OPERA. A dramatic composition set to music. It is sung on the stage, accompanied and interspersed with dialogue.

OPERA-GLASS. A kind of glass constructed in a small wooden tube, so as to view a person in a theatre, and, as the glass is made to point at a different object from that which is viewed, it may be used without any one knowing exactly who is observed.

OPHIOPHAGI. A name given by some to the eagle, vulture, and some other birds of prey which are sometimes seen to feed on serpents.

Pliny gives the name ophiophagi to a certain people of Ethiopia, whom he describes as very barbarous and savage, going always naked, and feeding on serpents, whence the appellation. Solinus, who generally copies Pliny but imperfectly, has perverted his meaning strangely in this passage, having placed the ophiophagi in Arabia Felix, instead of Ethiopia.

OPHIR. In Sacred Geography, the place from which Solomon procured the gold and other precious articles with which he enriched himself, and adorned the temple of Jerusalem.

Concerning the part of the world in which Ophir was situated, there have been many and various opinions and conjectures; some of them extremely fanciful, not to say absurd; and others supported and elucidated with no inconsiderable portion of ingenuity and learning: still, however, the exact situation of this place is undetermined, though the opinion that it was somewhere either on the eastern or western coast of Africa seems the most plausible, and to obtain the sanction of the most learned and well-informed writers, who have discussed or adverted to this point of sacred geography.

OPHTHALMIA. An inflammation of the membranes of the eye, a disease which particularly affected the English soldiers during their stay in Egypt.

this state opium is a tenacious substance, of a brownish color; has a peculiar smell, and a disagreeable bitter taste. It becomes soft with a moderate heat. It readily takes fire, and burns rapidly.

OPODELDOC. A solution of soap and alcohol, with the addition of camphor and volatile oils. It is used externally against rheumatic pains, sprains, bruises, and other like complaints.

OPOSSUM. An American animal that lives in holes and woody places. The female is remarkable for having two or three pouches, wherein the young conceal themselves in time of danger. It is found only in Virginia, and the vicinity.

OPPOSITION. In Astronomy, one of the aspects of the planets, when they are 180 degrees distant from each other, that is, in a diametrically opposite relation to each other.

OPTICAL ILLUSIONS. ‘I have observed,' says the traveller Patrin, 'in the sandy deserts of Asia and Africa, a phenomenon rendered singular by the illusive appearances which it presents. The traveller, in the midst of arid plains, imagines that he sees at the distance of a few hundred steps, a vast extent of waters, whose shores sometimes appear covered with trees and verdure. Charmed with this agreeable and unexpected aspect, he presses onward in the hope of finding that refreshinent and repose, which, in this scorching climate, he so greatly needs. But as he approaches in the direction of the object, it still retires before him, and finally vanishes.

This phenomenon, which has been called mirage and looming, has been observed by modern travellers in Egypt, and was formerly perceived by the army of Alexander, in the deserts of Sagdiana, to the east of the Caspian sea, as we learn from the history of that conqueror. Quintus Curtius thus describes this appearance: When the fierceness of the sun heated these deserts, it might have been said, that the whole country assumed the aspect of a general inflammation. The sky was darkened by the vapors which arose from the burning soil, and the sandy plain had the appearance of a vast and profound sea.'

OPIUM. In Chemistry and Medicine, an inspissated gummy juice, which is obtained from the head of the papaper somniferum.' It is im- At the commencement of the last century, an ported from Persia, Arabia, and other warm parts English traveller, on his route to Pekin, who found of Asia, in flat cakes covered with leaves to pre-himself, about the middle of October, in the sandy vent their sticking together. It has a reddish desert which separates Siberia from the frontiers brown color, and strong peculiar smell: its taste of China, witnessed the same spectacle, which he at first is nauseous and bitter; but this soon be-describes in the following terms;- Sometimes in comes acrid, and produces a slight warmth in the mouth. A peculiar substance has been detected in opium, to which it is supposed the properties it possesses of producing 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 different 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

the morning, I was agreeably surprised to see before us, at a little distance, what appeared a broad and beautiful river, bordered with ranges of fine trees. It was no more than an optical illusion, occasioned, as I imagine, by vapors which so magnified objects, as to transform the bushes, spread along the desert, into large trees.'

When the French troops entered the deserts of Egypt, they witnessed the same singular spectacle of mirage. Instead of an arid and sandy plain which really spread before them, they imagined that they saw a vast lake, in the midst of which were islands, with villages built upon them.

Mariners and travellers have presented us innu

merable instances of these optical deceptions from mirage, or looming. This spectacle is nowhere seen more frequently, or in greater perfection, than on the drier prairies of the Upper Missouri. A man seen through the fog, in certain aspects of the sky, seems as tall as the mast of a ship; and sandhill cranes, walking on the grass, might be mistaken for buffaloes.

so seductively visible, as to deceive the most experienced eye. We have no theory, which explains these appearances in a manner perfectly satisfactory. Quintus Curtius supposed that such illusions were caused by the burning vapors of the sands of Sagdiana. But a French traveller, as we have seen, had the same view in the sandy deserts of Siberia, when the air was so cold that the caraMariners at sea are often deceived by an optical van hastened its march, through fear of being deillusion, which may be viewed as a sort of reverse layed by snow. It is not then as Quintus Curtius of land mirage. This creates a sca in the midst | supposed, burning vapors, which present to vision of sandy deserts. The other shows in the wide a vast sheet of water in a desert of arid sands. wastes of the sea, lands, where are seen in the What is the true cause of this singular phenomeclearest manner shores, rocks, mountains, ravines non! We can only reply, that it is seldom oband trees. The illusion is so complete, that often served, except in sandy and dry plains; and that the most experienced mariners and the most en- the fog-banks, with their rocks and mountains, lightened philosophers have been completely de-are never seen, except on the open sea. ceived. The vessels have been directed to make There are other aerial phenomena, which profor the land, which fled before them, and finally | duce very singular optical illusions. Among thei entirely disappeared. The name of fog-banks has none is more striking, than that, which is seen been applied to them. from time to time, upon the strait, which separates The great navigator Cook was often deceived Sicily from Calabria. Swinburne, the English by these illusions, familiar as was his experience traveller, quotes father Angelluca as describing with all the phenomena of the seas. He was seek-one, of which he was an eye witness. The sea,' ing to make the isle of Pepys near the straits of he remarks, became suddenly as if in flames, and Magellan, in forty degrees of south latitude. appeared, in a line of ten miles extent, like a chain He judged that he saw land; and he coasted along of mountains of an obscure tint, while the waters the supposed shores two hours and a half. He of the shores of Calabria were apparently united, then discovered that the seeming land was only a and appeared like a highly polished mirror leaning phantom. This was not the fog of winter; for it against a curtain of hills. Upon this glass was was in the height of the summer of the southern seen painted a range of many thousands of pilasthemisphere. Another time he was deceived in ers, all alike in height, distance, and degree of the same way, in a clear and pleasant day, to the light and shade. A moment afterwards these pilsouth of the Indian Ocean. Still a third time he asters were transformed into arcades like the aquewas led astray by the same deception to the south ducts of Rome. Upon the summit of these arof Africa. These three deceptions were all expe- cades was a sweeping line of coruices, surmounted rienced in the southern hemisphere; and in the with a multitude of castles, which immediately afmiddle of the summer of that portion of the earth. ter transformed themselves to towers. These in How strong the illusion was, is proved by the cir- turn became colonnades, then ranges of windows, cumstance, that Cook had with him the most en- and finally pines and cypresses all of equal elevalightened of observers, who shared this error with tion. Previous to seeing this spectacle, called in the language of the country Fata Morgana, I had regarded the description of it, as a mere tale.'

him.

The great French navigator Peyrouse was completely deceived by these fantastic apparitions, as well as the learned philosophers and naturalists who accompanied him. He thus describes a deception of this sort. He was in latitude forty-four near the shores of Tartary. At four in the evening,' he remarks, the most beautiful sky succeeded to the thickest fog. We imagined we saw the continent; and in the south a great country, which appeared to join Tartary to the west, leav-bited by the wandering Arabs. ing between it and the continent an opening of about fifteen degrees. We distinguished the mountains, the ravines, in short all the details of a land view. We could not conceive how we had entered into this strait. In this position, I ordered the vessel to be directed towards the south east, and this supposed land. But soon the hills and ravines disappeared. It was a fog-bank, the most extraordinary I had ever seen, which had occasioned this error.'

The phenomenon of figures seen in the air, is an optical illusion so general, that almost every country has its own.

One of the most singular is that described by Diodorus Siculus, which was observed in that part of Africa, which extends from Tripoli to Barca, opposite the gulf named by the ancients the Syrtes of Africa. This desert country is at present inha

What surprises most, is the singular contrast between the mirage and the fog-banks. In the first instance, a parched and desert country presents the aspect of a great and beautiful lake, or a wide river; and the surface of the open ocean shows rocks, lands, mountains, and all these appearances

This author tells us, That in all seasons, when there is no wind, the air appears full of figures of animals, of which some seem motionless; and others to have the power of voluntary motion. They appear of an extraordinary size, and nothing is more capable of affrighting those, who are not habituated to this strange spectacle.'

Patrin relates, that he has remarked hundreds of times in northern Asia, in those vast plains, known by the name of Steppes, which are bare of trees, and bounded only by the horizon, corresponding exactly to our prairies, in autumnal evenings, when the air was perfectly calm, a belt of clouds elevated from fifteen to twenty degrees above the horizon. These clouds, seemingly thick, although the dome of the sky was perfectly

clear, were mixed with lights and shadows, like a design in India ink; and always represented human figures in different attitudes, oftener naked than clad; and of a size beyond the proportions of nature. The imitation at times appeared so perfect,' he remarks, 'that I began to fear that they were the phantoms of my own imagination. To satisfy myself on this head, I asked my guides and the inhabitants of the country, what they saw in the clouds, and the answers of these simple people described appearances precisely similar to those I saw myself.'

which most persons are supposed to see small obs jects best, is about seven inches; but by the help of convex glasses, we are enabled to view things clearly at a much shorter distance than this; for the nature of a convex lens is, to render an object distinctly visible to the eye at the distance of its focus. With a knowledge of this fact, we may easily determine the magnifying powers of glasses employed in Single Microscopes, which are small double convex lenses, having the object placed in the focus, and the eye at the same distance on the other side. If rays of light from an object are converged to a point at the distance of one inch OPTICAL INSTRUMENTS. As the sight is from the centre of the glass, or, in other words, if the most noble and extensive of all our senses; the focal distance of the lens is one inch, an object as we make the most frequent use of our eyes in may be seen through that lens at one inch distance all the actions and concerns of life; that instru- from the eye, and it will appear, in its diameter,— ment which relieves the eyes when decayed, and since the natural sight is seven inches,-seven supplies their defects, must be estimated as one of times larger than to the naked eye. But as the the greatest of advantages. Sight may be defec- object is magnified every way equally, in length tive in various ways. Some eyes are too flat, oth- as well as breadth, we must square this diameter, ers are too convex or round; in some, the humors to know really how much the object appears enlose a part of their transparency, and on that ac-larged; and we shall thus find that its surface is count, much of the light that enters the eye is magnified forty-nine times. If we suppose the stopped and lost in the passage, and every object appears dim. Spectacles are intended to collect the light and to bring it to a proper degree of convergency. The honor of their invention was claimed by Salvinus Armatus, a nobleman of Florence, who died in 1317, and the fact was inscribed on his tomb. When the eye is too flat, the rays proceeding from objects do not converge to a focus so soon as they reach the retina; in this case a convex glass is necessary, for it has the property of converging the rays, and of course, when suited to the eye, of bringing them to a focus, and forming an image on the retina. When the eye is too convex, the rays of light are converged to a focus before they reach the retina; to remedy this, a concave glass is used, which causes the rays to diverge, and prevents their coming to a focus too soon. Short-sighted persons bring objects close to their eyes; it has a similar effect to that produced by concave glasses; for the nearer an object is brought to the eye, the greater is the angle under which it is seen, that is, the extreme rays, and of course all the others, are made more divergent. But persons whose eyes are too flat, when examining an object, hold it at a distance, for the farther an object is held from their eyes, the less is the divergency of its rays, that is, the smaller is the angle under which it is seen: the focal distance is increased, and an image is properly formed on the retina. In considering vision as achieved by the means of an image formed at the bottom of the eye, we can never reflect, without wonder, upon the smallness, yet correctness of the picture, the subtilty of the touch, the fineness of the lines. A landscape of five or six square leagues is brought into a space of half an inch diameter; yet the multitude of objects which it contains are all preserved; are all discriminated in their magnitudes, positions, figures, colors.

Microscopes are instruments for viewing small objects; and they apparently magnify them, because they enable us to see them nearer than with the naked eye, without affecting the distinctness of vision. The distance from the naked eye, at

focus of a convex lens to be at one-tenth of an inch distant from its centre, in seven inches there are seventy such tenths of an ineb; and an object therefore may be seen through this lens seventy times nearer than it can, distinctly, by the naked eye. It will consequently appear seventy times longer and seventy times broader than it does to common sight; and as seventy multiplied by seventy makes four thousand nine hundred; so many times it really appears magnified. Those lenses, therefore, which have the shortest focus, will magnify the object most. Single microscopes of the greatest power may be made with a very small globule of glass, fixed in a thin plate of metal, so that the middle of it may be directly over the centre of an extremely small hole made in the plate.

The compound microscope consists of at least two lenses, by one of which an image is formed within the tube of the microscope; and this image is viewed through the eye-glass, instead of the object itself. The solar microscope is a kind of camera obscura, which, in a darkened chamber, throws the image on a wall or screen. It consists of two lenses fixed opposite to a hole in a board or window-shutter. There is also a plane reflector or mirror placed without, which may be so regulated as to throw the sun's rays upon the outer lens. A magic lantern is constructed on the same principles. The light is supplied by a lamp instead of the sun, and it is used for magnifying paintings on glass, and throwing their images upon a white screen in a darkened chamber.

OPTIC NERVES. The second pair of nerves of the brain, which perforate the bulb of the eye, and serve for the sense of sight.

OPTICS. Optics is the science which treats of light, and of the instruments by which it is applied to useful purposes. It is one of the most interesting branches of natural philosophy, but not one of the easiest to understand; it will be necessary, therefore, that you give to it the whole of your attention.

Light, when emanated from the sun, or any dium, and the more transparent the body, the other luminous body, is projected forwards in more perfect is the medium. But rays of light do straight lines in every possible direction; so that not pass through a transparent medium, (unless the luminous body is not only the centre from they fall perpendicularly upon it) in precisely the whence all the rays proceed, but every point of it same direction in which they were moving before may be considered as a centre which radiates light they entered it. They are bent out of their former in every direction. The particles of light are so course, and this is called refraction. When they extremely minute, that although they are projected pass out of a rarer into a denser medium, as from in different directions, and cross each other, yet air into water or glass, they are always refracted they are never known to interfere, and impede towards a perpendicular to the surface, and the each other's course. It is still a disputed point, refraction is, more or less, in proportion as the rays however, whether light be a substance composed fall, more or less, obliquely on the refracting surof particles like other bodies. In some respects it face. But when they pass from a denser into a is obedient to the laws which govern bodies; in rarer medium, as from glass or water into air, they others, it appears to be independent of them: thus, move in a direction farther from the perpendicular. though its course is guided by the laws of motion, If you put a piece of money into an empty basin, it does not seem to be influenced by the laws of and stand at such a distance that it may not be gravity. It has never been discovered to have visible; then let another person pour water into weight, though a variety of interesting experi- the basin, and the money will be seen; for the ments have been made with a view of ascertaining rays of light, in passing from a denser into a rarer that point. Some suppose that the rays of light, medium, are bent from the perpendicular, and thus instead of being particles, consist of the undula- are directed to your eye. The following, theretions of an elastic medium, which fills all space, fore, may be established as a sort of axiom in opand which produces the sensation of light to the tics: we see every thing in the direction of that eye, just as the vibrations of the air produce the line in which the rays approach us last. If you sensation of sound to the ear. Most of the phe- place a candle before a looking-glass, and stand nomena may be accounted for by either hypothe-before it, the image of the candle appears behind sis, but that of their being particles applies more happily to some of the facts respecting the modifications of light by refraction and reflection.

it; but if another looking-glass be so placed as to receive the reflected rays of the candle, and you stand before this second glass, the candle will When rays of light encounter an opaque body, appear behind that; because the mind imagines part of them are absorbed, and part are reflected, every object to be in the direction from which the and rebound just as an elastic ball which is struck rays come to the eye last. Hence, when the rays against a wall. A ray of light striking perpendic- of light coming from the celestial bodies, arrive at ularly upon a plane mirror, is reflected back in the our atmosphere, they are bent downward; and same direction; but those rays which strike it those bodies appear, when in the horizon, higher obliquely, are reflected back in an opposite direc- than they are. The effect of this refraction is about tion, but with the same obliquity; the angle of six minutes of time, but the higher they rise, the reflection, therefore, is exactly equal to the angle less are the rays refracted; and when they are in of incidence. If you stand directly before a look- the zenith, they suffer no refraction. The sun is ing-glass, you see your image reflected back to visible about three minutes before he rises, and you. If you stand a little to the side, you cannot about the same time after he sets; making in the see yourself; but a person who stands just as far course of a year about a day and a half. Twilight on the other side of it, can see your image in the is occasioned partly by refraction, but chiefly by glass, and you can see his. If you place a candle reflection of the sun's rays by the atmosphere, and a little to one side, you must go as far on the other it lasts till the sun is eighteen degrees below_the to see its image in the glass. This is the same horizon. Were there no atmosphere to reflect rule which takes place in the collision of elastic and refract the sun's rays, only that part of the bodies against any surface. If you strike an ivory heavens would be luminous in which the sun is ball or common marble perpendicularly against placed; and if we could live without air, and the wainscot, it returns to you; but if you make should turn our backs to the sun, the whole heait strike sideways, it goes off at the same angle vens would appear as dark as in the night. In with which it came to the wainscot. So it is with this case also, a sudden transition from the brightrays of light; the incident ray, or the ray which est sunshine to dark night would immediately take falls upon a surface, makes an angle with a per-place upon the setting of the sun. pendicular line, drawn from the point where it strikes, equal to that which the reflected ray makes OPTICS, HISTORY OF. The properties of with it. light naturally attracted, at an early period, the With respect to a looking-glass, it is the silvering attention of philosophers who made nature their on the glass which causes the reflection, otherwise study. Empedocles, who flourished upwards of the rays would pass through it without being stop-four hundred years before Christ, is said to have ped, and if they were not stopped they could not written a treatise on light; and the works of be reflected. No glass, however, is so transparent Aristotle present us with a number of questions but it reflects some rays; if you put your hand near a window, you clearly see its image on the other side, and the nearer the hand is to the glass, the more evident is the image. Whatever suffers the rays of light to pass through it is called a me

and observations concerning optical appearances, This philosopher was aware that it is the reflection of the light from the atmosphere which prevents total darkness after the sun sets, and in those places where he does not shine in the daytime. He also

considered that rainbows, halos, and mock sunsopher, who flourished in the twelfth century. He were all occasioned by the reflection of the sun-made many observations and experiments on the beams in different circumstances, by which an effects of refraction at the surface between air and imperfect image of his body was produced, the water, air and glass, and water and glass, from color only, and not his proper figure, being exhib- which he deduced that atmospherical refraction ited. Euclid wrote a treatise on optics and catop- increases the altitudes of all objects in the heavens. trics, in which he shows the chief properties of He also first observed that the stars are sometimes reflected rays in plane, convex, and concave sur- seen above the horizon by means of refraction, faces, in a geometrical manner, beginning with that when they are really below it; an observation concerning the equality of the angles of incidence confirmed by Vitellio and other opticians. He and reflection. He also takes some notice of di- likewise maintained that refraction contracts the optrics, and remarks on the effect of refraction in diameters and distances of heavenly bodies, and regard to an object at the bottom of a vessel, which, that it is the cause of the twinkling of the when water is poured in, is brought to view, that stars. was not to be seen above the edge of the vessel before the water was poured in. As to the effect of burning glasses, both by reflection and refraction, this is noticed not only by Euclid, but by many other of the ancients; and, if we give credit to historiaus, the exploits performed by Archimedes in setting fire to the vessels of the Romans before Syracuse, by means of burning mirrors, prove that his practical knowledge exceeded that of modern times. There is no doubt that he wrote a treatise on burning glasses, as also concerning the appear ance of a ring or circle under the water, which shows that this phenomenon had not escaped his notice. The ancients were also acquainted with the production of colors by means of refracted light. Seneca observes that when the light of the sun shines through an angular piece of glass, it shows all the colors of the rainbow; also that the colors seen in a pigeon's neck when it changes its position, are the effect of refraction, and on the same principle that a speculum, not having any color of its own, will assume that of any other body.

Besides, the ancients were not unacquainted with the magnifying power of glass globes filled with water, for the ancient engravers used to employ such a glass globe, in order, as is supposed, to nagnify the figures, that they might execute their work with more correctness. Ptolemy, who wrote a considerable treatise on optics, was well acquainted with the refraction of light, and deterinned the ratio of the angles of refraction, as compared with that of the angles of incidence, with such accuracy, that there is but a trifling difference between the results of his observations and those of Newton; not more than might arise from his having used glass and water of specific gravities something different from those employed by Newton. It appears also from this work of Ptolemy, as also from his Almagest, that he employed his knowledge of optics in his astronomical observations, for he was fully aware that refraction decreases from the horizon to the zenith, and that, by means of this refraction, the intervals between the stars appear less when near the horizon than in the meridian. He also accounts for the remarkably great apparent size of the sun and moon when seen near the horizon, by ascribing the appearance to the refraction of the rays by vapors, which actually enlarge the angle under which the Juminaries appear, just as the angle is enlarged by which an object is seen from under water.

The next writer of any importance on the science of optics was Alhazen, an Arabian philos

Besides, Alhazen treats largely on the magnifying power of glasses, so that probably his observations led to the invention of spectacles. In the next century followed Vitellio, a Pole, who digested the contents of Alhazen's work, and made many additional observations on the power of refraction. He gave a table of the results of his experiments on the refracting power of air, water, and glass, corresponding to different angles of incidence. Roger Bacon, a contemporary with Vitellio, also wrote on this science, and is generally considered to be the inventor of the magic lantern. Maurolycus, who followed these two at the distance of nearly two centuries, explains, in his treatise De Lumine et Umbrâ, the process of vision, showing that the crystalline humor of the eye is a lens which collects the rays of light issuing from the objects, and throws them on the retina, where the focus of each pencil is formed. From this principle he discovered the reason why some people have a short sight and others a long one; also why the former are assisted by concave glasses, and the latter by convex ones. John Baptista Porta, his contemporary, discovered the camera obscura, and took the first public notice of the magic lantern, the original invention of which has been ascribed to Roger Bacon. Kircher, who followed Baptista, enlarged on his hints, and put them into execution. He also made many experiments with the camera obscura, by which he satisfied himself 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. He considered the eye as a camera obscura, the pupil to be the hole in the window shutter, and the crystalline humor to correspond to the wall which receives the images; but in this latter point his idea has been proved, by closer observations, to be incorrect, for it is now known that this office is performned by the retina. The observations and experiments of this writer on the science of optics, and on the nature of vision, appear to have led the way to the discovery of telescopes, which was doubtless made very soon after his time. After this the writers on optics became very numerous, and their labors contributed to the confirmation and improvement of those who had preceded them. Among the works entitled to particular notice are Barrow's Optical Lectures, Huygen's Dioptrics, Hartsocker's Essaie Dioptrique, David Gregory's Elements of Dioptrics and Catoptrics, Dr. Smith's Optics, Wolfius's Dioptrics and Catoptrics, Harris's Optics, but above all, Newton's Treatise on Optics, and his Optical Lectures.