Telescope. OPE'RCULUM. (Lat. operio, / cover.) The lid of any tiling: applied by botanists to the cap of the pitcher of Nepenthes, to the loose apex of such fruits as tliat of Lecythis, to the conical limb of the calyx of Eticalyptus, and to the body which closes up the theca of a mo . Operculum. In Zoology, tliis term is applied to the apparatus supported by four bones which protects the gills of fishes; also to the horny or calcareous plate which closes the aperture of univalve shells; and to the four calcareous pieces which defend the entrance to the tube of Balanites or bell-barnacles. OPHICLEl'DE. (Gr. ixpt;, a serpent, and xXm, a key.) The largest bra wind instrument used in the orchestra of the trumpet species, and forming the ba to that cla of instruments: its compa is from double B b to \b above the line, in the ba clef, being three octaves. OPHI'DIANS, Ophidia. (Gr. (xp,s, and t/S«?, form.) The name of the order of reptiles which includes all the serpentiform species of that cla , corresponding to the Amphibia serpentes of Linnaeus. O'PHIOMANCY. (Gr. »(? «.) The name of an early sect of Christian heretics, who emanated from the Gnostics, K) callcjd from their worshipping the serpent that tempted Ere. They considered the serpent as the father of all the sciences, which, but for the tempUtion of our first parenU. would never have been known. (Mosheim £cei. Hist., vol. i.) ' OPHIU'CHUS. or OPHIU'CUS; also called Sernentnrtm. One of the constellations of the northern hcmlnphcro. OPHTHA'LMIA,orOPHTHALMI'TIS.(Gr. ' **»« neck. Emetics have some OPIUM. times been of service. When ail inflammatory symptoms have subsided, local astringents, and mild strengthening eyewaters, may be resorted to; but so long as any inflammation remains they should be most cautiously applied. Sometimes the iris is the seat of inflammation. This is attended with fever, great intolerance of light, and pain, and requires the same treatment. It sometimes terminates in a small absce , which discharges its contents into the interior chamber of the eye, and the sight is destroyed by the permanent damage done to the part. There is another form of ophthalmia in which the conjunctiva and inner membrane of the eyelids becomes inflamed and purulent, as in the Egyptian ophthalmia, which form of the disease is highly contagious. The swelling and purulent inflammation of the eyelids is so great as to close or distort them, and the ulceration sometimes extends over the cornea; the humours escape, the whole organ is frightfully disorganized, and the agony that attends the whole progre of the worst form of the malady is indescribable. The treatment originally resorted to consisted in bleeding and purging: the plan afterwards adopted was to give nauseating doses of tartar emetic, to remove the granulations by the sci ors or knife, and afterwards to apply solutions of nitrate of silver or alum to prevent their reproduction. There is a variety of ophthalmia called »weteste;/c,caused by the translation of some other disease, such as gout, or by the application of some morbid poison to the part. Its treatment depends upon its origin. The purulent opthalmia of new-born infants generally yields to the skilful application of mild astringent eyewaters, and to gentle aperients. After the small-pox and measles, and some fevers, especially in scrofulous habits, the glands of the eyelids form a morbid secretion, which irritates and glues together the parts. This disease has been termed psorophtkalmia; it frequently yields to fomentation with warm water, to a drop of vinous tincture of opium into tl;e eye at bedtime, and to the application of the ointment of nitrate of mercury or red precipitate. O'PIATE. {Gr. or OS, juice.) A medicine producing sleep. See Anodyne. OPISTHO'GRAPHUM. (Gr. »*i Page 860