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OPISTHODOMOS, the enclosed space behind a temple.The treasury at Athens was so called, because it stood behindthe temple of Minerva.

OPPOSITE ANGLES, those which are formed by twostraight lines crossing each other, but not two adjacentangles.

Opposite Cones, those to which a straight line can beeverywhere applied on the surfaces of both cones.

Opposite Sections, the sections made by a plane cuttingtwo opposite cones.

OPTIC PYRAMID. See Perspective.

Optic Rays. See Perspective.

OPTICS, (from the Latin, optica) is properly the science of direct vision. In a larger sense, the word is used for thescience of vision, or visibles in general; in which sense,optics includes catoptrics and dioptrics, and even per-spective.

In its more extensive acceptation, optics is a mixedmathematical science, which explains the manner by whichvision is performed in the eye; treats of sight in the general;gives the reasons of the several modifications or alterationswhich the rays of light undergo in the eye ; and shows whyobjects appear sometimes greater, sometimes smaller, some-times more distinct, sometimes more confused, sometimesnearer, and sometimes more remote. In this extensivesignification, it is considered by Sir Isaac Newton , in hisadmirable work called Optics .

Optics make a considerable branch of natural philosophy ;both as it explains the laws of nature, according to whichvision is performed ; and as it accounts for abundance of phy-sical phenomena, otherwise inexplicable.

From optics likewise arises perspective, all the rules ofwhich have their foundation in optics. Indeed, Taequetmakes perspective a part of optics; though John, Arch-bishop of Canterbury, in his Perspectiva Communis, callsoptics, catoptrics, and dioptrics, by the common name per-spective.

This art, for so it should be considered rather than as ascience, was revived, or re-invented, in the sixteenth century.It owes its birth to painting, and particularly to that branchof it which was employed in the decoration of the theatre.Vitruvius informs us, that Agnthnrchus, instructed by _ZEs-chylus, was the first who wrote upon this subject ; and thatafterwards the principles of this art were more distinctlytaught by Democritus and Anaxagoras , the disciples of Aga-tharchus. How they described the theory of this art we arenot informed, as their writings have been lost; however, therevival of painting in Italy was accompanied with a revivalof this art ; and the first person who attempted to lay downthe rules of perspective, was Pietro del Borgo, an Italian .He supposed objects to be placed beyond a transparent tab-let, and endeavoured to trace the images which rays of light,emitted from them, would make upon it. The book which hewrote upon this subject, is not now extant ; and this is themore to be regretted, as it is very much commended by thefamous Egnazio Dante. Upon the principles of Borgo, AlbertDurer constructed a machine, by which he could trace theperspective appearance of objects. Balthazar Parussi, havingstudied the writings of Borgo, endeavoured to make themmore intelligible. To him we owe the discovery of points ofdistance, to which all lines that make an angle of 45 degreeswith the ground-line are drawn. Soon after, Guido Ubaldi,another Italian , found that all the lines, which are parallel toeach other and to the horizon, if they be inclined to theground-line, converge to some point in the horizontal line ;and that through this point, also, a line drawn from the eye,parallel to them, will pass. These principles combined, en-

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abled him to make out a pretty complete theory of persp® . v . gGreat improvements were made in the rules of P e .T, fess°tby subsequent geometricians, particularly by P r j PGravesande, and still more by Dr. Brook Taylor , whose r

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ciples are, in a great measure, new, andthan those of any person before him. juiuuuy - t jt 13really invented this excellent method of perspective, y^ p,suggested by Mr. Robins, that the same method ' va Lrftlished by Guido Ubaldi, in his Perspective, printed a 1 j ea rly>in 1600. In this treatise the method is delivered ver ^j ie In* 1and confirmed by most excellent demonstrations. I n ^ Jiebook, Ubaldi applies his method to the delineation c0 „.scenes of a theatre ; and in this, as far as the practicecerned, he is followed by Signor Sabatellini, in his f pa-di Fabrica Scene, of which there was a new edition <venna in 1638 ; and to this was added a second booj’^ tl> etabling a description of the machines used for produ^®^.sudden changes in the decorations of the stage. I 11 11 ^ oflogue of the great Sir Isaac Newton ’s works, at the facto 11 *liis Life, is a work on perspective, written in Latin ; jii'Elementa Perspective Universalis, 1746. 8vo. , W ?nven* uSdehted to opticians of a much later period f°‘ ^ e sp6'

devices to apply the knowledge they had of optics, ancially of perspective, to the purposes of amusement-For the principles and practice of Perspective:

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article, where they will be fully treated of.

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ORANGERY, a gallery in a garden, or parterre, ge i

to the south, but well closed with a glass window, to Voranges in during the winter season. .. n t tl |ilt

The orangery of Versailles is the most magnin c ^g^a 1 *ever was built; it has wings, and is decorated withorder. tela 0 ' 0 ”

ORATORY, (from the Latin , oralorium, a ten, gl per, f uI "set or apartment in a large house, near a bed-elm-

nished with a small altar, or an image, for private

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among the Romanists. The ancient ^oratoriesivere^pels adjoining to monasteries, wherein the monkstheir prayers, before they had churches. . , er e 1* .

In the sixth and seventh centuries, oratories ei 1 ^churches built frequently in burial-grounds, wit ^ ^glioPbaptistry, cardinal, priest, or any public office,sending a priest to officiate occasionally. c fJo"' er \

ORB, (from the Latin , orbis, a sphere,) a knot 0 ^joii ^or herbs, in a Gothic ceiling, placed upon the -idj 0 ’ 11

several ribs, in older to cover the mitres of every

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ing ribs. This is otherwise called boss. .

ORCHESTRA, in ancient theatres, a place se ^^ c )\the chorus, and in modern theatres that division mmusicians are located. See Theatre. . . on of fk

ORDER, the perfect arrangement and comp 0 ® 1 ^ a se j flarchitectural work ; but the term is more espeem , o y e d 1designate the various methods of arrangement ? w #pP j flGrecian or Classical architecture, and is defim Wl |0 _ eto such a portion of a building as may comprehen ^ Jiodesign by a continuity and repetition of its P arts ‘ gupP^dwho put faith in Vitruvius , the Grecian orders a j lP t, a ito be but an imitation of the parts of a prinj 1 ’ ^ 0 f awhich, according to his theory, originally consis e tmor covering, supported by posts made of the ti u ^ . fl

in four rows, forming a quadrangular enclosure,laid upon the tops of the posts, in order to con gU ppoT £ ( j,gtheir longitudinal direction, in one body. *° #ci' oS9 ] I1 jdcovering, timbers were laid from beam to be a w e f e .i,etbreadth; and to throw off the wet, other . t( j n g f' 11 ' te dparallel to those resting upon the posts, but J u gUpP 0 , e dover on each side of the edifice ; and these &g a gf] j fo 1inclined timbers, which overhung their supp° r s ’