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mus with the Orchis. Linnaeus distinguished this natural order into genera from the nectarium, which in Orchis forms a horn or spur at the back of the flower, whereas the lip of it in this is a petal, hanging down, with a ridge or keel running along the back. O. nidis avis, bird's nest Ophrys, has the root composed of many strong fibres, from which arise two oval veined leaves, jointed at their base ; between these spring up a naked stalk about eight inches in height, terminated by a long spike of herbaceous flowers, resembling gnats, composed of five petals, with a long bifid lip to the nectarium, a crest or standard above, and two wings on the side; capsule angular, opening in six parts, filled with small seeds like dust. Native of several parts of Europe. OPHTHALMIA, in medicine, an inflammation of the membranes which inwest the eye. OPIUM, in chemistry and medicine, an inspissated gummy juice, which is obtained from the head of the “papaver somniferum.” It is imported from Persia, Arabia, and other warm parts of Asia, in flat cakes, covered with leaves to prewent their sticking together. It has a reddish brown colour, and strong peculiar smell: its taste at first is nauseous and bitter; but this soon becomes acrid, and produces a slight warmth in the mouth. A peculiar substance has been detected in opium, to which it is suppos. ed the properties it possesses of proucing sleep are owing. On account of this property, this substance has received the name of narcotic matter. It is obtained from the milky juices of some plants, as those of the poppy, lettuce, and some others. Opium, which is extracted from the poppy, is prepared by the following process. The heads of the White poppy, which is cultivated in difserent countries of the east for this purPose, are wounded with a sharp instrument; a milky juice flows out, which concretes, and is collected and formed into cakes. In this state opium is a tenacious substance, of a brownish colour; has a peouliar smell, and a disagreeable bitter taste. It becomes soft with a moderate heat. It readily takes fire, and burns rapidJ. By the analysis of opium, it appears to * composed of the o of lime and opotash, extractive matter, gluten, mu*ge, resinous matter, and an oil, be* des the narcotic matter to which its pejo, Suliar properties are owing. By digest2. "g opium in water, part of it is dissolv

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ed, and by evaporating the solution to the consistence of syrup, a gritty precipitate appears, which becomes more copious with the addition of water. This precipitate is composed of resinous and extractive matter, besides the peculiar narcotic matter which is crystallized. When alcohol is digested on this precipitate, the resinous and narcotic matters are dissolved, and the extractive matter remains behind. As the solution cools, the narcotic matter crystallizes; but the crystals are coloured with a portion of resin. By repeated solutions and crystallizations it may be obtained tolerably pure. If alcohol be digested on the residuum, it becomes of a deep red colour, the same crystals are deposited on cooling, and may be purified in the same way from the resinous matter with which they are contaminated. The narcotic matter, when properly purified, is of a white colour; crystallizes in right-angled prisms, with a rhomboidal base; and has neither taste nor smell. It is insoluble in cold water, and requires 400 parts of boiling water for its solution, from which it is precipitated by cooling. The solution does not redden the tincture of turnsole. It is soluble in 24 parts of boiling alcohol, and acquires about 100 parts when it is cold. When water is added to the solution in alcohol, it is precipitated in the form of a white opaque matter. One of the most decided characters of this substance is its easy solubility in all the acids, and without the aid of heat. It is precipitated from these solutions by means of an alkali, in the form of white powder. Pure alkalies increase the power of its solubility in water, and the acids, when not added in excess, occasion a precipitate. When nitric acid is poured on the crystals reduced to a coarse powder, it communicates to them a red colour, and readily dissolves them. When the solution is heated and evaporated, it yields crystals of oxalic acid in considerable quantity. The residuum has a very bitter taste. From the effects of heat and of nitric acid on this substance, it appears to be composed of oxygen, hydrogen, carbon, and azote. This narcotic substance is also found in the milky juice, and in the extracts which are obtained from several other plants, as from different species of lactuca, or lettuce; hyoscyamus niger, or henbane. The leaves of some plants also produce similar effects, as those of the deadly night-shade, fox-glove, and conium maculatum, or hemlock. See Poppy.

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OPOPANAX. See GUM resin. OpoSSUM. See DIDELPHIS. OPTICS, the science of vision, including Catoptrics and Dioptrics, and even Perspective; as also the whole doctrine of light and colours, and all the phenomena of visible objects. See PERspecTive. Optics, in its more extensiv acceptation, is a mixed mathematical science, which explains the manner in which vision is performed in the eye; treats of sight in general; gives the reasons of the several modifications or alterations which the rays of light undergo in the eye; and shows why objects appear sometimes greater, sometimes smaller, sometimes more distinct, sometimes more confused, sometimes nearer, and sometimes more remote. In this extensive signification it is considered by Sir Isaac Newton, in his Optics. Indeed Optics make a considerable branch of natural philosophy; both as it explains the laws of nature, according to which vision is performed, and as it acounts for abundance of physical phenomena, otherwise inexplicable. The reflection of the rays of light is, indeed, an occurrence too frequent and obvious to have escaped the notice even of the earliest observers; a river or some other piece of water was probably the first mirror; its effect was afterward imitated by metallic mirrors; hence was discovered the equality of the angles of incidence and reflection. It was known at an early period that an oar, or other straight piece of wood, partially immersed in water, no longer appeared straight, yet ages after this elapsed before any attempts were made to ascertain the relation between the angles of incidence and refraction. Empedocles was the first person on record that wrote systematically on light; and Euclid composed a treatise on the ancient optics and catoptrics; dioptrics being less known to the ancients, though it was not quite unnoticed by them; for among the phenomena at the beginning of that work, Euclid remarks the effect of bringing an object into view, by refraction, in the bottom of a vessel, by pouring water into it, which could not be seen over the edge of the vessel before the water was poured in ; and other authors speak of the then known effects of glass globes, &c. both as burning glasses, and as to bodies seen through them. Euclid's work, the genuineness of which has been doubted, is chiefly on catoptrics, or reflected rays; in which he shows the chief properties of them in plane, convex, and concave sur

faces, in his usual geometrical manner, beginning with that concerning the equality of the angles of incidence and reflection, which he demonstrates; and in the last proposition, showing the effect of a concave speculum, as a burning glass, when exposed to the rays of the sun. The effects of burning glasses, both by refraction and reflection, are noticed by several others of the ancients, and it has been thought that the Romans had a method of lighting their sacred fire by some such means. Aristophanes, in one of his comedies, introduces a person as making use of a globe filled with water to cancel a bond that was against him, by thus melting the wax of the seal. If we give credit to what some ancient historians are said to have written concerning the exploits of Archimedes, we shall be induced to think that he constructed some very powerful burning mirrors It is even allowed that this eminent geometrician wrote a treatise on the subject of them, though it be not now extant; as also concerning the appearance of a ring or circle under water, and therefore could not have been ignorant of the common phenomena of refraction. We find many questions concerning optical appearances in the works of Aristotle. This author was also sensible that it is the reflection of light from the atmosphere, which prevents total darkness after the sun sets, and in places where he does not shine in the day time. He was also of opinion, the rainbows, halos, and mock-suns, were all occasioned by the reflection of the sun-beams in different circumstances, by which an imperfect image of his body was produced, the colour only being exhibited, and not his proper figure. The ancients were not only acquainted with the more ordinary appearances of refraction, but knew also the production of colours by refracted light. Seneca says, that when the light of the sun shines through an angular piece of glass, it shows all the colours of the rainbow.— These colours, however, he says, are false, such as are seen in the pigeon’s neck when it changes its position; and of the same nature, he says, as a speculum, which, without having any colour of its own, assumes that of any other body. It appears also, that the ancients were not unacquainted with the magnifying power of glass globes filled with water, though they probably knew nothing of the reason of this power; and it is supposed that the ancient engravers made use of a glass globe filled with water to

magnify their figures, that they might work to more advantage. Ptolemy, about the middle of the second century, wrote a considerable treatise on optics. The work is lost; but from the accounts of others it appears that he there treated of astronomical refractions. The first astronomers were not aware, that the intervals between stars appear less when near the horizon than in the meridian; and on this account they must have been much embarrassed in their observations; but it is evident that Ptolemy was aware of this circumstance, by the caution which he gives to allow something for it, whenever recourse is had to ancient observations. This philosopher also advances a very remote hypothesis, to account for the remarkably great apparent size of the sun and moon when seen near the horizon. The mind, he says, judges of the size of objects by means of a preconceived idea of their distance from us; and this distance is fancied to be greater, when a number of objects are interposed between the eye and the body we are viewing, which is the case when we see the heavenly bo. dies near the horizon. In his Almagest, however, he ascribes this appearance to a refraction of the rays by vapours, which actually enlarge the angle under which the luminaries appear, just as the angle is enlarged by which an object is seen from under water. See ProLEMY. Alhazen, an Arabian writer, was the next author of consequence, who wrote about the year 1100. Alhazen made many experiments on refraction, at the surface between air and water, air and glass, and water and glass; and hence he deduced several properties of atmospherical refraction, such as, that it increases the altitudes of all objects in the heavens; and he first advanced that the stars are sometimes seen above the horizon by means of refraction, when they are really below it; which observation was confirmed by Vitellio, Walther, and especially by the observations of Tycho Brahe. Alhazen observed, that refraction contracts the diameters and distances of the heavenly bodies, and that it is the cause of the twinkling of the stars. This refractive power he ascribed, not to the vapours contained in the air, but to its different degrees of transparency. And it was his opinion, that so far from being the cause of the heavenly bodies appearing larger near the horizon, it would make them appear less; observing that two stars appear nearer together in the horizon than near the meridian, . This VOL. IX.

phenomenon he ranks among optical deceptions. We judge of distance, he says, by comparing the angle under which objects appear with their supposed distance; so that if these angles be nearly equal, and the distance of one object be conceived greater than that of the other, this will be imagined to be the larger. And he further observes, that the sky near the horizon is always imagined to be further from us than any other part of the concave surface. In the writings of Alhazen, too, we find the first distinct account of the magnifying power of glasses, and it is not improbable that his writings on this head gave rise to the useful invention of spectacles: for he says, that if an object be applied close to the base of the larger segment of a sphere of glass, it will appear magnified. . He also treats of the appearance of an object through a globe, and says that he was the first who observed the refraction of rays into it. In 1270, Vitellio, a native of Poland, published a treatise on optics, containing all that was valuable in Alhazen, and di. gested in a better manner. He observes, that light is always lost by refraction, which makes objects appear less luminous. He gave a table of the results of his experiments on the refractive powers of air, water, and glass, corresponding to different angles of incidence. He ascribes the twinkling of the stars to the motion of the air in which the light is refracted; and he illustrates this hypothesis by observing, that they twinkle still more when viewed in water put in motion. He also shows that refraction is necessary, as well as reflection, to form the rainbow: because the body which the rays fall upon is a transparent substance, at the surface of which one part of the light is always reflected, and another refracted. And he makes some ingenious attempts to explain refraction, or to ascertain the law of it. He also considers the foci of glass spheres, and the apparent size of objects seen through them, though with but little accuracy. See REFRAcTION. Contemporary with Vitellio was Roger Bacon, a man of very extensive genius, who wrote upon almost every branch of science; though it is thought his improvements in optics were not carried far beyond those of Alhazen and Vitellio: to him, however, has been attributed the invention of the MAG1c LANTERN, which see. One of the next who distinguished himself in this way was Maurolycus, teacher G

of mathematics at Messina. In a treatise, “De Lumine et Umbra,” published in 1575, he demonstrates, that the crystalline humour of the eye is a lens that collects the rays of light issuing from the objects, and throws them upon the retina, where the focus of each pencil is. From this principle he discovered the reason why some people are short-sighted, and others long-sighted; also why the former are relieved by concave glasses, and the others by convex ones. Contemporary with Maurolycus was John Baptista Porta, of Naples. He discovered the camera obscura, which throws considerable light on the nature of vision. His house was the constant resort of all the ingenious persons at Naples, whom he formed into what he called an Academy of Secrets, each member being obliged to contribute something that was not generally known, and might be useful. By this means he was furnished with materials for his “Magia Naturalis,” which contains his account of the camera obscura, the first edition of which was published, as he informs us, when he was not §. fifteen years old. He also gave the rst hint of the magic lantern, which Kircher afterwards followed and improved. His experiments with the camera obscura convinced him, that vision is performed by the intromission of something into the eye, and not by visual rays proceeding from it, as had been formerly imagined; and he was the first who fully satisfied himself and others upon this subject. He justly considered the eye as a camera obscura, and the pupil the hole in the window-shutter; but he was mistaken in supposing that the crystalline humour corresponds to the wall which receives the images; nor was it discovered, till the year 1604, that this office is performed by the retina. He made a variety of just remarks concerning vision, and particularly explained several cases in which we imagine things to be without the eye, when the appearances are occasioned by some af. fection of the eye itself, or by some motion within the eye. He remarked also, that in certain circumstances, vision will be assisted by convex or concave glasses; and he seems even to have made some small advances towards the discovery of telescopes. Other treatises on optics, with various and gradual improvements, were afterwards successively published by several authors, whose names, with the titles and brief accounts of their general works, would occupy a large space. We must, however, mention the excellent work on

optics, by Dr. Smith, 2 vols. 4to, ; an abridgement of which was made by Dr. Kipling, for the use of the students at the Universities, entitled “Elementary Parts of Dr. Smith's Optics,” &c. 1778; and an elaborate History of the Present State of Discoveries relating to Vision, Light, and Colours, by Dr. Priestley, 4to. 1772; a work highly instructive and entertaining to persons who have a taste for physics. The laws of optics depending upon the properties of light, the reader will do well, as introductory to this article, to refer to what has been said in our fourth volume on that subject. There will be found much curious speculation, and a variety of interesting facts relating to the nature of light, its velocity, and the direction which it takes in moving through free i. and through our atmosphere. We shall in this place give a few definitions necessary to the mere student. By a ray of light, is meant the motion of a single particle; and its motion is represented by a straight line. Any parcel of rays proceeding from a point, is called a pencil of rays. By a medium, is meant any pellucid or transparent body, which suffers light to pass through it. Thus, water, air, and glass, are called media. Parallel rays, are such as move always at the same distance from each other. If rays continually recede from each other, as from C to c d (Plate I. Optics, fig. 1.) they are said to diverge. If they continually approach towards each other, as in moving from c d to C, they are said to converge. The point at which converging rays meet is called the focus. The point towards which they tend, but which they are prevented from coming to, by some obstacle, is called the imaginary focus. When rays, after passing through one medium, or entering another medium of different density, are bent out of their former course, and made to change their direction, they are said to se refracted: thus A C (fig. 2,) is a ray, which, when it enters the medium H GK, instead of proceeding in the same direction C L, it is made to move in the direction C. S. When they strike against a surface, and are sent back again from the surface, they are said to be reflected. The incident ray, as A C, is that which comes from any luminous body, and falls upon the reflecting surface, as H. K, and C M is the reflected ray. The angle of incidence is that which is contained between the incident ray A C and a perpendicular to the reflecting surface in the point of reflection, as the angle A CD: The angle of reflection, is that contained

between the said perpendicular DC, and the reflected ray CM, viz. the angle DCM. The angle of refraction, is that contained between the refracted ray CS, and the perpendicular CN, viz. the angle FCK. The angle of deviation, is that which is contained between the line of direction of an incident ray AL, and the direction of the same ray CF, after it is refracted; thus the angle L C F is the angle of deviation. A lens, is glass ground into such a form as to collect or disperse the rays of light which pass through it. These are of different shapes, and from thence receive different names. A plano-convex, has one side flat, and the other convex, as A (fig. 3.) A plano-concave, is flat on one side, and concave on the other, as B. A double convex, is convex on both sides, as C. A double concave, is concave on both sides, as D. A meniscus, is convex on one side, and concave on the other, as E. A line passing through the centre of a lens, as FG, is called its axis. Of Refraction. If the rays of light, after passing through a medium, enter another of a different density perpendicular to its surface, they proceed through this medium in the same direction as before. Thus the ray OP (fig. 2.) proceeds to K, in the same direction. But if they enter obliquely to the surface of a medium, either denser or rarer than what they moved in before, they are made to change their direction in passing through that medium. If the medium which they enter be denser they move through it in a direction nearer to the perpendicular drawn to its surface. Thus, AC, upon entering the denser medium HGK, instead of proceeding in the same direction AL, is bent into the direction CF, which makes a less angle with the perpendicular OP. On the contrary, when light passes out ef a denser into a rarer medium, it moves in a direction farther from the perpendicular. Thus, if SC were a ray of light which had passed through the dense medium HGK, on arriving at the rarer medium it would move in the direction CA, which makes a greater angle with the perpendieular. This refraction is greater or less, that is, the rays are more or less bent or turned aside from their course, as the second medium through which they pass is more or less dense than the first. Thus, for instance, light is more refracted in passing from air into glass, than from air into water; glass being denser than water. And, in general, in any two given media, the sine of any one angle of incidence has the same ratio

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to the sine of the corresponding angle of refraction, as the sine of any other angle of incidence has to the sine of its corresponding angle of refraction. Hence, when the angle of incidence is increased, the corresponding angle of refraction is also increased; because the ratio of their sines cannot continue the same, unless they be both increased; and if two angles of incidence be equal, the angles of refraction will be equal. The angle of deviation must also vary with the angle of incidence. If a ray of light, AC, (fig. 2) pass obliquely out of air into glass, A D, the sine of the angle of incidence A CD, is to N S, the sine of the angle of refraction N C S, nearly as 3 to 2; therefore, supposing the sines proportional to the angles, the sine of FC L, the angle of deviation, is as the difference between A D and N S, that is as 3–2, or 1, whence the sine of incidence is to the sine of the angle of deviation as 3 to 1. In like manner it may be shown, that when the ray passes obliquely out of glass into air, the sine of the angle of incidence will be to that of deviation, as N S to A D – N S, that is, as 2 to 1. . In passing out of air into water, the sine of the angle of incidence is to that of refraction, as 4 to 3, and to that of deviation, as 4 to 4–3, or 1; and in passing out of water into air, the sine of the angle of incidence is to that of refraction, as 3 to 4, and to that of deviation, as 3 to 1. Hence a ray of light cannot pass out of water into air at a greater angle of incidence than 48° 36', the sine of which is to radius as 3 to 4. Out of glass into air the angle must not exceed 40°11', because the sine of 40° 11’ is to radius as 2. to 3 nearly; consequently when the sine has a greater proportion to the radius than that mentioned, the ray will not be refracted. It must be observed, that when the angle is within the limit for light to be refracted, some of the rays will be reflected. For the surfaces of all bodies are for the most part uneven, which occasions the dissipation of much light by the most transparent bodies; some being reflected, and some refracted, by the inequalities on the surfaces. Hence a person can see through water, and his image reflected by it at the same time. Hence also, in the dusk, the furniture in a room may be seen by the reflection of a window, while objects that are without are seen through it. Upon a smooth board, about the centre C, describe a circle H O K P ; draw two diameters of the circle, O P, H K, perpendicular to each other; draw A D M

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