Page:Cyclopaedia, Chambers - Volume 2.djvu/322

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OPT

Eclipfes of the Moon never happen but when (he is in Oppofition with the Sun, and when they both meet in the Ixodes of the Ecliptic. See Eclipse. Mars in his Opp op- tion to the Sun, is nearer the Earth than he is to the Sun.

OPPROBRIl Lafis, fee Lapis Vituperii.

OPSONOMUS, in Antiquity, a Magiftrate of Athens, whereof there were two, or three ; chofen out of the Senate, or Council. Their Office was to infpect the Fifh-Market, and to take care that every thing were done in Order, and according to the Laws.

OPTATIVE, in Grammar, the third Mood of the Conjugations of Verbs, ferving to exprefs an ardent Defire orWifh for any thing. See Mood.

Inftead of a particular Mood, or a particular Set of In- flexions to exprefs this Defire, the Ettglifi, Latins, &c. ex- prefs it by an Adverb of Wifhing prefix'd to it. The Latins by Vtinam ; the Trench by Flat a Dieu ; and the Englifi by Would" God, &c.

In thefe Languages, fetting afide the Adverb, the Op- tative is the fame with the Subjunct ive ; the Inflexions of the Verb, which make what we call the Moods, are the fame in both.

Indeed, in the Greek, the Wiih is exprefs'd by a par- ticular Inflexion, thence call'd Optative ; and in the French, Spamfi, and Italian, there is fomething like it ; their triple Tenfes ferving the fame purpofes. But the Optathe Mood maybe fafely tetrcnch'd from the Latin and Englifi. SeeStJBjoNCTivE.

OPTER1A, among the Antients, Prefents made to a Child, the firfttime a Perfon fawit.

The Word was alfo ufed for the Prefents which the Bridegroom made his Bride when (he was conducted to him ; 'this being the firtt time he faw her. See Bartbal. tie Fuerp.Vet.

The Word isform'd from the Greek tfro/^si, I !ec.

OPTICS, is properly the Science of direct Vifiou. See

Tho', fometimes, the Word is ufed in a larger Senfe for the Science of Vifion, or Vifibles in general : In which Senfeit includes Catoptrics, and Dioptrics ; and even Per- fpective. Sec Catoptrics, Dioptrics, and Ferspec-

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Optics in its more extenfive Acceptation, is a mix d Ma- thematical Science, which explains the manner wherein Vifion is perform'd in the Eye 5 treats of Sight in the ge- neral ; gives the Reafons of the feveral Modifications or Alterations which the Rays of Light undergo in the Eye ; and Ihews why Objects appear fometimes greater, fome- times fmaller, fometimes more diftinct, fometimes more confufed, fometimes nearer, fometimes more remote. See Light, Eye, ci?c.

In thisextenfive Signification, it is confider'd by Sir If. Newton in his admirable Work call'd Optics.

Optics makes a confiderable Branch of Natural Philofo- phy ; both as it explains the Laws of Nature, according to which Vifion is perform'd ; and as it accounts for abun. dance of Phyfical Phenomena, otherwife inexplicable. For what can be determined about Light, Colours, Tranf- parency, Opacity, Brightnefs, Meteors, the Rainbow, Parrhelia, &c. but on Principles of Optics? What about the Nature of the Stars \ The Structure of the Mundane Syflem ? The Motions of the Planets ? The Eclipfes of the Luminaries ? l$c.

Optics, thetefore, make a confiderable Part of Agro- nomy. See Astronomy.

From Optics likewife arifes FerfpeSlive ; all the Rules whereof have their Reafon or Foundation in Optics. Indeed TacqtKt makes Perfpeflive a part of Optics ; tho' Join Arch- bifhop of Canterbury, in his PerfpeBiva Communis, calls Op- tics, Catoptrics, and Dioptrics by the Name Ferfpective. See Perspective.

Euclid has wrote on the antient Optics and Catoptrics : Dioptrics were unknown to them. F. Honorat. Fabri has an Abridgment of Optics, Catoptrics, and Dioptrics. Father Efchinard a Century of Problems in Optics. Vitellio, and Alhazon have done well on the Principles of Optics. Fath er Kircher has a large Volume on the Secrets of Optics, of Lighr, and Shadow ; and its furprizing Effects, which pafsonthe People for Magic. We have alfo I'Optiaue ££? Catoptrique of F. Merfcnne, Paris io"ji. Dioptriaue Oculaire of Fath. Cherubin, Tarts 1*71, Fol. Cbrifl. Sche'meri Optica, Loiid.i6'jz. 3 acobi Gregorii Optica, Lond. 166;. Job. Bap. Fortti tie RefraBione Optices. Barrovii LeBiones Opticx, Lond. 1669. Principe Generate tie I'Optiqtte, by Mr. Leibnitz, in the Leipfic Acts, 1 68a. L'Occhiale alf Occbio, or Dioptrica PraBica, Carol. Ant. Mancini, Boulogne 1660, 4 Q . Pbyjico- Mathefi, de Limine, Coloribits jj? Jridi, per F. Mar. Grimaldi, Bononut 16*65, 4°. Cogitationes Phy/ico-Mechanic* de Natura yifionts, per 3oban. Ott. Scaphufam, Heidel. 1670, 4° • And, who ought to have been named firft, the great Sir J. Newton, in his Optics, Englifi and Latin, 4'. OPTIC-Neroft, the fecond Pair of Nerves, fpringing from

the Crura of the Medulla Oblongata, and paffing thence to the Eye. See Nerve. r 5

Thefe Nerves approach, by degrees, in their rccefs from their Origin ; and at length meet, in the Balis of the Brain, near the Infundibulum. Thence they again fepa- rate, but without decuffating ; and proceed, one to each Eye. See Eye.

They are cover 'd with two Coats, which they take from the Dura and Fia Mater ; and which, by their Expanfions, form the two Membranes of the Eye, call'd the Uvea and Cornea. See Uvea, t/c.

The Retina, which is a third Membrane, and the imme- diate Organ of Sight, is only an Expanfion of the fibrous, or inner, and medullary Part of thefe Nerves. See Re- tina.

The Conffruftion of the Optic-Nerve Teems to be different from that of the other Nerves, which all appear to confift of hard Fibres : For this, e'er it enters the Orbit of the Eye, is only a Coat or Cover form'd by the Pia Mater, and including a Production of the Medulla of the Brain ; which is eafily feparated from it. At its entrance into the Eye, it takes another Coat from the Dura Mater 3 which two Coats are bound together by exceedingly fine Filaments : That from the Fta Mater is continued in the Cboroides, and that from the Dura Mater in the Uvea.

From their Entrance within the Orbit, to the Ball of the Eye, the Medulla, enclofed under the two Coats, is fe- parated into a number of little Cells anfwering to each other. See Vision.

Olric-Pencil, or Pencil of Rays, is that Affemblage of Rays by means whereof any Point or Part of an Object is fecn. See Pencil and Radiant.

Some Optic Writers laugh at the Notion of Optic-Pencils, and maintain 'em mere Chimera's.

Oi r 1 c-Pyramid, in Ferfpective, is the Pyramid A BCO (Tab. Perspective, 'Fig. la.) whofe Bafe is the vifible Object ABC; and its Vertex, in the Eye O ; form'd by Rays drawn from the feveral Points of the Perimeter to the Eye.

Hence alfo appears, what is meant by OpiicTriangle.

Optic Rays, are particularly ufed for thofe wherewith nt\ Optic-Pyramid, ox Optic-Triangle, is terminated ; asOA, OC, OB.

Optic Axis, is a Ray paffing through the Centre of the Eye, or the middle of the Optic-Pyramid, &c.

Optic Chamber, fee Camera ObscurA.

Opt lc-Glaffes, are Glaffes ground either concave, or convex, fo as either to collect, or difperfe the Rays of Light ; by means whereof Vifion is improved, and the Eye ftrengthned, preferved,ci?c.

For the manner of Grinding and Polifhing Optic-Glares, fee Grinding, Polishing, Glass, £yc.

For their Phenomena, fee Lens, Mi rrcr, ci?c

The Principal among Optie-Glajfes, are Tele/copes, Micro- fcopes, .Spectacles, Reading-GlaJJ'es, Magic Lanthorns,&c. See the Construction and Ufe of each under its proper Article, Telescope, Microscope, Spectacle, Magic Lastu horn, &c.

Optic Place of a Star, £yc. is that Point of its Orbit in which it appears tobe, to our Eye. See Place.

This is either true j as when the Eye is fuppofed at the Centre of the Earth, or Planet it inhabits ; or apparent, as when at the Circumference. See Apparent, Planet, &c.

The difference between the two, is the Parallax. See Parallax.

OPTICAL Inequality, in Aftronomy, is an apparent Ir- regularity m the Motions of far diilant Bodies j fo call'd, becauie not really in the moving Bodies, but ariling from the Situation of the Spectator's Eye : So that were the Eye in the Centre, it would always fee the Motions uni- form.

The Optical Inequality may be thus illuftrated: Suppofea Body revolving in the Periphery of a Circle A B D E FGQ, (Tab. Optics, Fig. 40.) and moving through equal Arches AB, BD, DE, EF, in equal Times'; and fuppofe the Eye in the Plane of the fame Circle, butat a dirlance fromit, viewing the Motion of the BodyfromO: When the Body goes from A toE j its apparent Motion is mea- furcd by the Angle A OB, or the Arch H L, which it will feem to defcribe. But in an equal time, while it moves thro' the Arch B D, its apparent Motion will be determi- ned by the Angle BOD, or the Arch L M, which is lefs than the former Arch H L. And when arrived at D, it will be feen at the Point M of the Line N L M. But it fpends the fame time in defcribing D E, which is equal to A B or B D j and when arrived at E, is ftill feen at M 5 appearing Stationary in all the Space from Dto E. When it arrives at E, the Eye will fee it in L j and at G, will appear at H j fo that it will feem to have gone retrograde : And, laftly, from Q, to P, it will again appear Statio- nary.

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