O R A

14:3

O RA

ble object, at which it can i>e distinctly seen ; and let d representits distance trom the object-glass of the instrument. Let m be themagnifying power of the instrument; that is, let the visual anglesubtended at the eye by the object when at the distance/(, and view-ed without the instrument, be to the visual angle produced by theinstrument as 1 to m Let a be the diameter of the object-glass,andp he that of the pupil. Let the instrument be so constructed,that no parts of the pencils are intercepted for want of sufficientapeitures of the intermediate glasses. Lastly, let the light lostin reflection or refraction he neglected. The brightness of vision

through the instrument will be expressed by the fraction-

in p d’’

the brightness of natural vision being 1. But although this fractionmay exceed unity, the vision through the instrument will not hebrighter than natural vision. For, when (Ills is the case, the pupildoes not receive all the light transmitted through the instrument.In microscopes, n is tiie nearest limits of distinct vision,nearly editinches. But a difference in this circumstance, arising from a dif-ference in the eye, makes no change in the formula! because inchanges in the same proportion with «. In telescopes, n and

a z

d may be accounted equal, and the formula becomes -

m p z

4. The Magic Lanthorn. This corresponds with and is up-on the same principles as the solar microscope, and may be usedwith good effect for magnifying small transparent objects; but Ingeneral it is applied to the purpose of amusement, by casting du-nnage of a small transparent painting on glass upon a white wall orscreen, at a proper distance from the instrument. Let a caudle or

lamp C, fig. 13, Plate CXVIII. be placed in the inside of a boxso that the light may pass through the plano-convex lens NN,and strongly illuminate the object OB; which is a transparentpainting on glass, inverted and moveable beiore NN, by means ofa sliding piece in which the glass is set or fixed. This illumina-tion is still more increased by the reflection of light from a concavemirror SS, placed at the other end of the box.which causes the lightto fall upon the lens NN, as represented in the figure. Lastly, alens LL, fixed in a sliding tube, is brought to the requisited dis-tance from the object OB, and a large erect image IM is formedupon the opposite wall.

5. The Camera Ohscura. This instrument has the same rela-tion to the telescope as the solar microscope has to the commondouble microscope, and is thus constructed : Let CD, tig. 12,represent a darkened chamber perforated at L, where a convexlens is fixed, the curvature of which is such, that the focus of pa-rallel rays falls upon the opposite wall. Then if AB is an object atsuch a distance that the rays which proceed from any given pointof its surface to the lens L may be esteemed parallel, an invertedpicture will be formed on the opposite wall ; tor the pencil whichproceeds from A will converge to a, and the pencil which pro-ceeds from B will converge to b, and the intermediate points oi theobject will be depicted between a and b. For the use of paintersthese instruments are now constructed in a very convenient mode.The lens is made to slide in a small wooden box, so as to be easi-ly adjusted to a proper focus; and the image falls upon a planemirror, placed obliquely at the back part of the box, from whichit is reflected on a piece of ground-glass, or on a sheet of while pa-per extended over. The picture which is thus formed is verytender and beautiful. The moving objects give it animation; andthe outline formed is so perfect that it may be easily traced., evenby a person who is little skilled in drawing or perspective.

OPTIMATES, one of the divisions of the Roman people, op- ;posed to populares. They were warm supporters of the dignity 'of the chief magistrate, and promoters of the grandeur of the state,who-cared not if the inferior members suffered, provided the com-manding powers were advanced; whereas the populaces boldlystood up for the rights of the people, pleaded for larger urivile<*esand laboured to bring matters nearer to a level. 8 P

OPTION, in ecclesiastical polity. When a new suffraganbishop is consecrated, the archbishop of the province, by a cus-tomary prerogative, claims the collation of the first vacant benefice,or dignity, in that see, according as he shall choose; which choiceis called the archbishop’s option. But if the bishop die, or betranslated, before the incumbent of the promotion chosen by thearchbishop die or he removed, it is generally supposed that theoption is void ; as the granter, singly and by himself, could notconvey any right or title beyond the term of his continuance inthat see. And if the archbishop die before the avoidance happen,the right of filling up the vacancy shall go to his executors or ad-ministrators. j

OPUNTIA, the prickly pear-tree, a species of cactus; SeeCactus .

OR, the French word for gold, by which this metal is expressedin heraldry. In engraving it is noted by small points all over thefield or bearing. It may be supposed tosignify of itself, generosity,splendor, or solidity, ; according to G. Leigh, if it is compoundedwith Gules, it signifies Courage ; Azure, Trust; Vert, Joy; Pur-pure, Charity ; and Sable, Constancy.

ORA, in antiquity, was a term equivalent to an ounce; but ithas been much debated among antiquaries, whether the ora was acoin, or only money of account.

ORACH, a town of Bosnia , capital of a Sangiacat, 72 milesS. E. of Serajo, and 96 S. S. W. of Belgrade .

ORACLE, in sacred history, is sometimes used for the mercy-seat, or the cover of the ark of the covenant; and at other times itis taken for the sanctuary, or for the most holy place, whereih theark was deposited. Among the Jews we may distinguish severalsorts of real oracles. They had first oracles that were deliveredviva voce; as when God spake to Moses face to face, as one friendspeaks to another, (Numb. xii. 8.) 2. Prophetical dreams sent by

God: as the dreams which God sent to Joseph, and which foretoldhis future greatness, (Gen. xxxvii. 5, 6.) 3. Visions; as when a

prophet in an ecstacy, being neither properly asleep nor awake,had supernatural revelations. (Gen. xv. 1, xlvi. 2.) 4. The ora-

cle of Urim and Thummim, which was accompanied with theephod or the pectoral worn by the high-priest, and whic h God had endued with the gift of foretelling things to come, (Numb. xii.6, Joel ii. 28.) This manner of enquiring of the Lord was oftenmade use of from Joshua’s time to the erection of the temple atJerusalem . Fifthly, After the building of the temple, theygenerally consulted the prophets, who were frequent in Judah andIsrael. From Haggai, Zechariah, and Malachi , who are the last ofthe prophets that have any of their writings remaining, the Jews pretend that God gave them what they call Bathkol, the daughterof the voice, which was a supernatural manifestation of the w ill ofGod , performed either by a strong inspiration or internal voice,or else by a sensible and external voice, which was heard hv anumber of persons sufficient to bear testimony of it. For example,such was the voice that was heard at the baptism of Jesus Christ (Matth. iii. 17.) The scripture affords us examples likewise ofprofane oracles. Balaam , at the instigation of his own spirit, andurged on by his avarice, fearing to lose the recompense that hewait promised by Balak king of the Moabites , suggest a diabolicalexpedient to this prince, of making the Israelites fall into idolatryand fornication (Numb. xxii. 14, xxxi. 16,) by which he assureshim of a certain victory, or at least of considerable advantageagainst the people of God . See also 1 Kings xxii. 21, See. Thedialogue there recorded proves two tilings ; ' first, that the devilcould do nothing by his own power; and. secondly, that with thepermission of God , he could inspire the false prophets, sorcerers,and magicians, and make them deliver false oracles.

Oracle, among the heathens, was the answer, which the godswere supposed to give, to those who consulted them upon any af-fair of importance. The credit of oracles was so great, that in alldoubts and disputes their determinations were held sacred and in-violable: whence vast numbers docked to them for advice; and nobusiness of importance was undertaken, scarce any peace con-cluded, any war waged, or any new form of government instituted,without the advice and approbation of some oracle. The answers

were