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distance of bodies, which, however, in many cases, are corrected by experience. The nearer any object is to the eye, the larger is the angle by which it will appear in the eye, and therefore the greater will be the seeming magnitude of that body. In fig. 4 let A B be an object viewed directly by the eye QR. From each extremity draw the lines AN and B M, intersecting each other in the crystalline humor at I. Then draw the line I K in the direction in which the eye is supposed to look at the object. The angle AIB is then the optical or visual angle, and the line IK is called the optical axis, because it is the axis of the lens or crystalline humor continued to the object.

535. The apparent magnitude of objects then, depending thus on the angle under which they are seen, will evidently vary according to their distances. Thus different objects, as A B, C D, E F, the real magnitudes of which are very unequal, may be situated at such distances from the eye as to have their apparent magnitudes all equal; for if they are situated at such distances that the rays A N, B M, shall touch the extremities of each, they will then appear all under the same optical angle, and the diameter M N of each image on the retina will consequently be equal.

536. In the same manner objects of equal magnitude, situated at unequal distances, will appear unequal. For let A B and G H, two objects of equal size, be placed before the eye at different distances I K and IS; draw the lines GP and HO, crossing each other in I; then OP, the image formed by the objects G H on the retina, is evidently of a greater diameter than the image M N, which represents the object A B; in other words the object G H will appear as large as an object of the diameter TV situated at the same place as the object A B.

537. To render the subject still clearer, suppose the object H K, fig. 5, to be at 100 yards distance, it will form an angle in the eye at A. At 200 yards distance the angle it makes will be twice as small in the eye at B. Thus, to whatever moderate distance the object is removed, the angle it forms in the eye will be proportionably less, and therefore the object will be diminished in the same proportion.

538. Hence it follows that objects situated at different distances, whose apparent magnitudes are equal, are to each other as their distances from the eye; and, by the same rule, equal objects situated directly before the eye have their apparent magnitudes in a reciprocal proportion to their distances.

539. This last proposition must, however, be received with some allowance; for it is only applicable to very distant objects, and to those where the sense is not corrected by the judgment. For, if the objects are near, we do not judge of their magnitude according to the visual angle. Thus, if a man of six feet high is seen at the distance of six feet under the very same angle as a dwarf of only two feet high, at the distance of two feet, still the dwarf will not appear as large as the man, because the sense is corrected by the judgment. In most cases, however, where the distance is considerable, the rule will be found accurate.

540. If the eye is placed above a horizontal

plain, the different parts of this plain will appear elevated in proportion to their distance, till at length they will appear on a level with it. For, in proportion as the different parts are more distant, the rays which proceed from them to form angles with the optical axis IK, fig. 4, more and more acute, and at length become almost parallel. This is the reason why, if we stand on the sea-shore, those parts of the ocean which are at a great distance appear elevated; for the globular form of the earth is not perceptible to the eye, and, if it was, the apparent elevation of the sea is far greater than the arch which a segment of the globe would form within any distance that our eyes are capable of reaching.

541. For the same reason, if a number of objects are placed on the same plain, and at the same height below the eye, the more distant will appear taller than the others; and if the same objects are placed on a similar plane above the eye, the more distant will appear the lowest.

542. The distant parts of a long wall, for the same reason, appear to a person who stands near one end to curve or incline towards him. In the same manner, the high wall of a lofty tower seems to a spectator placed directly under it to bend over him, and threaten him with instant destruction. If any person who wishes to try the experiment will lie down on his back in a situation of this description, at the distance of five or six feet from the wall of which he contemplates the tremendous height, he will immediately be made sensible of the phenomenon.

543. If the distance between two objects forms an invisible angle, the objects, though in reality at some distance from each other, will appear contiguous. This is assigned by some astronomers as the reason why the ring or belt of Saturn appears as one mass of light, while they contend that it is formed from a number of little stars or satellites ranged within a certain distance of each other.

544. If the eye is carried along as in a boat, without being sensible of its own motion, the objects which are stationary on each side will appear to move in a contrary direction. Thus we attribute to the sun and the other heavenly bodies a diurnal motion, which only affects the earth which we inhabit.

545. If two or three objects at a considerable distance, and on which the eye of the spectator is fixed, move with equal velocity past a third object which is at rest, the moving objects will appear to be actually at rest, and that which is really stationary will appear in motion. Thus the clouds which pass over the face of the moon appear at rest, while the moon itself appears to proceed rapidly along in an opposite direction. This happens because the eye, which is fixed upon the clouds, follows their motion mechanically, and therefore the moon appears to move and not the clouds, as in the boat we do not perceive its motion, but conceive the banks are retiring behind us.

546. If the centre of the pupil, that is, the optic axis, is directed along the surface of any slender object in a perfectly straight line, this line will appear only a point, because, in fact, the extremities only are visible.

547. An extended and distant arch, viewed

by an eye which is exactly in the same line, will appear as a plane surface, because, all the parts appearing equally distant, the curvature will not be perceived.

548. If a circle is viewed obliquely it will appear an oval, because the diameter which is perpendicular to the eye is shortened; in other words, the rays which proceed from the extremities form an angle so much the more acute as the obliquity is greater; on the contrary the diameter which is parallel to the eye is apparently extended.

549. We are indebted to S. L. Kent, Esq., honorary secretary of the London Institution, for the following very important facts, which tend materially to illustrate the real arrangement of the prismatic spectrum, as it serves to reduce the number of colors to three; while Dr. Wollaston's latest experiments would indicate the number four, as their ultimate component parts. 550. The prism used in these experiments was five inches long, and the side planes one inch broad; the lens was six inches in diameter, having a focus of two feet three inches; and it was found requisite that the diameter of the lens should exceed the length of the prism in order to insure a good spectrum.

551. Experiment 1.-Mr. Kent threw the colors of the prism on a screen eleven feet distant; and having placed the lens between them, and only two inches from the prism, as in fig. 6, he found the prismatic colors magnified and in the same order, to the dimension of two feet six inches in width, and one foot three inches in depth. In this case the sun's rays were admitted through a Venetian blind; but, when admitted through a hole in a shutter of five inches by four, the dimension was only two feet by nine inches.

552. Experiment 2.-Having placed the lens at the distance of two feet six inches from the prism, as in fig. 7, the figure of the prism was clearly defined, but without exhibiting any prismatic colors whatever on the screen.

553. Experiment 3.-He placed the lens three feet from the prism, as in fig. 8, which produced only the figure of the prism having the violet ray at the bottom and the yellow above.

554. Experiment 4.-When the lens was five feet from the prism, as in fig. 9, the figure of it was distinctly seen with the prismatic colors reversed.

555. Experiment 5.-Mr. Kent placed the lens behind the prism, as in fig. 10, and threw the sun's rays on it at its focal distance, two feet three inches, when the prismatic colors were increased, both in brilliancy and magnitude, considerably more than in the first experiment.

556. Experiment 6.-The lens was put within the focal distance of the screen, as in fig. 11, when a small figure of the prism was seen very bright, but without any prismatic color.

557. Experiment 7.-Having placed the lens as in the second experiment, no prismatic spectrum was produced, but a perfect spectrum of the prism in a strong white light; Mr. Kent then placed another prism in the focus of the lens, as in fig. 12, which produced three colors only, viz. yellow, of a greenish tint, red, and

deep violet. Wishing to ascertain if those three colors were neutral, he tried them with a third prism, and found not the slightest alteration; and, having placed a card so as to receive them, he found, on giving it a whirling motion, that the colors were entirely lost.

558. In the most perfect form of the phantasmagoria lantern the sliders are made opaque, except where the figures are introduced, the glass being covered, in the light parts, with a more or less transparent tint, according to the effect required. Several pieces of glass may also be occasionally placed behind each other, and may be made capable of such motions as will nearly imitate the natural motions of the objects which they represent. The figures may also be drawn with water colors on their paper, and afterwards varnished. By removing the lantern to different distances, and altering at the same time more or less the position of the lens, the image may be made to increase or diminish, and to become more or less distinct at pleasure, so that to a person unaccustomed to the effects of optical instruments the figures may appear actually to advance and retire. In reality, however, these figures become much brighter as they are rendered smaller, while in nature the imperfect transparency of the air causes them to appear fainter when they are remote than when they are near: this imperfection might be easily remedied by the interposition of some semi-opaque substance, which might gradually be caused to admit more light as the figure becomes larger, or by uncovering a larger or smaller portion of the lamp, or of its lens. Sometimes, by throwing a strong light upon the actual opaque object, or on a living person, its image is formed on the curtain, retaining its natural motions; but in this case the object must be considerably distant, otherwise the images of its nearer and remoter parts will never be sufficiently distinct at once, the refraction being either too great for the remoter, or too small for the nearer parts and there must also be a second lens, placed at a sufficient distance from the first to allow an inverted image to be formed between them, and to throw a second picture of this image on the screen in its natural erect position; unless the object be of such a nature that it can be inverted without inconvenience. This effect was very well exhibited at Paris by Robertson; he also combined with his pictures the shadows of living objects, which imitate tolerably well the appearance of such objects in a dark night, or by moon-shine; and, while the room was in complete darkness, concealed screens were probably let down in various parts of it, on which some of the images were projected; for they were sometimes actually situated over the heads of the audience.

559. Dr. Young's suggestions may be thus illustrated :-The light of the lamp A, fig. 13, is thrown by the mirror B and the lenses C and D on the painted slider at E, and the magnifier T forms the image on the screen at G. This lens is fixed to a slider, which may be drawn out of the general support or box H: and, when the box is drawn back on its wheels, the rod IK lowers the point K, and by means of the rod K L adjusts the slider in such a manner that the

image is always distinctly painted on the screen G. When the box advances towards the screen G, in order that the images may be diminished and appear to vanish, the lens F suffers the screen M to fall and intercept a part of the light. The rod K N must be equal to IK, and the point I must be twice the focal length of the lens T, before the object, L being immediately under the focus of the lens. The screen M may have a triangular opening, so as to uncover the middle of the lens only, or the light may be intercepted in any other manner.

560. That the rays of lignt flow in all directions from different bodies, without interrupting one another, is plain from the following experiments:-Make a little hole in a thin plate of metal, and set the plate upright on a table, facing a row of lighted candles standing near together; then place a sheet of paper or pasteboard at a little distance from the other side of the plate, and the rays of all the candles, flowing through the hole, will form as many specks of light on the paper as there are candles before the plate; each speck as distinct and large as if there were only one candle to cast one speck; which shows that the rays do not obstruct each other in their motions, although they all cross in the same hole.

561. If three pieces of paper are fixed against the wall of a room at equal distances, at the height of the eye, the operator placing himself directly before them at a few yards distance, and the right eye being closed, he will perceive with his left only two of them,-suppose the first and second; let the position of the eye be altered, and the first and third will be seen; if altered a second time, the second and third; but never the whole three together.

562. An artificial halo may be formed by placing a candle on one side of a glass receiver, and, the spectator having placed himself at a distance on the other side, let the air be exhausted, and the light of the candle will be refracted in circles of various colors like the halo which encircles the sun.

563. Opticians sometimes grind a glass mirror concave in one direction only, or longitudinally; it is in fact a concave portion of a cylinder, the breadth of which may be considered that of the mirror. A person looking at his face in this mirror, in the direction of its concavity, will see it curiously distorted in a very lengthened appearance; and, by turning the cylindrical mirror a quarter round, his visage will appear distorted in another way, by an apparent increase in width only. If in a very near situation before it you put your finger on the right hand side of the nose, it will appear the same in the mirror; but if in a distant situation, somewhat beyond the centre of concavity, you again look at your face in the mirror, your finger will appear to be removed to the other side.

564. The thaumatrope, or wonder-turner, a very ingenious philosophical toy, invented we believe by Dr. Paris, is founded on the well known optical principle that an impression upon the retina continues for about the eighth part of a second after the object which produced it is withdrawn. The luminous rings formed by the whirling of a burning stick in the dark are well

known, and Homer has availed himself of the same principle in his description of the lengthened shadow of the flying javelin.

565. The thaumatrope consists of a number of circular pieces of card, about two inches and a half in diameter, which may be twirled round with great velocity by the application of the fingers to pieces of silk string attached to two opposite points of their circumference. On each side of the card is painted a part of a picture, so that, if we could see both sides at once, the two parts of the picture will form a whole picture. For example, let us suppose a card on one side of which is a cage, and on the other a bird. If we now take hold of each of the silk strings, and put it into a twirling motion, the bird and the cage will appear to the eye at the same moment, in consequence of the impression of each continuing on the retina for a short space of time. The following are some of the other devices on the cards of the thaumatrope :A rose-tree, with a garden-pot on the reverse. A horse, with a man on the reverse. A leafless branch, which becomes verdant on the twirling of the card.

A female in one dress on one side and another dress on the other.

The body of a Turk, with his head on the reverse.

Harlequin and Columbine on different sides, appear together by the revolution of the card. A comic head on one side, which, on turning round, becomes invested with a wig.

A man sleeping, and awakened by being turned round.

566. The principle of the thaumatrope may be extended to many other devices. Parts of a sentence may be written on one side, and the rest of the sentence on the other; and we may even put halves of the letters or words on one side and the other halves on the other side. This method of breaking down letters or words or sentences may be varied ad infinitum, and will furnish us with a variety of rotatory cyphers.

567. Those who have used the thaumatrope must have been dissatisfied with the general effect of the two combined pictures. There is a hobbling motion, arising from the imperfection of the method adopted to produce the rotatory motion, which entirely destroys the effect; and it is manifest that the rotatory motion should be produced by quite different means.

568. If strings are adopted they ought to be attached to the circular pieces of card so that the axis of rotation should be in the plane of the card; but a solid axis of rotation is decidedly preferable, and will produce much more pleasing combinations.

569. A curious optical deception may be thus produced. It is to exhibit an erect object which, when placed near a hole in a card, will appear to be on the other side, and also inverted and magnified. Let a card be perforated with a small hole, and placed opposite a white wall or window, the eye of the observer being situated on the other side of the card. A pin being then placed between the eye and card will be seen on the other side of the aperture, inverted and magnified as already described. The reason

of this is, as M. Le Cat has observed, that the eye in this case sees only the shadow of the pin on the retina; and, since the light which is stopped by the upper part of the pin or its head comes from the lower part of the white wall or window, whilst that which is stopped by the lower end of the pin comes from the upper part, the shadow must necessarily appear inverted with respect to the object.

570. Another variation of Le Cat's experiment is as follows:-Take a common pin and hold it in any position near the eye, so that the observer sees reflected from its head a faint circle of

light; then hold a second pin opposite to it, and an inverted image of the one pin will be seen in the head of the other. If the head of the first pin is round and well polished, the inverted and magnified image of the other will be more distinct. In this form of the experiment a diverging pencil of light, from the window or a candle, replaces the diverging pencil which proceeds from the perforation in the card, and of course produces the same effect. The little round knob by the pressure upon which the case of a watch is often opened will answer better than the finest pin head.

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BARTON, Mr., iris metal buttons, 337

BARTHOLINUS on double refraction, 95. BEAM of light, 72.

BECCARIA on double refraction, 153.

BINOCULAR telescope, 362.

KEPLER, astronomical telescope invented by, 360.

LAMBERT, his optical researches, 19.

LENS, 383. 400.

LIGHT, minuteness of its particles, 28.

MAGIC lantern improved, 559.

MARALDI'S experiments on light, 60.

MARINE rainbow, 315.

MERCURY, reflection of light from its surface, 40.
MICROSCOPE, 458.

BOUGUER'S table of the intensity of the sun's light, MINUTENESS of the particles of light, 28.

35. Treatise on optics by, 16.

BRADLEY, Dr., his optical discoveries, 15.

BREWSTER, Dr., his important discoveries in light,

218.

BURNING mirror formed by Archimedes, 4.

CALEIDOSCOPE improved, 499.

CAMEO converted into an intaglio, 295.

MIRAGE, 321. 332.

MONOCHROMATIC lamp, 463.

MOTHER-OF-PEARL, its peculiar optical properties,

229.

NEWTON'S experiments, 49.

OPTICAL deception in looking at a guinea, 288.

CARBONATE of lime, its double refracting properties, PARTICLES of light, their minuteness, 28.

171.

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PHOSPHORESCENCE of diamonds, 9. PHOTOMETER, 477.

PLATE glass, its refractive power, 41.

POLARISED light, 186.

PRISMATIC spectrum, 549.

PUPIL of the eye, 546.

QUADRANT invented by Hadley, 373.

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OPTIMACY, n. s. Į Lat. optimates. NobiOPTIM'ITY. lity; the body of nobles; one of the divisions of the Roman people: optimity, the state of being best.

In this high court of parliament there is a rare coordination of power, a wholesome mixture betwixt monarchy, optimacy, and democracy. Howel. OPTIMATES, in ancient history, a political party among the Roman people, opposed to populares. They were warm supporters of the dignity of the chief magistrates and promoters of the senatorial power; whereas the populares boldly stood up for the rights of the people, pleaded for larger privileges, and labored to bring matters nearer to a level. Cicero says, that the optimates were the best citizens, who wished to deserve the approbation of the better sort; and that the populares courted the favor of the populace, not so much considering what was right, as what would please the people, and gratify their own thirst of vain glory and empty applause.

OPTIO, an officer in the Roman army, an assistant or lieutenant to every centurion; so called because he was the choice or option of the centurion in later times: at first, however, he had been chosen by the tribune, or chief coinmander of the legion. These optiones are also sometimes called succenturiones and tergiductories; the last name was given them because their post was in the rear of the company. Some authors mention suboptiones, or sublieutenants.

OPTION, n. s. Lat. optio. Choice; election. Transplantation must proceed from the option of the people, else it sounds like an exile; so the colonies must be raised by the leave of the king, and not by his command.

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To begin with the supposed policy of gratifying only the rich and opulent :-does our wise man think that the grandee whom he courts does not see through all the little plots of his courtship? South. He had been a person not only of great opulency, but authority. Atterbury.

There in full opulence a banker dwelt, Who all the joys and pangs of riches felt; His side-board glittered with imagined plate, And his proud fancy held a vast estate. Swift. How few can rescue opulence from want!

Young. OPUNTIA, the prickly pear tree, a species of cactus; see CACTUS. The fruit of the opuntia is remarkable for coloring the juices of living animals, though it appears not to be poisonous or even hurtful to the body. The prickly pear grows in great abundance about Carolina; and, cochineal insects are found upon it, though no attempt has yet been made to cure them for use as the Spaniards do.

OR, conj. Sax. oben; Goth. odr; Teut. oder. Other; either (of both which it is used as a contraction); before a disjunctive particle denoting choice, distribution, and sometimes opposition. Learn before thou speak, and use physick or ever thou be sick. Eccles. xviii. 19.

The dead man's knell Is there scarce asked, for whom; and good men's lives

Expire before the flowers in their caps,
Dying or ere they sicken.

Shakspeare. Macbeth. How great soever the sins of any unreformed person are, Christ died for him because he died for all; only he must reform and forsake his sins, or else he He decrees to punish the contumacy finally, by shall never receive benefit of his death. Hammond. assigning them their own options.

Bacon.

Hammond.

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OPTION, in ecclesiastical polity. When a new suffragan bishop is consecrated, the archbishop of the province, by a customary prerogative, claims the collation of the first vacant benefice, or dignity, in that see, according as he shall choose; which choice is called the archbishop's option. But if the bishop dies, or is translated, before the incumbent of the promotion chosen by the archbishop die or be removed, it is generally supposed that the option is void; as the granter, singly and by himself, could not convey any right or title beyond the term of his continuance in that see. And, if the archbishop dies before the voidance happen, the right of filling up the vacancy shall go to his executors or administrators.

OPULENCE, n. s. Fr. opulence; Ital. OP'ULENCY, opulenza; Port. Span. OP'ULENT, adj. and Lat. opulentia. AfOP'ULENTLY, adv. fluence; riches; splendid style of living: opulent is, wealthy; rich; affluent.

It must be a discovery of the infinite flatteries that follow youth and opulency. Shakspeare. Timon.

He made him his ally, and provoked a mighty and opulent king by an offensive war in his quarrel.

Bacon.

For thy vast bounties are so numberless, That them or to conceal, or else to tell, Is equally impossible.

Cowley.

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