reach the retina before they are collected into a focus; in this case the defect is sup plied by a concave glass, a b,which makes the point whence the rays diverge, nearer than the object; consequently, the rays falling on the eye will now diverge more than before, so as when refracted through the humours not to come to their focus before they reach the retina.

Spectacles are constructed on the above principles, concave for short-sighted, and convex for long-sighted people. See SPEC. TACLES and VISION.

"Of microscopes and other optical in struments." The impediments to the vision of very near objects arise from too great a divergence of the rays in each pencil incident on the eye, and are remedied by the microscope. This instrument is of two kinds: 1, refracting; and, 2, reflecting.

The refracting microscope is either single or compound. The former is a small double convex lens, of a short focal length; the object is placed in its focus, by which disposition the rays of each pencil emerging from the lens become parallel, and so are brought to their respective foci on the retina by the humours of the eye: the magnifying power of the instrument appears from hence.

The apparent lineal magnitude of an object seen with this instrument, is to its lineal magnitude seen with the naked eye, as the least distance that admits of distinct vision with the naked eye, to the focal length of the lens; for these magnitudes are as the angles under which the object appears, i. e. inversely as the distances at which it is viewed.

A compound microscope is composed of two double convex glasses, the broader next the eye. In this instrument the distance of the object from the object-glass is to be made greater than the focal length of that lens; then the image will be formed at the focus conjugate to the place of the object, and the eye-glass being placed at its own focal distance from the image, will make the rays emerge parallel to each other, and consequently produce distinct vision. See MICROSCOPE.

To enlarge the field of the compound microscope, it is usual to insert a broad lens, as in the astronomical telescope, between the object-glass and the image.

The reflecting microscope is thus constructed: In the extremity of a broad tube insert a concave speculum N U (fig. 21); a point O in its axis, whose distance from

the vertex, V, is greater than the focal length of the concave, is the place for the object, whose image will consequently be formed at the focus, G, conjugate to the point 0: at the distance of its own focal length, LG, place a double convex lens, ab, by which the image will be seen distinctly. The object is illuminated by light admitted into the tube through a space, P R, adjoining to the speculum; and the illustration of the object may be rendered more intense by a concave speculum, A B, which shall reflect the light so admitted to a focus at the place of the object.

A solar microscope is constructed in the following manner: In the inside of a tube is placed a convex lens, A B (fig. 22); and at a distance a little greater than its focal length, but less than double of it, is fixed some transparent coloured object, QP, whose image will be painted much enlarged at the focus conjugate to the place of the object. A broad lens CD, is placed before the object to collect the solar rays, for the purpose of illuminating it more strongly, and consequently making the image more distinct and vivid. On the same principle is constructed the MAGIC LANTERN, which see.

The camera obscura is an instrument used to facilitate the delineation of prospects. It is constructed in the following manner: AC (fig. 23), represents a box of about a foot and a half square, shut on every side, except DC; OP is a smaller box placed on the top of the greater; MN is a double convex lens, whose axis makes an angle of 45° with BL, a plane mirror fixed in the box OP; the focal length of the lens is nearly equal to CS+ ST, i. e. to the sum of the distances of the lens from the middle of the mirror, and of the middle of the mirror from the bottom of the larger box. The lens being turned toward the prospect would form a picture of it, nearly at its focus; but the rays being intercepted by the mirror will form the picture as far before the surface as the focus is behind it, i. e. at the bottom of the larger box, a communication being made between the boxes by the vacant space (2 O. The draughtsman then putting his head and hands into the box through the open side, DC, and drawing a curtain round to prevent the admission of the light, which would disturb the operation, may trace a distinct outline of the picture that appears on the bottom of the box.

There is another kind of camera obscura, constructed thus. In the extremity of the

arm, PQ (fig. 24), that extends from the side of a small square box, BL, is placed a double convex lens, whose axis is inclined in an angle of 45° to a plane mirror BO: the focal length of the lens is equal to its distance from the side of the box OT; therefore, when the lens is turned towards the illuminated prospect, it would project the image on the side OT, if the mirror were removed, but this will reflect the image to the side M L, which is as far distant from the middle of the mirror, as this is from the side OT; it is there received on a piece of glass, rough at the upper side and smooth at the lower, and appears in its proper colours on the upper side of the plate. It is evident that in each of these instruments the image is inverted with respect to the object.

M S is a lid to prevent the admission of light during the delineation of the picture, and others for the same purpose are applied to the sides M R and N L.

Dr. Wollaston has recently invented a portable instrument for drawing in perspective, to which he has given the name of Camera Lucida. In this instrument two plane reflectors are fixed at such angles with regard to each other, that the objects intended to be delineated are seen after reflection from the second mirror, as though they were on the same plane as that whereon the paper is placed which is to contain the drawing. These plane reflectors may be either common mirrors with a silver coating at the back of each, or two contiguous faces of a glass prism, in which latter case the image will be produced by what is called prismatic reflection. In either case the most convenient position, in which the reflecting surfaces can be arranged, will be such as will cause the rays proceeding directly from the object and falling as incident rays upon the first surface, after reflection from thence to the second, to emerge from that second reflecting surface in angles of 90 degrees, with the direction of the original incident rays; for in these circumstances vertical objects may be projected upon a horizontal plane, and the in strument will be adapted to drawing upon a horizontal surface. Now, if two plane mirrors are used, the incident rays upon the first will make right angles with the emergent rays from the second, when those mirrors are fixed so as to make angles of either 45 or 135 degrees with each other. In this case the mirror which first receives the rays from the object may be entirely silvered at its back; but the second mirror is

only to have a sufficient portion silvered to reflect the image of the proposed object to the eye; and thus to allow the paper, on which the drawing is to be made, to be seen either through an opening of the silvering or past the edges of the same, by one portion of the eye, while the double reflected object is seen in the silvering by the other portion of the same eye. When prismatic reflection is employed, the prism must not be triangular as usually constructed, but quadrangular, and the two reflecting surfaces (to produce an angle of 90 degrees between the first incident and second emergent rays) must make an angle of 135 degrees, while the opposite angle must be one of 90 degrees, and the other two angles may be either respectively equal or unequal at pleasure; then one of the faces which make right angles with each other is to be turned towards the object or objects to be delineated, and the rays after passing through that surface and reflection from the two next faces, will emerge from the fourth under the proposed angle. The mirrors or other reflecting surfaces are mounted in a proper frame, and supported at a suitable distance from the paper intended to receive the drawing; and, when necessary, either a double concave or a double convex glass may be fixed in the frame and properly adjusted, to produce distinct vision when the apparatus is used by short-sighted or longsighted persons respectively. These concave or convex glasses may conveniently be made of twelve inches focal length; the instrument must then be supported at the distance of twelve inches from the paper; a distance which is convenient enough in other respects.

Dr. Wollaston has himself published a description of this instrument, in Nicholson's Philosophical Journal, where he like. wise institutes a comparison between the Camera Obscura and the Camera Lucida. The objection to the Camera Obscura are, 1. That it is too large to be carried abont with convenience. The Camera Lucida is as small and portable as can be wished. 2. In the former, all objects that are not situated near the centre of view are more or less distorted. In this, there is no distortion; so that every line, even the most remote from the centre of view, is as straight as those through the centre. 3. In that, the field of view does not extend beyond 30°, or at most 35o, with distinctness. But in the Camera Lucida as much as 70° or 80° might be included in one view.

Dr. Wollaston remarks further, that by a

proper use of the same instrument every purpose of the pentagraph may also be answered; as a painting may be reduced in any proportion required, by placing it at a distance in due proportion greater than that of the paper from the instrument. In this case a lens becomes requisite for enabling the eye to see at two unequal distances with equal distinctness; and in order that one lens may suit for all these purposes, there is an advantage in varying the height of the stand according to the proportion in which the reduction is to be effected.

OPTION, in law, every bishop, whether created or translated, is bound immediately after confirmation, to make a legal conveyance to the archbishop, of the next avoidance of such dignity or benefice belonging to the see, as the said archbishop shall choose, which is therefore called an option.

OR, in heraldry, denotes yellow, or gold colour. In the coats of noblemen it is blazoned topaz; and in those of sovereign princes, sol. It is represented in engraving by small points or dots, scattered all over the field or bearing.

ORATORIO, in music, a species of musical drama, originally an imitation of the serious opera, the subject of which is generally taken from scripture, and can be only treated properly by music of the sublimest style.

ORBIT, in astronomy, the path of a planet or comet, or the curve that it describes in its revolution round its central body: thus the Earth's orbit is the curve which it describes in its annual course, and usually called the ecliptic. The orbits of all the planets are ellipses having the Sun in their common focus; in which curve they move according to an invariable law. See ASTRONOMY. However, the orbit of the Earth is considerably disfigured by the action of the Moon; as is also the orbit of Saturn by the action of Jupiter, when they happen to be in conjunction. Though the orbits of the planets be elliptical, not circular, yet that they are very little so, even in the most eccentric orbit, as that of Mercury, will appear by comparing their eccentricities with their mean distances from the Sun. The orbits of the planets are not all in the same plane with the ecliptic, but are variously inclined to it, and to each other; but still the plane of the ecliptic intersects the plane of the orbit of every other planet, in a right line which

passes through the Sun, called the line of the nodes, and the points of intersection of the orbits themselves are called the nodes.

ORCHESTRA, in music, that enclosed part of the theatre between the audience and the curtain; in which the instrumental performers sit.

ORCHIDEÆ, in botany, the seventh order in Linnæus's Fragments of a Natural Method, consisting of Orchis, and the plants that resemble it in habit, powers, and sensible qualities. The flowers are hermaphrodite, and placed at the summit of the stalk, either in a spike, or in a panicle. Each flower is accompanied with a leaf that is smaller than the other leaves, and forms a sort of sheath round the stalk. The petals are five in number, and very irregular. The flowers of the different species are remarkably various and singular in their shape, resembling different kinds of animals or insects.

ORCHIS, in botany, a genus of the Gynandria Diandria class and order. Natural order of Orchideæ. Essential character: nectary a horn or spùr behind the flower. There are fifty species. Among which we shall notice the O. bifolia, butterfly orchis. This plant has ovate bulbs, tapering to a point at the base; thick fleshy fibres proceed above them from the base of the stem; one of these bulbs is always wrinkled and withered, whilst the other is plump and delicate; the first is the parent of the actual stem; the second is an offset, from the centre of which the stem of the succeeding year is destined to arise. Such are the means that nature uses, not only to disseminate plants, but to enable them to change their place, and thus to draw in fresh nutriment. The second root is always about half an inch from the centre of the first, so that in twenty years the plant will have marched ten inches from the place of its birth. This mode of increase is particularly necessary in a family of plants that rises with great difficulty, and very seldom by seed. O. conopsea, long-spurred Orchis, is distinguished by the remoteness of the cells or cases in which the stamens are lodged, and again by the colour of the corolla, the great length of the spur, the delicious fragrance of its flowers, vying with that of the honeysuckle, and particularly by the unusual structure of its flowers. Below the stigma, which is remarkably well defined in this species, there is a circular opening between the cavities containing the stamens; just above the stigma is a very con

spicuous ridge; the stamens soon change to a brownish hue; the anthers are club shaped, and are divided as in most others, the gland at the base of the filament is of a circular form, with a cavity on its inner side: the roots of this species are well caculated for making salep.

ORDEAL, was anciently a form of trial, and was of two sorts; either fire ordeal, or water ordeal; the former being confined to persons of higher rank, the latter to the common people. Both these might be performed by deputy, but the principal was to answer for the success of the trial; the deputy only venturing some corporeal pain for hire, or perhaps for friendship. Fire ordeal was performed either by taking up in the hand a piece of red hot iron, of one, two, or three pounds weight; or else by walking barefoot and blindfold over nine red hot ploughshares, laid at unequal distances; and if the party escaped unhurt, he was adjudged innocent; if not, he was condemned as guilty. Water ordeal was performed, either by plunging the bare arm up to the elbow in boiling water, and escaping unhurt thereby, or by casting the person suspected into a river or pond of water; and if he floated, without any action of swimming, it was deemed an evidence of his guilt; but if he sunk he was acquitted. This trial by ordeal was abolished by parliament anno 3 Henry III.

ORDER, in architecture, is a system of the several members, ornaments and proportions of columns and pilasters; or a regular arrangement of the projecting parts of a building, especially the column, so as to form one beautiful whole. There are five orders of columns, of which three are Greek, viz. the Doric, Ionic, and Corinthian, and two Roman, the Tuscan and Composite. The three Greek orders represent the three different manners of building, viz. the solid, the delicate, and that which is between the two. See ARCHItecture.

ORDER, in astronomy. A planet is said to go according to the order of the signs when it is direct, proceeding from Aries to Taurus, thence to Gemini, &c. It goes contrary to the order of the signs when it is retrograde, or goes backward from Pisces to Aquarius.

ORDER, in geometry, is denominated from the rank or order of the equation by which the geometrical line is expressed: thus the simple equation, or first power, denotes the first order of lines, which is a right line: the quadratic equation, or second

power, defines the second order of lines, which are the conic sections and circle: the cubic equation, or third power, defines the third order of lines; and so on. Or the orders of lines are denominated from the number of points in which they may be cut by a right line. Thus the right line is of the first order, because it can be cut only in one point by a right line: the circle and conic sections are of the second order, because they can be cut in two points by a right line; while those of the third order are such as can be cut in three points by a right line.

ORDER, in botany, the first subdivision of a class in the Linnæan system, founded on the number of styles or female organs. The orders of Linnæus are all expressed by a single term, which, like the names of the classes, is of Greek etymology, and is significant of the character of the order to which it is applied. The names of these orders are often different in different classes, because the same idea predominates in their institution.

ORDINANCE, or Ordonnance, a law, statute, or command of a sovereign or supe

rior.

ORDINARY, in the civil law, signifies any judge that hath authority to take cognizance of causes in his own right, as he is a magistrate, and not by deputation; but in the common law it is taken for him who has exempt and immediate jurisdiction in causes ecclesiastical.

ORDINARY, or honourable Ordinary, in heraldry, a denomination given to certain charges properly belonging to that art. The honourable ordinaries are ten in number; viz. the chief, pale, bend, sesse, bar, cross, saltier, chevron, bordure, and orie. For which see HERALDRY, &c.

ORDINATES, in geometry, are right lines drawn parallel to each other, and cutting the curve in a certain number of points. Parallel ordinates are usually all cut by some other line, which is called an absciss. When this line is a diameter of the curve, the property of the ordinates is then the most remarkable; for, in the curves of the first kind, or the conic sections and circle, the ordinates are all bisected by the diameter, making the part on one side of it equal to the part on the other side of it; and in the curves of the second order, which may be cut in three points by an ordinate, then of the three parts of the ordinate, lying between these three inter

sections of the curve and the intersection with the diameter, the part on one side the diameter is equal to both the two parts on the other side of it. And so for curves of any order, whatever the number of inter. sections may be, the sum of the parts of any ordinate, on one side of the diameter, is equal to the sum of the parts on the other side of it. The use of ordinates in a curve, and their abscisses, is to define or express the nature of a curve by means of the general relation or equation between them; and the greatest number of factors, or the dimensions of the highest term, in such equation, is always the same as the order of the line; that equation being a quadratic, or its highest term of two dimensions, in the lines of the second order, being the circle and conic sections; and a cubic equation, or its highest term containing three dimensions, in the lines of the third order, and so on. Thus, y denoting an ordinate and x its absciss, also ab c, &c. given quantities: then yax2+ bx + c is the general equation for the lines of the second order; and xy2 — ey = ax3 + bx2 + c x +d is the equation for the lines of the third order, and so on.

ORDINATION, the act of conferring holy orders, or of initiating a person into the priesthood by prayer, and the laying on of hands. Ordination has always been es. teemed the principal prerogative of bishops, and they still retain the function as a mark of spiritual sovereignty in their diocese. Without ordination, no person can receive any benefice, parsonage, vicarage, &c. A clerk must be twenty-three years of age before he can have any share in the ministry; and twenty-four before he can be ordained, and by that means be permitted to administer the sacraments. A bishop, on the ordination of clergymen, is to examine them in the presence of the ministers who assist him at the imposition of hands: and in case any crime, as drunkenness, perjury, forgery, &c. be alleged against any one that is to be ordained, either priest or deacon, the bishop ought to desist from ordaining him. The person to be ordained is to bring a testimonial of his life and doctrine to the bishop, and give an account of his faith in Latin, and both priests and deacons are obliged to subscribe the thirty nine articles.

ORDNANCE, a general name for all sorts of great guns used in war.

ORDNANCE, office of, an office kept with

in the Tower of London, which superintends and disposes of all the arms, instruments, and utensils of war, both by sea and land, in all the magazines, garrisons, and forts, in Great Britain.

The officers of the ordnance are: 1. The Master General, from whom are derived all orders and dispatches relating to the same. 2. The Lieutenant General, who receives orders from the Master General, and sees them duly executed; orders the firing of guns on days of rejoicing, and sees the train of artillery fitted out when ordered to the field. 3. The Surveyor General, who has the inspection of the ordnance, stores, and provisions of war in the custody of the store-keepers: he allows all bills of debt, keeps a check on labourers, &c. 4. The Treasurer, through whose hands passes the money of the whole office, as well for payment of salaries as debentures; as also a Clerk of the Ordnance, and a Clerk of the Deliveries, for which see the articles CLERK of the ordnance, &c.

ORES, in mineralogy. An ore is a metal in the state in which it exists in the earth. It may be either native, that is pure, and uncombined with any other substance, or of an oxide, or a sulphuret, or a carburet, alloyed with another metal; or in a state or of a metallic salt. It is also mixed in most instances with various earthy minerals. The ores of metals may be analyzed in two modes, in the humid, and the dry way. The first is effected with the aid of acids, and of other liquid agents, and may often be accomplished by very simple means, and without the aid of a bulky and expensive apparatus. If sulphur be present, it impedes the action of acids, and should be separated by roasting the ore on a muffle, or by projecting it mixed with twice its weight of nitre into a red-hot crucible, washing off the alkali afterwards with hot water. No solvent will act upon all the metals. Thus nitric acid does not act on gold and platina; and the nitro-muriatic acid, which on silver. Hence the necessity of varying dissolves these metals, has no solvent action the solvent according to the nature of the ore, under examination. We shall give a derstand the theory, and may be enabled few instances by which the reader will unto verify the facts by practice.

For "ores of gold and platina," the nitromuriatic acid is the most proper solvent. A given weight of the ore may be digested with this acid, as long as it extracts any thing. The solution is to be evaporated to