OPTICS

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OPTICS ; the science of vision, which treats of the changes which light undergoes, in its qualities, or in its duration, when passing through bodies of different kinds and shapes, when reflected from their surfaces, or when moving past them at short distances. (For an account of the nature and more general properties of light, see the article under that word.) Preliminary to the present article, we give the following definitions: 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, passing 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, they are said to diverge ; if they continually approach towards each other, 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, on entering another medium, of different density, are bent out of their former course, and made to change their direction, they are said to be refracted; when they strike against a surface, and are sent back again from the surface, they are said to be re fleeted. 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 planoconvex lens has one side flat, and the other convex ; a planoconcave lens is flat on cne side, and concave on the other; a double convex lens is convex on both sides; a double concave lens is concave on both sides; a meniscus is convex on one side, and concave on the other. A line passing through the centre of a lens is called its axis.Of Refraction. Although a ray of light will always move in the same straight line, when it is not interrupted, yet every person knows, that when light falls upon a drop of water, or a piece of glass, or a bottle 'containing any fluid which allows the light to pass, it does not reach the eye, or illuminate a piece of paper placed behind those bodies, in the same manner as before they were put in its way. This obviously arises from the direction of the light being changed, by some power which resides in the bodies. The explanation of the law, or rule, by which this change in the direction of a ray takes place, constitutes that part of the science of OPTICs called dioptrics, from two Greek words, one of which signifies through, and the other to see, because the bodies which produce this change are those through which we can see, or through which light passes. 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 their original 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 they enter be denser, they move through it in a direction nearer to the perpendicular drawn to its surface. On the contrary, when light passes out of a denser into a rarer medium, it moves in a direction farther from the perpendicular. 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. To prove this, in a satisfactory way, take an upright empty vessel into a darkened room, which admits but a single beam of light obliquely through a hole in a window shutter. Let the empty vessel stand on the floor, a few feet hi advance of the window which admits the light, and let it be so arranged, that as the Deam of light descends towards the floor, it just passes over the top of the side of the vessel next the window, and strikes the bottom on the side farthest from the window. Let the spot where it falls be marked. Now, on filling the vessel with water, the ray, instead of striking the original spot, will fall considerably nearer the side towards the window. And if "we add a quantity of salt to the vessel of water, so as to form a dense solution, the point where the ray strikes the bottom will move still nearer to the window. In like manner, if we draw off the salt water, and supply its place with alcohol, the beam of light will be still more highly refracted; and oil will refract yet more than alcohol. In these experiments, if the room be filled with dust, the rays will be rendered much more visible. Although, in most cases, there is a connexion between the refractive power and the density of bodies, yet refraction does not invariably increase with their density. In the case of oily substances and inflammable bodies, such as hydrogen, phosphorus, sulphur, diamond, bees' wax, amber, spirit of turpentine, linseed oil, olive oil, camphor, their refractive powers are from two to seven times greater, in respect to their density, than those of most other substances. Sir Isaac Newton observed this fact with respect to the last live substances, which, he says, are "fat,su]phureous,unctuous bodies," and, as he observed the same high refractive power in the diamond, he inferred that it was probably an unctuous substance coagulated. This law, however, at one time, seemed to be overturned by an observation of doctor Wollaston, that phosphorus, one of the most inflammable substances in nature, had a very low refractive power ; but doctor Brewster, confiding in the truth of the law, examined the refractive power of phosphorus by forming it into prisms and lenses, and he found it to be nearly as high as diamond, and fully twice that of diamond compared with its densityan observation which reestablished and extended the general principle respecting the refractive power of inflammable substances. (For an account of double refraction, see Refraction, Double.)Of Reflection. When light falls upon a body, a portion of it is thrown back, or reflected from its surface, according to a regular law, the explanation of which constitutes that branch of OPTICs called catoptrics, a word derived from two Greek words, one of which signifies from, or against, and the other to see, because things are seen by light reflected from bodies. When a ray of light falls upon any body it is reflected so that the angle of incidence is equal to the angle of reflection; and this is the fundamental fact upon which all the properties of mirrors depend. Let a ray of light, passing through a small hole into a dark room, be reflected from a plane mirror; at equal distances from the point of reflection, the incident and the reflected ray will be at the same height from the surface. The same is found to hold in all cases, when the rays are reflected at a curved surface, whether it be convex or concave. The rays which proceed from any remote terrestrial object, are nearly parallel at the mirror ; not strictly so, but come diverging to it in several pencils, or, as it were, bundles of rays, from each point of the side of the object next the mirror; therefore they will not be converged to a point at the distance of half the radius of the mirror's concavity from its reflecting surface, but in separate points, at a greater distance from the mirror. And the nearer the object is to the mirror, the farther these points will be from it; and an inverted image of the object will be formed in it which will seem to hang pendent in the air, and will be seen by an eye placed beyond it (with regard to the mirror), in all respects like the object, and as distinct as the object itself. If a man place himself directly before a large concave mirror, but farther from it than the centre of its concavity, he will see an inverted image of himself in the air, between him and the mirror, of a less size than himself; and if he hold out his hand towards the mirror the hand of the image will come ou towards his hand, and coincide with it, of an equal bulk, when his hand is in the centre of concavity, and he will imagine that he may shake hands with his image, If he reach his hand farther, the hand of the image will pass by his hand, and come between it an4 his body ; and if he move his hand towards either side, the hand of the image will move towards the other; so that, whatever way the object moves, the image will move the contrary way. A bystander will see nothing of the image, because none of the reflected rays that form it enter his eyes. The images form ed by convex specula are in positions similar to those of their objects; and those also formed by concave specula, when the object is between the surface and the principal focus: in these cases, the image is only imaginary, as the reflected rays reflection from concave specula, the images are in positions contrary to those of their objects; and these images are real, for the rays, after reflection, do come to their respective foci. Colors. The origin of colors is owing to the composition which takes place in the rays of light, each heterogeneous ray consisting of innumerable rays of different colors: this is evident from the separation that, ensues in the wTell known experiment of the prism. That branch of optics which treats of the colors of light, of their physical properties, and of the laws according to which light is decomposed, and recomposed from its elements, is called chromatics, from a Greek word signifying color. A ray being let into a darkened room, through a small aperture, and falling on a triangular glass prism, is, by the refraction of the prism, considerably dilated, and will exhibit, on the opposite wall, an oblong image, called a spectrum, variously colored, the extremities of which are bounded by semicircles, and the sides are rectilinear. The colors are seven in number, which, however, have various shades, gradually intermixing at their juncture. Their order, beginning from the side of the refracting angle of the prism, is red, orange, yellow, green, blue, purple, violet. The obvious conclusion from this experiment is, that the several component parts of solar light have different degrees of refrangibility, and that each subsequent ray, in the order above mentioned, is more refrangible than the preceding. Their different degrees of refrangibility may be proved by admitting rays of red, orange, yellow, green, blue, indigo and violet light, through a small aperture, into a darkened room, prepared as in the experiment for showing the refractive power of water, alcohol, &c, above described. We shall find that each color has a different refractive power of its own, that of the red being the least, and that of the violet the greatest. The following table exhibits the result of such an experiment with water: Red,................1.3310 Orange,..............1.3317 Yellow,..............1.3336 Green,...............1.3358 Blue,............... 1.3378 Indigo,...............1.3413 Violet,...............1.3442 Either of these rays, on being made to traverse another prism, remains unalter34 * other. This opinion is heightened by the fact, that they undergo no manner of change by reflection; for if any colored body be placed in simplified, homogeneous light, it will always appear of the same color as the light in which it is placed. White is compounded of all the primary colors, mixed in their due proportion ; for if a solar ray be separated, by the prism, into its components, and, at a proper distance, a lens be so placed as to collect the diverging rays again into a focus, a paper, placed perpendicularly to the rays in this point, will exhibit whiteness. The same conclusion may be drawn from mixing together paints of the same color as the parts of the spectrum, and in the same proportion; the mixture will be white, though not of a resplendent whiteness, because the colors mixed are less bright than the primary ones: this may likewise be proved by fixing pieces of cloth, of all the seven different colors, on the rim of a wheel, and whirling it round with great velocity; it will appear to be white. Though seven different colors are distinguishable in the prismatic spectrum, yet, upon a closer examination, we shall see that there are, in fact, only three original colorsred, blue, yellow; for the orange, being situated between the red and yellow, is only the mixture of these two ; the green, in like manner, arises from the blue and yellow ; and the violet from the blue and red. As the color of a body, therefore, proceeds from a certain combination of the primary rays which it reflects, the combination of rays flowing from any point of an object will, when collected by a glass, exhibit the same compound color in the corresponding point of the image. Hence appears the reason why the images, formed by glasses, have the colors of the object which they represent. When a prism of solid glass is employed for separating the rays of light, the lengths of the colors are expressed as follows : red, 45; orange, 27 ; yellow,; 40; green, 60; blue, 60; indigo, 48 ; violet, 80; or 360 in all. But these spaces vary with prisms of different substances, and as they are not separated by distinct limits, but shade gradually into one another, it is almost impossible to obtain any thing like an accurate measure of their relative extents. Vision. Objects presented to the ey§ have their images painted on the back part of the retina, the rays of the incident pencils converging to their proper foci there, by the refraction of the different humors, for which purpose they are admirably adapted; for, as the distance between the back and front of the eye is very small, and the rays of each of the pencils that form the image fall parallel, or else diverging, on the eye, a strong refractive power is necessary for bringing them to their foci at the retina; but each of the humors, by its peculiar form and density, contributes to cause a convergence of the rays ; the aqueous, from its convex form; the crystalline, by its double convexity and greater density than the aqueous ; and the vitreous, by a less density than the crystalline, joined to its concave form. The structure of the eyes is, in general, adapted to the reception of parallel rays; but, as the distances of visible objects are various, so the eye has powers of accommodating itself to rays proceeding from different distances, by altering the distances of the crystalline from the retina, which is done by the action of the ciliary ligaments. Defective sight arises from an incapacity of altering the position of the crystalline within the usual limits: 1. when it cannot be brought close enough to the cornea, near objects appear indistinct; to this defect people in years are generally subject; 2. when the crystalline cannot be drawn sufficiently near to the retina, remote objects appear indistinct ; this is the defect under which shortsighted people labor. In each of these cases, the images of the different points in the object would be diffused over small circles on the retina, and so, being intermixed and confounded with each other, would then form a very confused picture of the object. For, in the former case, the image of any point would be formed behind the retina, as the refraction of the eye is not sufficiently strong to bring the rays (diverging so much as they do in proceeding from a near point) to a focus at the retina. This defect will therefore be remedied by a convex glass, which makes the point whence the rays now proceed more distant than the object; therefore the rays, falling on the eye, will now diverge less than before, or else be parallel, and will, of course, be brought to a nearer focus, viz. at the retina. In the latter case, the image is formed before the retina, because the refractive power of the eye is too great "to permit rays so little diverging (as they do in proceeding from a distant point) to reach the retina, before they are collected into a focus: in this ease, the defect is supplied by a concave glass, 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 humors, not to come to their focus before they reach the retina. Therefore spectacles are constructed concave for shortsighted, and convex for longsighted people. And the frames of spectacles should be so bent that the axes of both glasses may be directed to the same point, at such a distance as you generally look with spectacles. By this means the eyes will fall perpendicularly upon both glasses, and make the object appear distinct; whereas, if they fell obliquely upon the glasses, the object will appear confused and indistinct. Cause of Squinting. A person is said to squint, when both eyes do not seem to be directed to the object at which he is looking. When either of the eyes has a less perfect vision, or a different focal length, or when there is any weakness in its external muscles, we are apt to make use only of the good eye; and when we acquire the habit of doing this, the imperfect eye is left at rest, and will sometimes cease even to follow the movements of the other. If the good eye is shut, and the bad one forced to exert itself, the iris will be placed symmetrically between the eyelids, and the squint formerly seen in the eye will disappear. Should the eye, in this case, still squint, the cause of it must be sought either in the central hole of the retina not being at the extremity of the axis, or in some malconformation, by which the retina is not perpendicular to the axis of the eye, at the point where they meet. This disease of the eye, which is so generally neglected, might be frequently cured, even in adults, by those who are thoroughly acquainted with the structure and functions of this organ. Accidental Colors. One of the most curious affections of the eye is that which gives rise to ocular spectra, or accidental colors. If we place a red wafer on a sheet of white paper, and, closing one eye, keep the other directed, for some time, to the centre of the wafer, then, if we turn the same eye to another part of the paper, we shall see a green wafer, the color of which will grow fainter and fainter as we continue to look at it. This green image of the wafer is called an ocular spectrum, or the accidental, or opposite color of red. By using different colored wafers, we obtain the following results: White,.....Black. Red,.......Bluish green. Orange,.....Blue. Yellow,.....Indigo. Green,.....Violet, with a little red. Blue,......Orange red. Indigo,.....Orange yellow. Violet,.....Bluish green. When a strong impression of white light is made upon the eye, a succession of remarkable spectra is visible. When the sun was near the horizon, M. iEpinus fixed his eye steadily upon it for fifteen seconds. Upon shutting his eye, he saw an irregular, pale greenishyellow image of the sun, surrounded with a faint red border. When he opened his eye, and turned it to a white ground, the image of the sun was brownishred, and its border skyblue. With his eye again shut, the image appeared green, and the border a red, different from the last. On opening his eye, and turning it to a white ground, as before, the image was more red than formerly, and the border a brighter skyblue. His eye being again shut, the image was green, approaching to skyblue, and the border a red, still differing from the former. When his eye was again opened upon a white ground, the image was still red, and its border skyblue, but with different shades from the last. At the end of four or five minutes, when his eye was shut, the image was a fine skyblue, and the border a brilliant red; and, upon opening his eye, as formerly, upon a white ground, the image was a brilliant red, and the border a fine skyblue. Experiments of a similar kind were made by doctor Brewster, by looking at a brilliant image of the sun's disk, formed by a concave mirror. With his right eye tied up, he viewed this luminous disk with the left, through a blackened tube, to prevent any extraneous light from falling upon the retina. When the retina was highly excited by this intense light, he turned his left eye to a white ground, and perceived the following spectra, by alternately opening and shutting his eye: Spectra with the left Eye open. ^eflE^'1. Pink, surrounded with ) Qreen green, $2. Orange, mixed with pink, Blue.3. Yellowish brown, .... Bluish pink.4. Yellow,..........Lighter blue.5. Pure red,........Skyblue.6. Orange,..........Indigo. when one of these spectra is visible, we press the eye to one side, the spectrum will appear to be absolutely immovable if the experiment is not made with much attention. It will be found, however^ by pressing both the eyes at once, and by due attention to their corresponding motions, that the spectrum does move, and that it is seen by the eye in the same manner as if it were the image of an external object, conformably to the law of visible direction. By means of pressure upon the eyeball, ocular spectra may be produced; and when spectra, produced by external impressions of light, are seen by the eye, their colors are changed by pressure on the eyeball. The pressure of the bloodvessels on the back of the eye often produces spectra, in particular states of the stomach. In slight affections, these spectra are floating masses of blue light, which appear and disappear in succession ; but, in severe ones, they become green, and sometimes rise to yellow: hence it follows, that pressure upon the retina creates the sensation of light and colors.Colors produced by the unequal Action of lAght upon the Eyes, If we hold a slip of white paper vertically, about a foot from the eye, and direct both eyes to an object at some distance beyond it, so as to see the slip of paper double, then, when a candle is brought near the right eye, so as to act strongly upon it, while the left eye is protected from its light, the left hand slip of paper will be of a tolerably bright green color, while the right hand slip of paper, seen by the left eye, will be of a red color. If the one image overlaps the other, the color of the overJapping parts will be white, arising from a mixture of the complementary red and green. When equal candles are held equally near each eye, each of the images of the slip of paper is white. If, when the paper is seen red and green, by holding the candle to the right eye, we quickly take it to the left eye, we shall find that the left image of the slip of paper gradually changes from green to red, and the right one from red to green, both of them having the same tint during the time in which the change is going on. This experiment seems to confirm the observation of doctor Brewster, that in certain highly excited states of one eye, the reverse impression may be conveyed from the one eye to the other.Insensibility of certain Eyes to particular Colors. Various cases have been described, in which persons capable of performing the most delicate functions of vision are unable to distinguish particular colors, and, what is certainly a most remarkable fact, this imperfection runs in families. A shoemaker by the name of Harris, at Allonby, in Cumberland, could only distinguish black and white. He was unable, when a child, to distinguish the cherries on a tree from the leaves, except by their shape and size. He had two brothers whose perception of colors was almost equally defective, one of whom constantly mistook orange for grass green, and light green for yellow. He had two other brothers and sisters, who, as well as his parents, had no such defect. Another case of a Mr. Scott is described by himself in the Philadelphia Transactions for 1778. He did not know any green color; a pink color and a pale blue were perfectly alike to him. A full red and a full green were so alike that he often thought them a good match; but yellows, light, dark and middle, and all degrees of blue, except pale skyblue, he knew perfectly well; and he could discern, with particular niceness, a deficiency in any of them. A full purple and a deep blue, however, sometimes baffled him. Mr. Scott's father, his maternal uncle, and one of his sisters, and her two sons, had all the same defect. Mr. Dugald Stewart, Mr. Dalton and Mr. Troughton are examples of the same inability to distinguish certain colors. Mr. Harvey has described, in the Edinburgh Transactions, the case of a tailor, now alive, and aged sixty, who could distinguish with certainty only White, yellow and gray. On one occasion, he repaired an article of dress with crimson in place of black silk; and, on another occasion, he patched the elbow of a blue coat with a piece of crimson cloth. He regarded indigo and prussian blue as black, purple as a modification of blue, while green puzzled him extremely. He considered carmine, lake and crimson to be blue. The solar spectrum appeared to him to consist only of yellow and light blue. None of the family of this person had the same defect. In these various cases, the persons are insensible to red light, and all the colors into which it enters. Mr. Dalton thinks it probable that the red light is, in these cases, absorbed bv the vitreous humor, which he supposes may have a blue tint. If, which is probable, the choroid coat be essential to vision, we may ascribe the loss of red light in certain eyes to the retina itself having a blue tint. If the dissection of the eye of any person who possesses this peculiarity shall not establish either of these two suppositions, we must content ourselves with supposing that the retina is insensible to the colors at the end of the spectrum, just as the ear of certain persons has been proved by doctor Wollaston to be insensible to sounds at one extremity of the scale of musical notes, while it is perfectly sensible to all other sounds. Optical Instruments. The impediments to the vision of very near objects arise from two great a divergence of the rays in each pencil incident on the eye, and are remedied by the microscope. The most powerful single microscopes are very small globules of glass, which any person may make for himself, by melting the ends of fine threads of glass in the flame of a candle; or by taking a little fine powdered glass on the point of a very small needle, and melting it into a globule before a smooth blowpipe. It was with such microscopes as these that Leuwenhoek made all his wonderful discoveries, most of which are deposited in the British museum. The double or compound microscope differs from the preceding in this respectthat it 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 eyeglass instead of the object itself, as in the single microscope. In this respect, the principle is analogous to that of the telescope, only that, as the latter is intended to view distant objects, the objectlens is of a long focus, and consequently of a moderate magnifying power, and the eyeglass of a short focus, which magnifies considerably the image made by the objectlens; whereas, the microscope being intended only for minute objects, the objectlens is consequently of a short focus, and the eyeglass, in this case, is not of so high a magnifying power. The solar microscope is a kind of camera obscura, w7hich, in a darkened chamber, throws the image on a wall or screen. It consists of two lenses fixed opposite a hole in a board or windowshutterone which condenses the light of the sun upon the object (which is placed between them), and the other which forms the image. There is also a plain reflector placed without, moved by a wheel and pinion, which may be so regulated as to throw the sun's rays upon the outer lens. Mr. Adam's most ingenious inventionthe lucernal microscopeis also to be considered as a kind of camera 6b~ scura, only the light, in this latter case, proceeds from a lamp instead of from the said on the nature of the microscope, the principle of the telescope may be easily understood. Telescopes are of two kinds ¦the one depending on the principle of refraction, and called the dioptric telescope, the other on the principle of reflection, and therefore termed the reflecting telescope. (For a further account of this instrument, see Telescope.) I?ifl.edion of Light The direction of the rays of light is changed, as we have seen, in their approach to certain bodies, by reflection and refraction; and, consequently, we must admit that there is some power in these bodies by which such effects are universally produced. If reflection was produced simply by the impinging of particles of light on hard or elastic bodies, or if they were in themselves elastic, the same effects would follow as in the impulse of other elastic bodies ; but the angle of incidence could not be equal to the :\ngle of reflection, unless the particles of light were perfectly elastic, or the bodies on which they impinged were perfectly elastic. Now, we know that the bodies on which these particles impinge are not perfectly elastic ; and also that, if the particles of light were perfectly elastic, the diffusion of light from the reflecting bodies would be very different from its present appearance; for, as no body can be perfectly polished, the particles of light, which are so inconceivably small, would be reflected back by the inequalities on the surface in every direction ; consequently we are led to this conclusionthat the reflecting bodies are possessed of a power which acts at some little distance from their surfaces. If this reasoning is allowed to be just, it necessarily follows that, if a ray of light, instead of impinging on a body, should pass so near to it as to be within the sphere of that power which the body possesses, it must necessarily suffer a change in its direction. Actual experiments confirm the truth of this position; and to the change in the direction of a particle of light, owing to its nearness to a body, we give the name of inflection. From one of these experiments, made by sir Isaac Newton, the whole of this subject will be easily understood. At the distance of two or three feet from the window of a darkened room, in which was a hole three fourths of an inch broad to admit the light, he placed a black sheet of pasteboard, having in the middle a hole about a quarter of an inch square, and behind the hole the blade passed through the hole. The planes of the pasteboard and blade were parallel to each other; and, when the pasteboard was removed at such a distance from the window as that all the light coming into the room must pass through the hole in the pasteboard, he received what came through this hole on a piece of paper, two or three feet beyond the knife, and perceived two streams of faint light shooting out both ways from the beam of light into the shadow. As the brightness of the direct rays obscured the fainter light, by making a hole in his paper, he let them pass through, and had thus an opportunity of attending closely to the two streams, which were nearly equal in length, breadth, and quantity of light. That part which was nearest to the sun's direct light wTas pretty strong for the space of about a quarter of an inch, decreasing gradually till it became imperceptible ; and, at the edge of the knife, it subtended an angle of about 12°, or at most 14°. Another knife was then placed opposite to the former, and he observed that, when the distance of their edges w7as about the £^th part of an inch, the stream divided in the middle, and left a shadow between the two parts, which was so dark that all light passing between the knives seemed to be bent aside to one knife or the other. As the knives were brought nearer to each other, this shadow grew7 broader, till, upon the contact of the knives, the whole light disappeared. Pursuing his observations upon this appearance, he perceived fringes, as they may be termed, of different colored light, three made on one side by the edge of one knife, and three on the other side by the edge of the other; and thence concluded that, as, in refraction, the rays of light are differently acted upon, so are they at a distance from bodies by inflection ; and by many other experiments of the same kind, he supported his position, which is confirmed by all subsequent experiments. We may naturally conclude that, from this property of inflection, some curious changes will be produced in the appearance of external objects. If we take a piece of wire of a less diameter than the pupil of the eye, and place it between the eye and a distant object, the latter will appear magnified ; for the rays by which the object, would have been otherwise seen are intercepted by the wire, and it is now seen by inflected rays, which make a greater angle than tne direct rays. Natural Phenomena. The most interesting of these is the rainbow, which consists of two bows, or arches, extended across the part of the sky, which is opposite to the sun. The innermost of the bows, and which is most commonly seen by itself, it being the principal rainbow, is part of a circle whose diameter is 82°, and is nothing more than an infinite number of prismatic spectra of the sun arranged in the circumference of a circle, the colors being the very same, and occupying the same space as in the spectrum produced from the sun's light. The red rays form the outermost portion, and the violet rays the innermost portion of the bow. The external or secondary bow is much fainter than the other, and has the violet outermost and the red innermost. It is part of a circle 104° in diameter. As the rainbow is never seen unless when the sun is shining, and when rain is falling between the spectator and the part of the horizon where the bow is seen, it is obvious that it depends upon the decomposition of the white light of the sun, by the refraction of the drops of rain and their subsequent reflection within the dropsan,, explanation sufficiently adequate, from the fact that rainbows are produced by the spray of waterfalls, and may be made artificially by scattering water with a brush or syringe when the sun is shining. The primary bow is the effect of one reflection and two refractions of the sun's rays by the drops of rain : the secondary one is formed by two reflections and two refractions. Within the primary rainbow, and immediately in contact with it, there have been seen what are called supeimumerai'y rainbows, each of which consists of red and green. Their origin has not been explained. Lunar rainbows have been seen ; but they differ in no respect from those formed by the solar rays, excepting in the faintness of their light. A halo is a circle, either composed of white light, or consisting of the prismatic colors, which is occasionally seen round the sun or moon. Parhelia are mock suns, which appear at places where two haloes or arches of luminous circles about the sun intersect each other. The prismatic haloes which are sometimes visible about the sun and moon, in fine weather, when white, thin, fleecy clouds are floating in the atmosphere, are called corona. Owing to the dazzling effect of the sun's rays, the haloes which surround his disk may be seen to most advantage by reflection in a pool of water. These phenomena are attributed to the crystals of ice and snow floating in the atmosphere, and, in some cases, to the ao tion of drops of rain of different sizes, The elevation of coasts, ships and mountains above their usual level, when seen in the distant horizon, has been long known and described under the name of looming. The name of mirage has been applied by the French to the same class of phenome na; and the appellation of fata morgana has been bestowed by the Italians to the singular appearances of the same kind, which have repeatedly been seen in the straits of Messina. When the rising sun throws his rays at an angle of 45° on the sea of Reggio, and neither wind nor rain ruffle the smooth surface of the water in the bay, the spectator, on an eminence in the city, who places his back to the sun and his face to the sea, observes, as it were upon its surface, numberless series of pilasters, arches and castles distinctly delineated; regular columns, lofty towers, superb palaces, with balconies and windows; extended valleys of trees, delightful plains, with herds and flocks; armies of men on foot and horseback, and many other strange figures, in their natural colors and proper actions, passing one another in rapid succession. When vapors and dense exhalations, rising to the height of about twenty feet, appear, then the same objects are seen depicted, as it were in the vapor, and suspended in the air, though with less distinctness than before. Captain Scoresby, when navigating the Northern seas, was able to recognise his father's ship when below the horizon, from the inverted image of it which appeared in the air. " It was," says he, " so well defined, that I could distinguish, by a telescope, every sail, the general rig of the ship, and its particular character, insomuch that I confidently pronounced it to be my father's ship, the Fame, which it afterwards proved to be; though, in comparing notes with my father, I found that our relative position at the time gave our distance from one another very nearly 30 miles, being about 17 miles beyond the horizon, and some leagues beyond the limit of direct vision." In the sandy plains of Egypt the mirage is seen to great advantage. These plains are often interrupted by small eminences, upon which the inhabitants have built theii villages, in order to escape the inundations of the Nile. In the morning and evening, objects are seen in their natural form and position ; but where the surface of the sandy ground is heated by the sun, the land seems terminated, at a particular distance, by a general inundation, the vil village an inverted image of it is seen. This optical deception has been noticed from the remotest times. The prophet [saiah alludes to it, when he says, " and the parched ground shall become a pool." The cause of these phenomena consists in variations in the refractive power of the atmosphere, which may be proved by actual experiment. This has been done in a variety of ways ; but we shall only mention the method adopted by doctor Brewster. He held a heated iron above a mass of water bounded by parallel plates of glass; as the heat descended slowly through the fluid, a regular variation of density, diminishing from the bottom to the surface, took place. On withdrawing the heated iron, and putting a cold body in its place, or even on allowing the air to act alone, the superficial stratum of water gave out its heat so as to produce a decrease of density from the surface to a certain depth below it. Through the medium thus constituted, the phenomenon of the mirage was observable in the finest manner.Colors of the Atmosphere. As the earth is surrounded with an atmosphere, varying in density from the surface of the globe, where it is the densest, to the height of about 45 miles, where it is extremely rare, and just able to reflect the rays of the setting sun, the rays of the sun, moon and stars are refracted into curve lines, unless when they are incident upon it perpendicularly. Hence the apparent altitude of the celestial bodies is always greater than their real altitude, and they appear above the horizon when they are actually below it. But while the solar rays traverse the earth's atmosphere, they suffer another change from the resisting medium which they encounter. When the sun, or any of the heavenly bodies, is considerably elevated above the horizon, its light is transmitted to the earth without any perceptible change; but when these bodies are near the horizon, their light must pass through a long tract of air, and is considerably modified before it reaches the eye of the observer. The momentum of the red, or greatest refrangible rays, being greater than the momentum of the violet, or least refrangible rays, the former will force their way through the resisting medium, while the latter will be either reflected or absorbed. A white beam of light will therefore be deprived of a portion of its blue rays by its horizontal passage through the atmosphere, and the resiiituig color will be either orange or their course ; hence the rich and brilliant hue with which nature is gilded by the setting sun, and hence the glowing red which tinges the morning and evening cloud. We have already seen that the red rays penetrate through the atmosphere, while the blue rays, less able to surmount the resistance which they meet, are reflected or absorbed in their passage. It is to this cause that we must ascribe the blue color of the sky, and the bright azure which tinges the mountains of the distant landscape. As we ascend in the atmosphere, the deepness of the blue tinge dies away; and to the aeronaut who has soared above the denser strata, or to the traveller who has ascended the Alps or the Andes, the sky appears of a deep black, while the blue rays find a ready passage through the attenuated strata of the atmosphere. It is owing to the same cause, that the diver at the bottom of the sea is surrounded with the red light which has pierced through the superincumbent fluid, and that the blue rays are reflected from the surface of the ocean. Were it not for the reflecting power of the air, and of the clouds which float in the lower regions of the atmosphere, we should be involved in total darkness by the setting of the sun, and all the objects around us would suffer a total eclipse by every cloud that passed over his disk. It is to the multiplied reflections which the light of the sun suffers in the atmosphere that we are indebted for the light of day, when the earth is enveloped with impenetrable clouds. From the same cause arises the sober hue of the morning and evening twilight, which increases as we recede from the equator, till it blesses with perpetual day the inhabitants of the polar regions.Colored Shadoivs. The shadows of bodies placed only in one light, and at a distance from all other bodies capable of reflecting light, must necessarily be black. In a summer morning, or evening, however, the shadows of bodies formed either by the light of the sun, or by that of a candle, have been observed to be blue: this obviously arises from the shadows being illuminated with the light of the blue sky. The colors thus produced vary in different countries, and at different seasons of the year, from a pale blue to a violet black; and when there are yellow vapors in the horizon, or yellow light reflected from the lower part of the sky, either at sunrise or at sunset, the shadows have a tinge of green, arising from the union of these accidental rays with the blue tint of the shadow. If the light of the sun or of the candle be faint, then the shadow of the body, formed by the light of the sky, will be visible also, and the two shades will be the one blue and the other a pale yellow. This fact has been ascribed to the circumstance of the light of the candle and. that of the rising and setting sun being of a yellowish tinge ; but though this will increase the effect, it is not the main cause of it, as one of the shadows would be yellow, even if the light of the sun and the candle had been perfectly white. The phenomena of colored shadows are sometimes finely seen in the interior of a room, the source of one of the colors being sometimes the blue sky, and the other the green window blinds, the painted walls, or the colored furniture.Converging and diverging Beams. When the sun is descending in the west, through masses of open clouds, the diverging of his beams, rendered visible by their passage through numerous openings, forms frequently a very beautiful phenomenon. It is sometimes accompanied with one of an opposite kind, viz. the convergency of beams to a point in the eastern horizon opposite to the sun, and as far beneath the horizon as the sun is above it, as if another sun, throwing out divergent beams, were about to rise in the east. This phenomenon is rarely seen in perfection, and has never been observed until within a few years. In order to explain it, let us suppose a line to join the eye of the observer and the sun. Let beams issue from the sun in all possible directions, and let us suppose that planes pass through these beams, and through the line joining the eye of the observer and the sun, which will be their common intersection, like the axis of an orange, or the axis of the earth, through which there pass all the septa of the former, and all the planes passing through the meridians of the latter. An eye, therefore, situated in this line, or common intersection of all the planes, will, when looking at a concave sky, apparently spherical, see them diverging from the sun on one side, and converging towards the opposite point, just as an eye in the axis of a large globe would perceive all the planes passing through the meridians diverging on one side and converging on another.