  Light is that invisible etherial matter which renders objects perceptible by the visual organs. It appears to be distributed throughout the immensity of the universe, and is essentially requisite to the enjoyment of every rank of perceptive existence. It is by the agency of this mysterious substance, that we become acquainted with the beauties and sublimities of the universe, and the wonderful operations of the Almighty Creator. Without its universal influence, an impenetrable veil would be thrown over the distant scenes of creation; the sun, the moon, the planets, and the starry orbs, would be shrouded in the deepest darkness, and the variegated surface of the globe on which we dwell, would be almost unnoticed and unknown. Creation would disappear, a mysterious gloom would surround the mind of every We have never yet known what it is to live in a world deprived of this delightful visitant; for in the darkest night we enjoy a share of its beneficial agency, and even in the deepest dungeon its influence is not altogether unfelt.1 The blind, indeed, do not directly enjoy the advantages of light, but its influence is reflected upon them, and their knowledge is promoted through the medium of those who enjoy the use of their visual organs. Were all the inhabitants of the world deprived of their eye-sight, neither knowledge nor happiness, such as we now possess, could possibly be enjoyed. There is nothing which so strikingly displays the beneficial and enlivening effects of light, as the dawn of a mild morning after a night of darkness and tempest. All appears gloom and desolation, in our terrestrial abode, till a faint light begins to whiten the eastern horizon. Every succeeding The whole of this splendid scene, which light produces, may be considered as a new creation, no less grand and beneficent than the first creation, when the command was issued, “Let there be light, and light was.” The aurora and the rising sun cause the earth and all the objects which adorn its surface, to arise out of that profound Such, then, are the important and beneficent effects of that light which every moment diffuses its blessings around us. It may justly be considered as one of the most essential substances connected with the system of the material universe, and which gives efficiency to all the other principles and arrangements of nature. Hence we are informed, in the sacred history, that light was the first production of the Almighty Creator, and the first born of created beings; for without it the As light is an element of so much importance and utility in the system of nature, so we find that arrangements have been made for its universal diffusion throughout all the worlds in the universe. The sun is one of the principal sources of light to this earth on which we dwell, and to all the other planetary bodies. And, in order that it may be equally distributed over every portion of the surfaces of these globes, to suit the exigencies of their inhabitants, they are endowed with a motion of rotation, by which every part of their surfaces is alternately turned towards the source of light; and when one hemisphere is deprived of the direct influence of the solar rays, its inhabitants derive a portion of light from luminaries in more distant regions, and have their views directed to other suns and systems dispersed, in As the science of astronomy depends solely on the influence of light upon the organ of vision, which is the most noble and extensive of all our senses; and as the construction of telescopes and other astronomical instruments is founded upon CHAPTER I.GENERAL PROPERTIES OF LIGHT.It is not my intention to discuss the subject of light in minute detail—a subject which is of considerable extent, and which would require a separate treatise to illustrate it in all its aspects and bearings. All that I propose is to offer a few illustrations of its general properties, and the laws by which it is refracted and reflected, so as to prepare the way for explaining the nature and construction of telescopes, and other optical instruments. There is no branch of natural science more deserving of our study and investigation than that which relates to light—whether we consider its beautiful and extensive effects—the magnificence and grandeur of the objects it unfolds to view—the numerous and diversified phenomena it exhibits—the optical instruments which a knowledge of its properties has enabled us to construct—or the daily advantages we derive, as social beings, from its universal diffusion. If air, which serves as the medium of sound, and the vehicle of speech, enables us to carry on an interchange of thought and affection with our fellow-men; how much But, what is the nature of this substance we call light, which thus unfolds to us the scenes of creation? On this subject two leading opinions have prevailed in the philosophical world. One of those opinions is, that the whole sphere of the universe is filled with a subtle matter, which receives from luminous bodies an agitation which is incessantly continued, and which, by its vibratory motion, enables us to perceive luminous bodies. According to this opinion, light may be considered as analogous to sound, which is conveyed to the ear by the vibratory motions of the air. This was the hypothesis of Descartes, which was adopted, with some modifications, by the celebrated Euler, Huygens, Franklin, and other philosophers, To the first hypothesis, it is objected that, if true, ‘light would not only spread itself in a direct line, but its motion would be transmitted in every direction like that of sound, and would convey the impression of luminous bodies in the regions of space beyond the obstacles that intervene to stop its progress.’ No wall or other opaque body could obstruct its course, if it undulated in every direction like sound; and it would be a necessary consequence, that we should have no night, nor any such phenomena as eclipses of the sun or moon, or of the satellites of Jupiter and Saturn. This objection has never been very satisfactorily answered. On the other hand, Euler brings forward the following objections against the Newtonian doctrine of emanation. 1. That, were the sun emitting continually, and in all directions, such floods of luminous matter with a velocity so prodigious, he must speedily be exhausted, or at least, some alteration must, after the lapse of so many ages, be perceptible. 2. That the sun is not the only body that emits rays, but that all the stars have the same quality; and as every where the rays of the sun must be crossing the rays of the stars, their collision must be violent in the extreme, and that their direction must be changed by such a collision.2 To the first of these objections it is answered—that so vast is the tenuity of light, that it utterly exceeds the power of conception: the most delicate instrument having never been certainly put in motion by the impulse of the accumulated sun-beams. It has been calculated that in the space of 385,130,000 Egyptian years, (of 360 days,) the sun would lose only the 1/1,217,420th of his bulk from the continual efflux of his light. And, therefore, if in 385 millions of years the sun’s diminution would be so extremely small, it would be altogether insensible during the comparatively short period of five or six thousand years. To the second objection it is replied—that the particles of light are so extremely rare that their distance from one another is incomparably greater than their diameters—that all objections of this kind vanish when we attend to the continuation of the impression upon the retina, and to the small number of luminous particles which are on that account necessary for producing constant vision. For it appears, from the accurate experiments of M. D’Arcy, that the impression of light upon the retina continues eight thirds, and as a particle of light would move through 26,000 miles in that time, constant vision would be maintained by a succession of luminous particles twenty-six thousand miles distant from each other. Without attempting to decide on the merits of these two hypotheses, I shall leave the reader to adopt that opinion which he may judge to be attended with the fewest difficulties, and proceed to illustrate some of the properties of light:—and in the discussion of this subject, I shall generally adhere to the terms employed by those who have adopted the hypothesis of the emanation of light. 1. Light emanates or radiates from luminous bodies in a straight line. This property is proved by the impossibility of seeing light through bent tubes, or small holes pierced in metallic plates placed one behind another, except the holes be placed in a straight line. If we endeavour to look at the sun or a candle through the bore of a bended pipe, we cannot perceive the object, nor any light proceeding from it, but through a straight pipe the object may be perceived. This is likewise evident from the form of the rays of light that penetrate a dark room, which proceed straight forward in lines proceeding from the luminous body; and from the form of the shadows which bodies project, which are bounded by right lines passing from the luminous body, and meeting the lines which terminate the interposing body. This property may be demonstrated to the eye, by causing light to pass through small holes into a dark room filled with smoke or dust. It is to be understood, however, that in this case, the rays of light are considered as passing through the same medium; for when they pass from air into water, glass, or other media, they are bent at the point where they enter a different medium, as we shall afterwards have occasion to explain. 2. Light moves with amazing velocity. The ancients believed that it was propagated from the sun and other luminous bodies instantaneously; but the observations of modern astronomers have demonstrated that this is an erroneous hypothesis, and that light, like other projectiles, occupies a certain time in passing from one part of space to another. Its velocity, however, is prodigious, and exceeds that of any other body with which we are acquainted. It flies across the earth’s orbit—a space 190 millions of miles in extent, in the We have many proofs, besides the above, that the particles of light are next to infinitely small. We find that they penetrate with facility the hardest substances, such as crystal, glass, various kinds of precious stones, and even the diamond itself, though among the hardest of stones; for such bodies could not be transparent, unless light found an easy passage through their pores. When a candle is lighted in an elevated situation, in the space of a second or two, it will fill a cubical space (if there be no interruption) of two miles around it, in every direction, with luminous particles, before the least sensible part of its substance is lost by the candle:—that is, it will in a short instant, fill a sphere four miles in diameter, twelve and a half miles in circumference, and containing thirty-three and a half cubical miles with particles of light; for an eye placed in any part of this cubical space would perceive the light emitted by the candle. It has been calculated that the number of particles of light contained in such a space cannot be less than four hundred septillions—a number which is six billions of times greater than the number of grains of sand which could be contained in the whole earth considered as a solid globe, and supposing each cubic inch of it to contain ten hundred thousand grains. Such is the inconceivable tenuity of that substance which emanates from all luminous bodies, and which gives beauty and splendour to the universe! This may also be evinced by the following experiment. Make a 3. Light is sent forth in all directions from every visible point of luminous bodies. If we hold a sheet of paper before a candle, or the sun, or any other source of light, we shall find that the paper is illuminated in whatever position we hold it, provided the light is not obstructed by its edge or by any other body. Hence, wherever a spectator is placed with regard to a luminous body, every point of that part of its surface which is toward him will be visible, when no intervening object intercepts the passage of the light. Hence, likewise, it follows, that the sun illuminates, not only an immense plane extending along the paths of the planets, from the one side of the orbit of Uranus to the other, but the whole of that sphere, or solid space, of which the distance of Uranus is the radius. The diameter of this sphere is three 4. The effect of light upon the eye is not instantaneous, but continues for a short space of time. This may be proved and illustrated by the following ‘With respect to the duration of the impression of light, it has been observed that the teeth of a cog-wheel in a clock were still visible in succession, when the velocity of rotation brought 246 teeth through a given fixed point in a second. In this case it is clear that if the impression made on the The following experiment has likewise been suggested as a proof of the impression which light makes upon the eye. If a card, on both sides of which a figure is drawn, for example, a bird and a cage, be made to revolve rapidly on the straight line which divides it symmetrically, the eye will perceive both 5. Light, though extremely minute, is supposed to have a certain degree of force or momentum. In order to prove this, the late ingenious Mr. Mitchell contrived the following experiment. He constructed a small vane in the form of a common weather-cock, of a very thin plate of copper, about an inch square, and attached to one of the finest harpsicord wires, about ten inches long, and nicely balanced at the other end of the wire, by a grain of very small shot. The instrument had also fixed to it in the middle, at right angles to the length of the wire, and in an horizontal direction, a small bit of a very slender sewing needle, about half an inch long, which was made magnetical. In this state the whole instrument might weigh about ten grains. The vane was supported in the manner of the needle in the mariner’s compass, so that it could turn with the greatest ease; and to prevent its being affected by the vibrations of the air, it was enclosed in a glass case or box. The rays of the sun were then thrown upon the broad part of the vane or copper plate, from a concave mirror of about two feet diameter, which, passing through the front glass of the box, were collected into the focus of the On the above experiment, the following calculation has been founded: If we impute the motion produced in this experiment to the impulse of the rays of light, and suppose that the instrument weighed ten grains, and acquired a velocity of one inch in a second, we shall find that the quantity of matter contained in the rays falling upon the instrument in that time amounted to no more than one twelve hundred-millionth part of a grain, the velocity of light exceeding the velocity of one inch in a second in the proportion of about 12,000,000,000 to 1. The light in this experiment was collected from a surface of about three square feet, which reflecting only about half what falls upon it, the quantity of matter contained in the rays of the sun incident upon a foot and a half of surface in one second of time, ought to be no more than the twelve hundred-millionth part of a grain. But the density of the rays of light at the surface of the sun is greater than that at the earth in the proportion of 45,000 to 1; there ought therefore to issue from one square foot of the sun’s surface in one second of time, in order to supply the waste by light 1/45,000th part of a grain of matter, that is, a little more than two grains a day, or about If the above experiment be considered as having been accurately performed, and if the calculations founded upon it be correct, it appears that there can be no grounds for apprehension that the sun can ever be sensibly diminished by the immense and incessant radiations proceeding from his body on the supposition that light is a material emanation. For the diameter of the sun is no less than 880,000 miles; and, before this diameter could be shortened, by the emission of light, one English mile, it would require three millions, one hundred and sixty-eight thousand years, at the rate now stated; and, before it could be shortened ten miles, it would require a period of above thirty-one millions of years. And although the sun were thus actually diminished, it would produce no sensible effect or derangement throughout the planetary system. We have no reason to believe that the system, in its present state and arrangements, was intended to endure for ever, and before that luminary could be so far reduced, during the revolutions of eternity, as to produce any irregularities in the system, new arrangements and modifications might be introduced by the hand of the All Wise and Omnipotent Creator. Besides, it is not improbable that a system of means is established by which the sun and all the luminaries in the universe receive back again a portion of the light which they are continually emitting, either from the planets from whose surfaces it is reflected, or from the millions of stars whose rays are continually traversing the 6. The intensity of light is diminished in proportion to the square of the distance from the luminous body. Thus, a person at two feet distance from a candle, has only the fourth part of the light he would have at one foot, at three feet distance the ninth part, at four feet the sixteenth part, at five feet the twenty fifth part, and so on for other distances. Hence the light received by the planets of the Solar system decreases in proportion to the squares of the distances of these bodies from the sun. This may be illustrated by the following figure, Figure 1. Suppose the light which flows from a point A, and passes through a square hole B, is received upon a plane C, parallel to the plane of the hole—or, let the figure C be considered as the shadow of the plane B. When the distance of C is double of B, the length and breadth of the shadow C will be each double of the length and breadth of the plane B, and treble when AD is treble of AB, and so on, which may be easily In conformity with this law, the relative quantities of light on the surfaces of the planets may be easily determined, when their distances from the sun are known. Thus, the distance of Uranus from the sun is 1,800,000,000 miles, which is about nineteen times greater than the distance of the earth from the same luminary. The square of 19 is 361; consequently the earth enjoys 361 times the intensity of light when compared with that of Uranus; in other words, this distant planet enjoys only the 1/361 part of the quantity of light which falls upon the earth. This quantity, however, is equivalent to the light we should enjoy from the combined effulgence of 348 full moons; and if the pupils of the eyes of the inhabitants of this planet be much larger than ours, and the retina of the eye be endued with a much greater degree of nervous sensibility, they may perceive objects with as great a degree of splendour 7. It is by light reflected from opake bodies that most of the objects around us are rendered visible. When a lighted candle is brought into a dark room, not only the candle but all other bodies in the room become visible. Rays of the sun passing into a dark room render luminous a sheet of paper on which they fall, and this sheet in its turn enlightens, to a certain extent, the whole apartment, and renders objects in it visible, so long as it receives the rays of the sun. In like manner, the moon and the planets are opake bodies, but the light of the sun falling upon them, and being reflected from their surfaces, renders them visible. Were no light to fall on them from the sun, or were they not endued with a power of The intensity of reflected light is very small, when compared with that which proceeds directly from luminous bodies. M. Bouguer, a French philosopher, who made a variety of experiments to ascertain the proportion of light emitted by the heavenly bodies, concluded from these experiments, that the light transmitted from the sun to As the moon and the planets are rendered visible to us only by light reflected from their surfaces, so it is in the same way that the images of most of the objects around us are conveyed to our organs of vision. We behold all the objects which compose an extensive landscape,—the hills and vales, the woods and lawns, the lakes and rivers, and the habitations of man—in consequence of the capacity with which they are endued of sending forth reflected rays to the eye, from every point of their surfaces and in all directions. In connection with the reflection of light, the following curious observation may be stated. Baron Funk, visiting some silver mines in Sweden, observed, that, 8. It is supposed by some philosophers that light is subject to the same laws of attraction that govern all other material substances—and that it is imbibed and forms a constituent part of certain bodies. This has been inferred from the phenomena of the Bolognian stone, and what are generally called the solar phosphori. The Bolognian stone was first discovered about the year 1680, by Leascariolo, a shoe-maker of Bologna. Having collected together some stones of a shining appearance at the bottom of Monte Paterno, and being in quest of some alchemical secret, he put them into a crucible to calcine them—that is, to reduce them to the state of cinders. Having taken them out of the crucible, and exposed them to the light of the sun, he afterwards happened to carry them into a dark place, when to his surprise, he observed that they possessed a self-illuminating power, and continued to emit faint rays of light for some hours afterwards. In consequence of this discovery, the Bolognian spar came into considerable demand among natural philosophers and the curious in general; and the best way of preparing it seems to have been hit upon by the family of Zagoni, who supplied all Europe with Bolognian phosphorus, till the discovery of more powerful phosphoric substances put an end to their monopoly.—In the year 1677, Baldwin, a native In 1730 M. du Fay directed his attention to this subject, and observed that all earthy substances susceptible of calcination, either by mere fire, or when assisted by the previous action of nitrous acid, possessed the property of becoming more or less luminous, when calcined and exposed for a short time in the light—that the most perfect of these phosphori were limestones, and other kinds of carbonated lime, gypsum, and particularly the topaz, and that some diamonds were also observed to be luminous by simple exposure to the sun’s rays. Sometime afterwards, Beccaria discovered that a great variety of other bodies were convertible into phosphori by exposure to the mere light of the sun, such as, organic animal remains, most compound salts, nitre and borax—all the farinaceous and oily seeds of vegetable substances, all the gums and several of the resins—the white woods and vegetable fibre, either in the form of paper or linen; also starch and loaf-sugar proved to be good phosphori, after being made thoroughly dry, and exposed to the direct rays of the sun. Certain animal substances by a similar treatment were also converted into phosphori; particularly bone, sinew, glue, hair, horn, hoof, feathers, and fish-shells. The same property was communicated to rock crystal and some other of the gems, by rubbing them against each other so as to roughen their surfaces, and then placing them for some minutes in the focus of a lens, by which the rays of light were concentrated upon In the year 1768 Mr. Canton contributed some important facts in relation to solar phosphori, and communicated a method of preparing a very powerful one, which, after the inventor, is usually called Canton’s phosphorus. He affirms that his phosphorus, enclosed in a glass flask, and hermetically sealed, retains its property of becoming luminous for at least four years, without any apparent decrease of activity. It has also been found that, if a common box smoothing-iron, heated in the usual manner, be placed for half a minute on a sheet of dry, white paper, and the paper be then exposed to the light, and afterwards examined in a dark closet, it will be found that the whole paper will be luminous, that part, however, on which the iron had stood being much more shining than the rest. From the above facts it would seem that certain bodies have the power of imbibing light and again emitting it, in certain circumstances, and that this power may remain for a considerable length of time. It is observed that the light which such bodies emit bears an analogy to that which they have imbibed. In general, the illuminated phosphorus is reddish; but when a weak light only has been admitted to it, or when it has been received through pieces of white paper, the emitted light is pale or whitish.—Mr. Morgan, in the seventy-fifth volume of the Philosophical Transactions, treats the subject of light at considerable length; and as a foundation for his reasoning, he assumes the following data:—1. That light is a body, and like all others, subject to the laws of attraction. 2. That light is a heterogeneous body; and that the same attractive power operates with different 9. Light is found to produce a remarkable effect on Plants and Flowers, and other vegetable productions. Of all the phenomena which living vegetables exhibit there are few that appear more extraordinary than the energy and constancy with which their stems incline toward the light. Most of the discous flowers follow the sun in his course. They attend him to his evening retreat, and meet his rising lustre in the morning with the same unerring law. They unfold their flowers on the approach of this luminary; they follow his course by turning on their stems, and close them as soon as he disappears. If a plant, also, is shut up in a dark room, and a small hole afterwards opened by which the light of the sun may enter, the plant will turn towards that hole, and even alter its own shape in order to get near it; so that though it was straight before, it will in time become crooked, that it may get near the light. Vegetables placed in rooms where they receive light only in one direction, always extend themselves in that direction. If they receive light in two directions, they direct their course towards that which is strongest. It is not the heat but the light of the sun which the plant thus covets; for, though a fire be kept in the room, capable of giving a much stronger heat than the sun, the plant will turn away from the fire in order to enjoy the solar light. Trees growing in thick forests, where they only receive light from above, direct their shoots almost invariably upwards, and therefore become much taller and less spreading than such as stand single. The green colour of plants is likewise found to depend on the sun’s light being allowed to shine on them; for without the influence of the solar light, they are always of a white colour. It is found by experiment that, if a plant which has It is likewise found that the perspiration of vegetables is increased or diminished, in a certain measure by the degree of light which falls upon In connection with this subject the following curious phenomenon may be stated, as related by M. Haggern, a Lecturer on Natural History in Sweden. One evening he perceived a faint flash 10. Light has been supposed to produce a certain degree of influence on the PROPAGATION OF SOUND?—M. Parolette, in a long paper in the The author then proceeds to give a description of a very delicate instrument, and various apparatus for measuring the propagation and intensity of sound, and the various experiments both in the dark, and in day-light, and likewise under different changes of the atmosphere, which were made with his apparatus—all of which tended to prove that light had a sensible influence in the propagation of sound. But the detail of these experiments and their several results would be too tedious to be here transcribed.—The night has generally been considered as more favourable than the day for the transmission of sound. ‘That this is the case (says Parolette) with respect to our ears cannot be doubted; but this argues nothing against my opinion. We hear further by night on account of the silence, and this always contributes to it, while the noise of a wind favourable to the propagation of a sound, may prevent the sound from being heard.’ In reference to the cause which produces the effect now stated, he proposes the following queries. ‘Is the atmospheric air more dense on the appearance of light than in darkness? Is this greater density of the air or of the elastic fluid that is subservient to the propagation of sound, the effect of aeriform substances kept in this state through the medium of light?’ He is disposed, on the whole, to conclude, that the effect in question is owing to the action of light upon the oxygen of the atmosphere, since oxygen gas is found by experiment to be best adapted to the transmission of sound. Our author concludes his communication with the following remarks:—‘Light has a velocity 900,000 times as rapid as that of sound. Whether it emanate from the sun and reach to our earth, or act by means of vibrations agitating the Such is a brief description of some of the leading properties of light. Of all the objects that present themselves to the philosophic and contemplative mind, light is one of the noblest and most interesting. The action it exerts on all the combinations of matter, its extreme divisibility, the rapidity of its propagation, the sublime wonders it reveals, and the office it performs in what constitutes the life of organic beings, lead us to consider it as a substance acting the first part in the economy of nature. The magic power which this emanation from the heavens exerts on our organs When the sun is said “to rule over the day,” it In the formation of light, and the beneficent effects it produces, the wisdom and goodness of the Almighty are conspicuously displayed. Without the beams of the sun and the influence of light, what were all the realms of this world, but an undistinguished chaos and so many dungeons of darkness? In vain should we roll our eyes around to behold, amidst the universal gloom, the flowery fields, the verdant plains, the flowing streams, the expansive ocean, the moon walking in brightness, the planets in their courses, or the innumerable host of stars. All would be lost to the eye of man, and the “blackness of darkness” would surround him for ever. And with how much wisdom has every thing been arranged in relation to the motion and minuteness of light? Were it capable of being transformed into a solid substance, and retain its present velocity, it would form the most dreadful and appalling element in nature, and produce universal terror and destruction throughout the universe. That this is not impossible, and could easily be effected by the hand of Omnipotence, appears from such substances as phosphorus, where light is supposed to be concentrated in a solid state. But in all its operations and effects, as it is now directed by unerring wisdom and beneficence, it exhibits itself as the most benign and delightful element connected with the constitution of the material system, diffusing splendour and felicity wherever its influence extends. CHAPTER II.ON THE REFRACTION OF LIGHT.Refraction is the turning or bending of the rays of light out of their natural course. Light, when proceeding from a luminous body—without being reflected from any opake substance or inflected by passing near one—is invariably found to proceed in straight lines without the least deviation. But if it happens to pass obliquely from one medium to another, it always leaves the direction it had before and assumes a new one. This change of direction, or bending of the rays of light, is what is called Refraction—a term which probably had its origin from the broken appearance which a staff or a long pole exhibits, when a portion of it is immersed in water—the word, derived from the Latin frango, literally signifying breaking or bending. When light is thus refracted, or has taken a new direction, it then proceeds invariably in a straight line till it meets with a different medium,7 when it is again turned out of its course. It must be observed, however, that though we may by this means cause the rays of light to make any number of angles in their course, it is impossible There are two circumstances essential to refraction. 1. That the rays of light shall pass out of one medium into another of a different density, or of a greater or less degree of resistance. 2. That they pass in an oblique direction. The denser the refracting medium, or that into which the ray enters, the greater will be its refracting power; and of two refracting mediums of the same density, that which is of an oily or inflammable nature will have a greater refracting power than the other. The nature of refraction may be more particularly explained and illustrated by the following figure and description. Let ADHI fig. 2, be a body of water, AD its surface, C a point in which a ray of light BC enters from the air into the water. This ray, by the greater density of the water, instead of passing straight forward in its first direction to K, will be bent at the point C, and pass along in the direction CE, which is called the refracted ray. Let the line FG be drawn perpendicular to the surface of the water in C, then it is evident that the ray BC, in passing out of air, a rare medium, into a dense medium, as water, is refracted into a ray CE which is nearer to the perpendicular CG than the incident ray BC, and on the contrary, the ray EC figure 2. The same thing may be otherwise illustrated as follows:—suppose a hole made in one of the sides of the vessel as at a, and a lighted candle placed within two or three feet of it, when empty, so that its flame may be at L, a ray of light proceeding from it will pass through the hole a in a straight line LBCK till it reach the bottom of the vessel at K, where it will form a small circle of light. Having put a mark at the point K, pour water into the vessel till it rise to the height AD, and the round spot that was formerly at K, will appear at E; that is, the ray which went straight forward, when the vessel was empty, to K, has been bent at the point C, where it falls into the the water, into the line CE. In this experiment it is necessary that the front of the vessel should be of glass, in order that the course of the ray may be seen; and if a little soap be mixed with the water so as to give it a little mistiness, the ray CE will be distinctly perceived. If, in place of fresh water we pour in salt water, it will be found that the ray BC is more bent at C. In like manner The angle of refraction depends on the obliquity of the rays falling on the refracting surface being always such, that the sine of the incident angle is to the sine of the refracted angle, in a given proportion. The incident angle is the angle made by a ray of light and a line drawn perpendicular to the refracting surface, at the point where the light enters the surface. The refracted angle is the angle made by the ray in the refracting medium with the same perpendicular produced. The sine of the angle is a line which serves to measure the angle, being drawn from a point in one leg perpendicular to the other. The following figure (fig. 3.) will tend to illustrate these definitions. figure 3. In this figure BC is the incident ray, CE the refracted ray, DG the perpendicular, AD the sine It may be also proper to remark, that a ray of light cannot pass out of a denser medium into a rarer, if the angle of incidence exceed a certain limit. Thus a ray of light will not pass out of glass into air, if the angle of incidence exceed 40° 11´; or out of glass into water, if the angle of incidence exceed 59° 20´. In such cases refraction will be changed into reflection. The following common experiments, which are easily performed, will illustrate the doctrine of refraction. Put a shilling or any other small object which is easily distinguished, into a bason or any other similar vessel, and then retire to such a distance as that the edge of the vessel shall just hide it from your sight. If then you cause another person to fill the vessel with water, you will then The same principle is illustrated by the following experiment. Place a bason or square box on a table, and a candle at a small distance from it; lay a small rod or stick across the sides of the bason, and mark the place where the extremity of the shadow falls, by placing a shilling or other object at the point; then let water be poured into the bason, and the shadow will then fall much nearer to the side next the candle than before. This experiment may likewise be performed by simply observing the change produced on the shadow of the side of the bason itself. Again, put a long stick obliquely into deep water, and the stick will seem to be broken at the point where it appears at the surface of the water—the part which is immersed in the water appearing to be bent upwards. Hence every one must have observed that, in rowing a boat, the ends of the oars appear bent or broken every time they are immersed in the water, and their appearance at such times is a representation of the course of the refracted rays. Again, fill a pretty deep jar with water, and you will observe the bottom of the jar considerably elevated, so that it appears much shallower than it The refraction of light explains the causes of many curious and interesting phenomena both in the heavens and on the earth. When we stand on the banks of a river, and look obliquely through the waters to its bottom, we are apt to think it is much shallower than it really is. If it be eight feet deep in reality, it will appear from the bank to be only six feet; if it be five feet and a half deep, it will appear only about four feet. This is owing to the effects of refraction, by which the bottom of the river is apparently raised by the refraction of the light passing through the water into air, so as to make the bottom appear higher than it really is, as in the experiment with the jar of water. This is a circumstance of some importance to be known and attended to in order to personal safety. For many school-boys and other young persons have lost their lives by attempting to ford a river, the bottom of which appeared to be within their reach, when they viewed it from its banks: and even adult travellers on horseback have sometimes fallen victims to this optical deception; It is likewise owing to this refractive power in water, that a skilful marksman who wishes to shoot fish under water, is obliged to take aim considerably below the fish as it appears, because it seems much nearer the top of the water than it really is. An acquaintance with this property of light is particularly useful to divers, for, in any of their movements or operations, should they aim directly at the object, they would arrive at a point considerably beyond it; whereas, by having some idea of the depth of the water, and the angle which a line drawn from the eye to the object makes with its surface, the point at the bottom of the water, between the eye and the object at which the aim is to be taken, may be easily determined. For the same reason, a person below water does not see objects distinctly. For, as the aqueous humour of the eye has the same refractive power as water, the rays of light from any object under water will undergo no refraction in passing through the cornea, and aqueous humour, and will therefore meet in a point far behind the retina. But if any person accustomed to go below water should use a pair of spectacles, consisting of two convex lenses, the radius of whose surface is three tenths of an inch—which is nearly the radius of the convexity of the cornea—he will see objects as distinctly below water as above it. It is owing to refraction, that we cannot judge so accurately of magnitudes and distances in water as in air. A fish looks considerably larger in water than when taken out of it. An object plunged vertically into water always appears contracted, and the more so as its upper extremity Refraction likewise produces an effect upon the heavenly bodies, so that their apparent positions are generally different from their real. By the refractive power of the atmosphere, the sun is seen before he comes to the horizon in the morning, and after he sinks beneath it in the evening; and hence this luminary is never seen in the place in which it really is, except when it passes the zenith at noon, to places within the torrid zone. The sun is visible, when actually thirty-two minutes of a degree below the horizon, and when the opake rotundity of the earth is interposed between our eye and that orb, just on the same principle as, in the experiment with the shilling and basin of water, the shilling was seen when the edge of the basin interposed between it and the sight. The refractive power of the atmosphere has been found to be much greater, in certain cases, than what has been now stated. In the year 1595 a company of Dutch sailors having been wrecked on the shores of Nova Zembla, and having been obliged to remain in that desolate region during a night of more than three months—beheld the sun make his appearance in the horizon about sixteen days before the time in which he The illumination of the heavens which precedes the rising of the sun, and continues sometime after he is set—or, what is commonly called the morning and evening twilight—is likewise produced by the atmospherical refraction—which circumstance forms a very pleasing and beneficial arrangement in the system of nature. It not only prolongs to us the influence of the solar light, and adds nearly two hours to the length of our day, but prevents us from being transported all at once from the darkness of midnight to the splendour of noon-day, and from the effulgence of day to the gloom and horrors of the night—which would bewilder the traveller and navigator in their journeys by sea or land, and strike the living world with terror and amazement. The following figure will illustrate the position now stated, and the manner in which the refraction figure 4. The same phenomena happen in relation to the moon, the planets, the comets, the stars, and every other celestial body, all of which appear more elevated, especially when near the horizon, than their true places. The variable and increasing refraction from the zenith to the horizon, is a source of considerable trouble and difficulty in making astronomical observations, and in nautical calculations. For, in order to determine the real altitudes of the heavenly bodies, the exact degree of refraction, at the observed elevation, must be taken into account. To the same cause we are to ascribe a phenomenon that has sometimes occurred—namely, that the moon has been seen rising totally eclipsed, while the sun was still visible in the opposite quarter of the horizon. At the middle of a total eclipse of the moon, the sun and moon are in opposition, or 180 degrees asunder; and, therefore, Extraordinary cases of refraction in relation to terrestrial objects.In consequence of the accidental condensation of certain strata of the atmosphere, some very singular effects have been produced in the apparent elevation of terrestrial objects to a position much beyond that in which they usually appear. The following instance is worthy of notice. It is taken from the Philosophical Transactions of London for 1798, and was communicated by W. Latham, Esq., F.R.S., who observed the phenomenon from Hastings, on the south coast of England:—‘On July 26, 1797, about five o’clock in the afternoon, as I was sitting in my dining-room in this place, which is situated upon the Parade, close to the sea-shore, nearly fronting the south, my attention was excited by a number of people running down to the sea-side. Upon inquiring the reason, I was informed, that the coast of France was plainly to be distinguished by the naked eye. I immediately went down to the shore, and was surprised to find that, even without the assistance of a telescope, I could very plainly see the cliffs on the opposite coast, which, at the nearest part, are between forty and fifty miles distant, and are not to be discerned from that low situation by the aid of the best glasses. They appeared to be only a few This singular phenomenon was doubtless occasioned by an extraordinary refraction produced either by an unusual expansion, or condensation of the lower strata of the atmosphere, arising from circumstances connected with the extreme heat of the season. The objects seem to have been apparently The following are likewise instances of unusual refraction:—When Captain Colby was ranging over the coast of Caithness, with the telescope of his great Theodolite, on the 21st of June, 1819, at eight o’clock, P.M. from Corryhabbie Hill, near Mortlich, in Banffshire, he observed a brig over the land of Caithness, sailing to the westward in the Pentland Frith, between the Dunnet and Duncansby heads. Having satisfied himself as to the fact, he requested his assistants, Lieutenants Robe and Dawson, to look through the telescope, which they immediately did, and observed the brig likewise. It was very distinctly visible for several minutes, while the party continued to look at it, and to satisfy themselves as to its position. The brig could not have been less than from ninety to one hundred miles distant; and, as the station on Corryhabbie is not above 850 yards above the sea, the phenomenon is interesting. The thermometer was at 44°. The night and day preceding the sight of the brig had been continually rainy and misty, and it was not till 7 o’clock of the evening of the 21st that the clouds cleared off the hill.8 Captain Scoresby relates a singular phenomenon of this kind, which occurred while he was traversing the Polar seas. His ship had been separated by the ice from that of his father for a considerable Mrs. Somerville states, that a friend of her’s, while standing on the plains of Hindostan, saw the whole upper chain of the Himalaya mountains start into view, from a sudden change in the density of the air, occasioned by a heavy shower, after a long course of dry and hot weather. In looking at distant objects through a telescope, over the top of a ridge of hills, about two miles distant, I have several times observed, that some of the more distant objects which are sometimes hid by the interposition of a ridge of hills, are, at other times, distinctly visible above them. I have sometimes observed, that objects near the middle of the field of view of a telescope, which was in a fixed position, have suddenly appeared to descend to the lower part, or ascend to the upper part of the field, while the telescope remained unaltered. I have likewise seen, with a powerful telescope, the Bell Rock Lighthouse, at the distance of about twenty miles, to appear as if contracted to less than two-thirds Such are some of the striking effects produced by the refraction of light. It enables us to see objects in a direction where they are not; it raises, apparently, the bottoms of lakes and rivers: it magnifies objects when their light passes through dense mediums: it makes the sun appear above the horizon, when he is actually below it, and thus increases the length of our day: it produces the Aurora and the evening twilight, which forms, in many instances, the most delightful part of a summer day: it prevents us from being involved in total darkness, the moment after the sun has descended beneath the horizon: it modifies the appearances of the celestial bodies, and the directions in which they are beheld: it tinges the sun, moon, and stars, as well as the clouds, with a ruddy hue, when near the horizon: it elevates the appearance of terrestrial objects, and, in certain extraordinary cases, brings them nearer to our view, and enables us to behold them when beyond the line of our visible horizon. In combination with the power of reflection, it creates visionary landscapes, and a variety of grotesque and extraordinary appearances, which delight and astonish, and sometimes appal the beholders. In short,—as we shall afterwards see more particularly—the refraction of light through glasses of different figures, forms the principle on which telescopes and microscopes are constructed, by which both the remote and the minute wonders of creation have been disclosed to view. So that had there been no bodies In the operation of the law of refraction in these and numerous other instances, we have a specimen of the diversified and beneficent effects which the Almighty can produce by the agency of a single principle in nature. By the influence of the simple law of gravitation, the planets are retained in their orbits, the moon directed in her course around the earth, and the whole of the bodies connected with the sun preserved in one harmonious system. By the same law the mountains of our globe rest on a solid basis, the rivers flow through the plains toward the seas, the ocean is confined to its prescribed boundaries, and the inhabitants of the earth are retained to its surface and prevented from flying upwards through the voids of space. In like manner the law by which light is refracted produces a variety of beneficial effects essential to the present constitution of our world and the comfort of its inhabitants. When a ray of light enters obliquely into the atmosphere, instead of passing directly through, it bends a little downwards, so that the greater portion of the rays which thus enter the atmospheric mass, descend by inflection to the earth. We then enjoy the benefit of that light which would otherwise have been totally lost. We perceive the light of day an hour before the solar orb makes its appearance, and a portion of its light is still retained The refraction of light by the atmosphere, combined with its power of reflecting it, is likewise the cause of that universal light and splendour which appears on all the objects around us. Were the earth disrobed of its atmosphere, and exposed naked to the solar beams—in this case, we might see the sun without having day, strictly so called. His rising would not be preceded by any twilight as it now is. The most intense darkness would cover us till the very moment of his rising; he would then suddenly break out from under the horizon with the same splendour he would exhibit at the highest part of his course, and would not change his brightness till the very moment of his setting, when in an instant all would be black as the darkest night. At noon day we should see the sun like an intensely brilliant globe shining in a sky as black as ebony, like a clear fire in the night seen in the midst of an extensive field, and his rays would show us the adjacent objects immediately around us; but the rays which fall on the objects remote from us would be for ever lost in the expanse of the heavens. Instead of the beautiful azure of the sky, and the colours which distinguish the face of nature by day, we should see nothing but an abyss of darkness, and the stars shining from a vault as dark as chaos. Thus there would be no day, such The effect of refraction, in respect to terrestrial objects, is likewise of a beneficial nature. The quantity of this refraction is estimated by Dr. Maskelyne at one-tenth of the distance of the object observed, expressed in degrees of a great circle. Hence, if the distance be 10,000 fathoms, its tenth part 1000 fathoms, is the sixtieth part of a degree, or one minute, which is the refraction in altitude. Le Gendre estimates it at one fourteenth; De Lambre at one eleventh; and others at a twelfth of the distance; but it must be supposed to vary at different times and places according to the varying state of the atmosphere. This refraction, as it makes objects appear to be raised higher than they really are, enlarges the extent of our landscapes, and enables us to perceive distant objects which would otherwise have been invisible. It is particularly useful to the navigator at sea. It is one important object of the mariner when traversing his course, to look out for capes and headlands, rocks and islands, so as to descry them as soon as they are within the reach of his eye. Now, by means of refraction, the tops of hills and the elevated parts of coasts, are apparently raised into the air, so that they may be discovered several leagues further off on the sea than they would be, did no such refractive power exist. This circumstance is therefore a considerable benefit to the science of navigation, in enabling the mariner to steer his course aright, and to give him the most In short, the effects produced by the refraction and reflection of light on the scenery connected with our globe, teach us that these principles, in the hand of the Almighty, might be so modified and directed, as to produce the most picturesque, the most glorious and wonderful phenomena, such as mortal eyes have never yet seen, and of which human imagination can form no conception; and in other worlds, more resplendent and magnificent than ours, such scenes may be fully realized, in combination with the operation of physical principles and agents, with which we are at present unacquainted. From what we already know of the effects of the reflection and the refraction of light, it is not beyond the bounds of probability to suppose, that in certain regions of the universe, light may be reflected and refracted through different mediums, in such a manner, as to present to the view of their inhabitants the prominent scenes connected with distant systems and worlds, and to an extent, as shall infinitely surpass the effects produced by our most powerful telescopes. CHAPTER III.ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANSPARENT SUBSTANCES, OR LENSES.It is to the refraction of light that we are indebted for the use of lenses or artificial glasses to aid the powers of vision. It lays the foundation of telescopes, microscopes, camera obscuras, phantasmagorias, and other optical instruments, by which so many beautiful, useful, and wonderful effects have been produced. In order therefore to illustrate the principles on which such instruments are constructed, it is necessary to explain the manner in which the rays of light are refracted and modified, when passing through spherical mediums of different forms. I do not intend however to enter into the minutiÆ of this subject, nor into any abstract mathematical demonstrations, but shall simply offer a few explanations of general principles, and several experimental illustrations, which may enable the general reader to understand the construction of the optical instruments to be afterwards described. A lens is a transparent substance of a different density from the surrounding medium, and terminating in two surfaces, either both spherical, or one figure 5.
The axis of a lens is a straight line drawn through the center of its spherical surface; and as the spherical sides of every lens are arches of circles the axis of the lens would pass through the centre of that circle of which its sides are segments. Rays are those emanations of light which proceed from a luminous body, or from a body that is illuminated. The Radiant is that body or object which emits the rays of light—whether it be a self-luminous body, or one that only reflects the rays of light. Rays may proceed from a Radiant in different directions. They may be either parallel, converging, or diverging. Parallel rays are those which proceed equally distant from each other through their whole course. Rays proceeding from the sun, the planets, the stars, and distant terrestrial objects are considered as parallel, as in fig. 6. Converging rays are such as, proceeding from a body, approach nearer and nearer in their progress, tending to a certain point where they all unite. Thus, the rays proceeding
Fig. 9, shows the effects of parallel rays, KA, DE, LB, falling on a convex glass AB. The rays which fall near the extremities at A and B, are bent or refracted towards CF, the focus, and centre of convexity. It will be observed, that they are less refracted as they approach the center of the lens, and the central ray DEC, which is called the axis of the lens, and which passes through its center, suffers no refraction. Fig. 10, If the lens A B, fig. 9, on which parallel rays are represented as falling, were a plano-convex, as represented at A, fig, 5, the rays would converge to a point P, at double the radius, or the The effects of concave lenses are directly opposite to those of convex. Parallel rays, striking one of those glasses, instead of converging towards a point, are made to diverge. Rays already divergent are rendered more so, and convergent rays are made less convergent. Hence objects seen through concave glasses appear considerably smaller and more distant than they really are. The following diagram, fig. 12, represents the course of parallel rays through a double concave lens, where the parallel rays T A, D E, I B, &c., when passing through the concave glass A B, diverge into the rays G L, E C, H P, &c., as if they proceeded from F, a point before the lens, which is the principal focus of the lens. figure 12. The principal focal distance E F, is the same as in convex lenses. Concave glasses are used to correct the imperfect vision of short-sighted persons. To find the focal distance of a concave glass. Take a piece of paste-board or card paper, and cut a round hole in it, not larger than the diameter of the lens; and, on another piece of paste-board, describe a circle whose diameter is just double the diameter of the hole. Then apply the piece with the hole in it to the lens, and hold them in the sun-beams, with the other piece at such a distance behind, that the light proceeding from the hole may spread or diverge so as precisely to fill the circle; then the distance of the circle from the lens is equal to its virtual focus, or to its radius, if it be a double concave, and to its diameter, if a plano-concave. Let d, e, (fig. 12,) represent the diameter of the hole, and g, i, the diameter of the circle, then the distance C, I, is the virtual focus of the lens.9 The meniscus represented at E, fig. 5, is like the crystal of a common watch, and as the convexity is the same as the concavity, it neither magnifies Of the IMAGES formed by convex lenses.It is a remarkable circumstance, and which would naturally excite admiration, were it not so common and well known, that when the rays of light from any object are refracted through a convex lens, they paint a distinct and accurate picture of the object before it, in all its colours, shades, and proportions. Previous to experience, we could have had no conception that light, when passing through such substances, and converging to a point, could have produced so admirable an effect,—an effect on which the construction and utility of all our optical instruments depend. The following figure will illustrate this position. Let L, N, represent a double convex lens, A, C, figure 13. It is obvious, from the figure, that the image of the object is formed in the focus of the lens, in an inverted position. It must necessarily be in this position, as the rays cross at C, the centre of the lens; and as it is impossible that the rays from the upper part of the object O, can be carried by refraction to the upper end of the image at M. This is a universal principle in relation to convex lenses of every description, and requires to be attended to in the construction and use of all kinds of telescopes and microscopes. It is easily illustrated by experiment. Take a convex lens of eight, twelve, or fifteen inches focal distance, such as a reading glass, or the glass belonging to a pair of spectacles, and holding it, at its focal distance from a white wall, in a line with a burning candle, the flame of the candle will be seen depicted on the wall in an inverted position, or turned upside down. The same experiment may be performed with a window-sash, or any other bright object. But, the most beautiful exhibition of the images of objects formed by convex lenses, is made The following principles in relation to images formed by convex lenses may be stated. 1. That the image subtends the same angle at the centre of the glass as the object itself does. Were an eye placed at C, the centre of the lens LN, fig. 13, it would see the object OB, and the image IM under the same optical angle, or, in other words, they would appear equally large. For, whenever right lines intersect each other, as OI and BM, the opposite angles are always equal, that is, the angle MCI is equal to the angle OCB. 2. The length of the image formed by a convex lens, is to the length of the object, as the distance of the image is to the distance of the object from the lens: that is, MI is to OB :: as Ca to CA. Suppose the distance of the object CA from the lens, to be forty-eight inches, the length of the object OB = sixteen inches, and the distance of the image from the lens, six inches, then the length of the image will be found by the following proportion, 48 : 16 :: 6 : 2, that is, the length of the image, in such a case, is two inches. 3. If the object be at an infinite distance, the image will be formed exactly in the focus. 4. If the object be at the same distance from the lens as its focus, the image Reflections deduced from the preceding subject.Such are some of the leading principles which require to be recognised in the construction of refracting telescopes, microscopes, and other dioptric instruments whose performance chiefly depends on the refraction of light.—It is worthy of particular notice that all the phenomena of optical lenses now described, depend upon that Without this property of the rays of light we should likewise have wanted the use of the microscope—an instrument which has disclosed a world invisible to common eyes, and has opened to our view the most astonishing exhibitions of Divine mechanism, and of the wisdom and intelligence of the Eternal Mind. We should have been ignorant of those tribes of living beings, invisible to the unassisted eye, which are found in water, vinegar, and many other fluids—many of which are twenty thousand times smaller than the least visible point, and yet display the same admirable skill and contrivance in their construction, as are manifested in the formation of the larger animals. We should never have beheld the purple tide of life, and even the globules of the blood rolling In this instance, as well as in many others, we behold a specimen of the admirable and diversified effects, which the Creator can produce from the agency of a single principle in nature. By means of optical instruments, we are now enabled to take a more minute and expansive view of the amazing operations of nature, both in heaven and on earth, than former generations could have surmised. These views tend to raise our conceptions of the attributes of that Almighty Being, who presides over all the arrangements of the material system, and to present them to our contemplation in a new, a more elevated, and expansive point of view. There is, therefore, a connection which may be traced between the apparently accidental principle of the rays of light forming images of objects, and the comprehensive views we are now enabled to CHAPTER IV.ON THE REFLECTION OF LIGHT.The reflection of the rays of light is that property by which—after approaching the surfaces of bodies, they are thrown back, or repelled. It is in consequence of this property that all the objects around us, and all the diversified landscapes on our globe, are rendered visible. It is by light reflected from their surfaces that we perceive the planetary bodies and their satellites, the belts of Jupiter, the rings of Saturn, the various objects which diversify the surface of the Moon, and all the bodies in the universe which have no light of their own. When the rays of light fall upon rough and uneven surfaces, they are reflected very irregularly and scattered in all directions, in consequence of which thousands of eyes, at the same time, may perceive the same objects, in all their peculiar colours, aspects, and relations. But, when they fall upon certain smooth and polished surfaces, they are reflected with regularity, and according to certain laws. Such surfaces, when highly polished, are called Mirrors or Speculums; and it is to the reflection of light from such surfaces, and the Mirrors or Specula, may be distinguished into three kinds, plane, concave, and convex, according as they are bounded by plane or spherical surfaces. These are made either of metal or of glass, and have their surfaces highly polished for the purpose of reflecting the greatest number of rays. Those made of glass are foliated or quicksilvered on one side; and the metallic specula are generally formed of a composition of different metallic substances, which, when accurately polished, is found to reflect the greatest quantity of light. I shall, in the first place, illustrate the phenomena of reflection produced by plane-mirrors. When light impinges, or falls, upon a polished flat surface, rather more than the half of it is reflected, or thrown back in a direction similar to that of its approach; that is to say, if it fall perpendicularly on the polished surface, it will be perpendicularly reflected; but if it fall obliquely, it will be reflected with the same obliquity. Hence, the following fundamental law, regarding the reflection of light, has been deduced both from experiment and mathematical demonstration, namely, that the angle of reflection is, in all cases, exactly equal to the angle of incidence. This is a law which is universal in all cases of reflection, whether it be from plane or spherical surfaces, or whether these surfaces be concave or convex, and which requires to be recognized in the construction of all instruments which depend on the reflection of the rays of light. The following figure (fig. 14) will illustrate the position now stated. Let AB represent a plane mirror, and CD a line or ray of light perpendicular to it. Let FD represent the incident ray from any object, then figure 14. In viewing objects by reflection we see them in a different direction from that in which they really are, namely, along the line in which the rays come to us last. Thus, if AB (fig. 15) represent a plane mirror, the image of an object C appears to the eye at E behind the mirror, in the direction EG, and always in the intersection G of the perpendicular CG, and the reflected ray EG—and consequently at G as far behind the mirror, as the object C is before it. We therefore see the image in the line EG, the direction in which the reflected rays proceed. A plane mirror does not alter the figure or size of objects; but the whole image is figure 15. Mr. Walker illustrates the manner in which we see our faces in a mirror by the following figure (16). AB represents a mirror, and OC, a person looking into it. If we conceive a ray proceeding from the forehead CE, it will be sent to the eye at O, agreeably to the angle of incidence and reflection. But the mind puts CEO into one line, and the forehead is seen at H, as if the lines CEO had turned on a hinge at E.—It seems a wonderful faculty of the mind to put the two oblique lines CE and OE into one straight line OH, yet it is seen every time we look at a mirror. For the ray has really travelled from C to E, and from E to O, and it is that journey which determines the distance of the object; and hence we see ourselves as far beyond the mirror as we stand from it. Though a ray is here taken only from one part of the face, it may be easily figure 16. In every plain mirror, the image is always equal to the object, at what distance soever it may be placed; and as the mirror is only at half the distance of the image from the eye, it will completely receive an image of twice its own length. Hence a man six feet high may view himself completely in a looking glass of three feet in length, and half his own breadth; and this will be the case at whatever distance he may stand from the glass. Thus, the man AC (fig. 17) will see the whole of his own image in the glass AB, which is but one half as large as himself. The rays from the head pass to the mirror in the line Aa, perpendicular to the mirror, and are returned to the eye in the same line; consequently, having travelled twice the length Aa, the man must see his head at B. From his feet C rays will be sent to the bottom of the mirror at B; these will be reflected at an equal angle to the eye in the direction BA, as if they had proceeded in the direction DbA, so that the figure 17. A person when looking into a mirror, will always see his own image as far beyond the mirror as he is before it, and as he moves to or from it, the image will, at the same time, move towards or from him on the other side; but apparently with a double velocity, because the two motions are equal and contrary. In like manner, if while the spectator is at rest, an object be in motion, its image behind the mirror will be seen to move at the same time. And if the spectator moves, the images of objects that are at rest will appear to approach, or recede from him, after the same manner as when he moves towards real objects; plane mirrors reflecting not only the object, but the distance also, and that exactly in its natural dimensions—The following principle is sufficient for explaining most of the phenomena seen in a plane mirror, namely;—That the image of an object seen in a plane mirror, is always in a perpendicular to the mirror joining the object and the image, and that the image is as much on one side the mirror, as the object is on the other. Reflection by Convex and Concave Mirrors.Both convex and concave mirrors are formed of portions of a sphere. A convex speculum is ground and polished in a concave dish or tool which is a portion of a sphere, and a concave speculum is ground upon a convex tool. The inner surface of a sphere brings parallel rays to a focus at one fourth of its diameter, as represented in the following figure, where C is the centre of the sphere on which the concave speculum AB is formed, and F the focus where parallel rays from a distant object would be united, after reflection, that is, at one half the radius, or one fourth of the diameter from the surface of the speculum. Were a speculum of this kind presented to the sun, F figure 18. Properties of Convex Mirrors.From a convex surface, parallel rays when reflected are made to diverge; convergent rays are reflected less convergent; and divergent rays are rendered more divergent. It is the nature of all convex mirrors and surfaces to scatter or disperse the rays of light, and in every instance to impede their convergence. The following figure shows the course of parallel rays as reflected from a convex mirror. AEB is the convex surface of the mirror; and KA, IE, LB, parallel rays falling upon it. These rays, when they strike the mirror, are made to diverge in the direction AG, BH, &c. and both the parallel and divergent rays are here represented as they appear in a dark chamber, when a convex mirror is presented to the solar rays. The dotted lines denote only the course or tendency of the reflected rays, towards the virtual focus F, were they not intercepted by the mirror. This virtual focus is just equal to half the radius CE. figure 19. The following are some of the properties of convex mirrors: 1. The image appears always erect, and behind the reflecting surface. 2. The image is always smaller than the object, and the diminution is greater in proportion as the object is further from the mirror, but if the object touch the mirror, the image at the point of contact is of the same size as the object. 3. The image does not appear so far behind the reflecting surface as in a plain mirror. 4. The image of a straight object, placed either parallel or oblique to the mirror is seen curved in the mirror; because the different points of the object are not all at an equal distance from the surface of the mirror. 5. Concave mirrors have a real focus where an image is actually formed; but convex specula have only a virtual focus, and this focus is behind the mirror; no image of any object being formed before it. The following are some of the purposes to which convex mirrors are applied. They are frequently employed by painters for reducing the proportions of the objects they wish to represent, as the images of objects diminish in proportion to the smallness of the radius of convexity, and to Properties of Concave speculums.Concave specula have properties very different from those which are convex; they are of more importance in the construction of reflecting telescopes and other optical instruments; and therefore require more minute description and illustration. Concave mirrors cause parallel rays to converge; they increase the convergence of rays that are already converging; they diminish the divergence of diverging rays; and, in some cases, render them parallel and even convergent; which effects are all in proportion to the concavity of the mirror. The following figures show the course of diverging and parallel rays as reflected from concave mirrors. Fig. 20 represents the course of parallel rays, and AB, the concave mirror on which they fall. In this case, they are reflected so as to unite at F, which point is distant from its surface one fourth of the diameter of the sphere of the mirror. This point is called the focus of parallel rays, or the true focus of the mirror. And, since the sun figure 20. figure 21. When converging rays fall upon a concave mirror, they are reflected more converging and unite at a point between the focus of parallel rays and the mirror; that is, nearer the mirror than one half the radius; and their precise degree of convergency will be greater than that wherein they converged before reflection. Of the images formed by Concave Mirrors.If rays proceeding from a distant object fall upon a concave speculum, they will paint an image or representation of the object on its focus before the mirror. This image will be inverted, because the rays cross at the points where the image is formed. We have already seen that a convex glass forms an image of an object behind it; the rays of light figure 22. If we suppose a real object placed at IM, then OB will represent its magnified image, which will be larger than the object, in proportion to its distance from the mirror. This may be experimentally illustrated by a concave mirror and a candle. Suppose a concave mirror whose focal distance is five inches, and that a candle is placed before it, at a little beyond its focus, (as at IM)—suppose at five and a half inches,—and that a wall or white screen receives the image, at the distance of five feet six inches from the mirror, an image of the candle will be formed on the wall which will be twelve times longer and broader than the candle itself. In this way concave mirrors may be made to magnify the images of objects to an indefinite extent. This experiment is an exact counterpart of what is effected in similar circumstances by a convex lens, as described p. 74; the mirror performing the same thing by reflection, as the lens did by refraction. From what has been stated in relation to concave mirrors it will be easily understood how they make such powerful burning-glasses. Suppose the focal distance of a concave mirror to be twelve inches, and its diameter or breadth twelve inches. When the sun’s rays fall on such a mirror, they Were we desirous of forming an image by a concave speculum which shall be exactly equal to the object, the object must be placed exactly in the centre; and, by an experiment of this kind, the centre of the concavity of a mirror may be found. In the cases now stated, the images of objects are all formed in the front of the mirror, or between it and the object. But there is a case in which the image is formed behind the mirror. This happens when the object is placed between the mirror and the focus of parallel rays, and then the image is larger than the object. In fig. 23, GF is a concave mirror, whose focus of parallel rays is at E. If an object OB be placed a little within this focus, as at A, a large image IM will be seen behind the mirror, somewhat curved and erect, which will be seen by an eye looking directly into the front of the mirror. Here the image appears at a greater distance behind the mirror than the object is before it, and the object appears magnified in proportion to its distance from the focus figure 23. figure 24. If we suppose the side T U to represent a convex mirror, and the figure D Q a head of an ordinary size, then the figure A will represent the diminished appearance which a person’s face exhibits, when viewed in such a mirror. It will not only appear reduced, but somewhat distorted; because from the form of the mirror, one part of the object is nearer to it than another, and consequently will be reflected under a different angle. The effect we have now mentioned as produced by concave mirrors, will only take place when the eye is nearer the mirror than its principal focus. If the spectator retire beyond this focus—suppose to the distance of five or six feet, he will not see the image behind the mirror; but he will see his image in a diminished form, hanging upside down, and suspended in the air, in a line between his figure 25. In consequence of the properties of concave mirrors, now described, many curious experiments and optical deceptions have been exhibited. The appearance of images in the air, suspended between the mirror and the object, have sometimes been displayed with such dexterity and an air of mystery, as to have struck with astonishment those who were ignorant of the cause. In this way birds, flying angels, spectres and other objects have been exhibited, and when the hand attempts to lay hold on them, it finds them to be nothing, and they seem to vanish into air. An apple or a beautiful flower is presented, and when a spectator attempts to touch it, it instantly vanishes, and a death’s head immediately appears, and seems to snap at his fingers. A person with a drawn sword appears before him, in an attitude as if about to run him through, or one terrific phantom starts up after another, or sometimes the resemblances of deceased persons are made to appear, as if, by the art of conjuration, they had been forced to return from the world of spirits. In all such exhibitions, a very large concave mirror is requisite, a brilliant light must be thrown upon the objects, and every figure 26. figure 27. figure 28. There is another experiment, made with a concave mirror, which has somewhat puzzled philosophers to account for the phenomena. Take a glass bottle AC, (fig. 27) and fill it with water to the point B; leave the upper part BC empty, and cork it in the common manner. Place this bottle opposite a concave mirror, and beyond its focus, that it may appear reversed, and, before the mirror place yourself still further distant from the bottle, and it will appear in the situation A B C. Now, it The following experiments are stated by Mr. Ferguson in his “Lectures on select Subjects,” &c. Many other illustrations of the effects of concave specula might have been given, but I shall conclude this department by briefly stating some of the general properties of speculums. 1. There is a great resemblance between the properties of convex lenses and concave mirrors. They both form an inverted focal image of any remote object, by the convergence of the pencil of rays. In those instruments whose performances are the effects of reflection, as reflecting telescopes, the concave mirror is substituted in the place of the convex lens. The whole effect of these instruments, in bringing to view remote objects in heaven and on earth, entirely depends on the property of a concave mirror in forming images of objects in its focus. 2. The image of an object placed beyond the centre, is less than the object; if the object be placed between the principal focus and the centre, the image is greater than the object. In both cases the image is inverted. 3. When the object is placed between the focus and the Quantity of light reflected by polished surfaces.As this is a circumstance connected with the construction of reflecting telescopes, it may not be Using a smooth piece of glass, one line in thickness, he found that, when it was placed at an angle of fifteen degrees with the incident rays, it reflected 628 parts of 1000 which fell upon it; at the same time, a metallic mirror which he tried in the same circumstances, reflected only 561 of them. At a less angle of incidence much more light was reflected; so that at an angle of three degrees, the glass reflected 700 parts, and the metal something
With regard to such mirrors as the specula of reflecting telescopes, it will be found, in general, that they reflect little more than the one half of the rays which fall upon them. Uncommon appearances in nature produced by the combined influences of Reflection and Refraction.The reflection and refraction of the rays of light frequently produce phenomena which astonish the beholders, and which have been regarded by the ignorant and the superstitious, as the effects of supernatural agency. Of these phenomena I shall state a few examples. One of the most striking appearances of this kind is what has been termed the Fata Morgana, or optical appearances of figures in the sea and the air, as seen in the Faro of Messina. The following account is translated from a work of Minasi, who witnessed the phenomenon, and wrote a dissertation It is somewhat difficult to account for all the appearances here described; but, in all probability, they are produced by a calm sea, and one or more strata of superincumbent air differing in refractive and consequently in reflective power. At any rate reflection and refraction are some of the essential causes which operate in the production of the phenomena. The Mirage, seen in the deserts of Africa, is a phenomenon, in all probability produced by a similar cause. M. Monge, who accompanied the French army to Egypt, relates that, when in the desert between Alexandria and Cairo, the mirage of the blue sky was inverted, and so mingled with the sand below, as to give to the desolate and arid wilderness an appearance of the most rich and beautiful country. They saw, in all directions, green islands, surrounded with extensive lakes of pure, transparent water. Nothing could be conceived more lovely and picturesque than the landscape. In the tranquil surface of the lakes, the trees and houses with which the islands were covered, were strongly reflected with vivid and varied hues, and the party hastened forward to enjoy the cool refreshments of shade and stream which these populous villages proffered to them. When they arrived, the lake on whose bosom they floated, the trees among whose foliage they were embowered, and the people who stood on the shore inviting their approach, had all vanished, and nothing remained but an uniform and irksome desert of sand and sky, with a few naked huts and ragged Arabs. Had they not been undeceived by their nearer approach, there was not a man in the French army who would not have sworn that the visionary trees and lakes had a real existence in the midst of the desert. Dr. Clark observed precisely the same appearances at Rosetta. The city seemed surrounded with a beautiful sheet of water; and so certain was his Greek interpreter—who was unacquainted with the country—of this fact, that he was quite indignant at an Arab who attempted to explain to him that it was a mere optical delusion. At length they reached Rosetta in about two hours, without meeting with any water; and on looking back on the sand they had just crossed, it seemed to them as if they had waded through a vast blue lake. figure 29. On the 1st of August, 1798, Dr. Vince observed at Ramsgate a ship which appeared as at A, (fig. 29.) the topmast being the only part of it that was seen above the horizon. An inverted image of it was seen at B, immediately above the real ship A, and an erect image at C, both of them The following phenomenon, of a description nearly related to the above, has been supposed to be chiefly owing to reflection. On the 18th of November, 1804, Dr. Buchan, when watching the rising sun, about a mile to the east of Brighton, just as the solar disk emerged from the surface of the water, saw the face of the cliff on which he was standing, a windmill, his own figure and the figure of his friend, distinctly represented, precisely opposite, at some distance from the ocean. This appearance lasted about ten minutes, The following appearance most probably arose chiefly from the refraction of the atmosphere. It was beheld at Ramsgate, by Dr. Vince of Cambridge and another gentleman. It is well known that the four turrets of Dover castle are seen at Ramsgate, over a hill which intervenes between a full prospect of the whole. On the 2nd of August, 1806, not only were the four turrets visible, but the castle itself appeared as though situated on that side of the hill nearest Ramsgate, and so striking was the appearance, that for a long time the Doctor thought it an illusion; but at last, by accurate observation, was convinced that it was an actual image of the castle. He, with another individual, observed it attentively for twenty minutes, but were prevented by rain from making further observations. Between the observers and the land from which the hill rises, there were about six miles of sea, and from thence to the top of the hill there was about the same distance, their own height above the surface of the water was about seventy feet.—The cause of this phenomenon was, undoubtedly, unequal refraction. The air being more dense near the ground and above the sea than at greater heights, reached the eye of the observer, not in straight but in curvilinear lines. If the rays from the castle had in their path struck an eye at a much greater distance than Ramsgate, the probability is, that the image of the castle would have been inverted in the air; but in the To similar causes as those now alluded to are to be attributed such phenomena as the following: The Spectre of the Brocken. This is a wonderful and, at first sight, a terrific phenomenon, which is sometimes seen from the summit of one of the Hartz mountains in Hanover, which is about 3,300 feet above the level of the sea, and overlooks all the country fifteen miles round. From this mountain the most gigantic and terrific spectres have been seen, which have terrified the credulous, and gratified the curious, in a very high degree. M. HawÉ who witnessed this phenomenon, says, the sun rose about four o’clock, after he had ascended to the summit, in a serene sky, free of clouds; and about a quarter past five, when looking round to see if the sky continued clear, he suddenly beheld at a little distance, a human figure of a monstrous size turned towards him, and glaring at him. While gazing on this gigantic spectre, with a mixture of awe and apprehension, a sudden gust of wind nearly carried off his hat, and he clapt his hand to his head to detain it, when to his great delight, the colossal spectre did the same. He changed his body into a variety of attitudes,all which the spectre exactly imitated, and then suddenly vanished without any apparent cause, and, in a short time as suddenly appeared. Being joined by another spectator, after the first visions had disappeared, they kept steadily looking for the aËrial spectres, when two gigantic monsters suddenly appeared. These spectres had been long considered as preternatural, by the inhabitants of the adjacent districts, and the whole country had been filled with awe and terror. Some of the lakes of Ireland are found to be susceptible There can be little doubt that most of those visionary appearances which have been frequently seen in the sky and in mountainous regions, are phantoms produced by the cause to which I am adverting, such as armies of footmen and horsemen, which some have asserted to have been seen in the air near the horizon. A well authenticated instance of this kind occurred in the Highlands of Scotland:—Mr. Wren of Wetton Hall, and D. Stricket his servant, in the year 1744, were sitting at the door of the house in a summer evening, when they were surprised to see opposite to them on the side of Sonterfell hill—a place so extremely steep, that scarce a horse could walk slowly along it—the figure of a man with a dog pursuing several horses, all running at a most rapid pace. Onwards they passed till at last they disappeared at the lower end of the Fell. In expectation of finding the man dashed to pieces by so tremendous a fall, they went early next morning and made a search, but no trace of man or horse, or the prints of their feet on the turf could be found. Sometime afterwards, about seven in the evening, on the same spot, they beheld a troop of horsemen advancing in close ranks and at a brisk I shall mention only another instance of this description which lately occurred in France, and for a time caused a powerful sensation among all ranks. On Sunday the 17th of December, 1826, the clergy in the parish of MignÉ, in the vicinity of Poictiers, were engaged in the exercises of the Jubilee which preceded the festival of Christmas, and a number of persons to the amount of 3000 souls assisted in the service. They had planted as part of the ceremony, a large cross, twenty-five feet high, and painted red, in the open air beside the church. While one of the preachers, about five in the evening, was addressing the multitude, he reminded them of the miraculous cross which appeared in the sky to Constantine and his army, and the effect it produced—when suddenly a similar celestial cross appeared in the heavens just Such phenomena as we have now described, and the causes of them which science is able to unfold, The late ingenious Dr. Wollaston illustrated the causes of some of the phenomena we have described, in the following manner. He looked along the side of a red hot poker at a word or object ten or twelve feet distant; and at a distance less than three eights of an inch from the line of the poker, an inverted image was seen, and within and without that image, an erect image, in consequence of the change produced, by the heat of the poker, in the density of the air. He also suggested the following experiment as another illustration of the same principle, namely, viewing an object through a stratum of spirit of wine lying figure 30. It is not unlikely that phenomena of a new and different description from any we have hitherto observed, may be produced from the same causes to which we have adverted. A certain optical writer remarks—‘If the variation of the refractive power of the air takes place in a horizontal line perpendicular to the line of vision, that is, from right to left, then we may have a lateral Mirage, that is, an image of a ship may be seen on the right or left hand of the real ship, or on both, if the variation of refractive power is the same on each side of the line of vision, and a fact of this kind was once observed on the Lake of Geneva. If there should happen at the same time, both a vertical and a lateral variation of refractive power in the air, and if the variation should be such as to expand or elongate the object in both directions, then the object would be magnified as if seen through a telescope, and might be seen and recognized at a distance at which it would not otherwise have been visible. If the refracting power, on the contrary, varied, so as to construct the object in both directions, the image of it would be diminished as if seen through a concave lens.’ Remarks and Reflections, in reference to the phenomena described above.Such, then, are some of the striking and interesting effects produced by the refraction and the reflection of the rays of light. As the formation of the images of objects by convex lenses, lays the foundation of the construction of refracting telescopes and microscopes, and of all the discoveries they have brought to light, so the property of concave specula, in forming similar The facts we have stated above, and the variety of modes by which light may be refracted and reflected by different substances in nature, lead us to form some conceptions of the magnificent and diversified scenes which light may produce in other systems and worlds, under the arrangements of the all-wise and Beneficent Creator. Light, in all its modifications and varieties of colour and reflection, may be considered as the beauty and glory of the universe, and the source of unnumbered enjoyments to all its inhabitants. It is a symbol of the Divinity himself; for “God is In the arrangements connected with the planet Saturn, and the immense rings with which it is encompassed, and in the various positions which its satellites daily assume with regard to one another, to the planet itself, and to these rings—there is, in all probability, a combination of refractions, reflections, light, and shadows, which produce scenes wonderfully diversified, and surpassing in grandeur what we can now distinctly conceive. In the remote regions of the heavens, there are certain bodies composed of immense masses of luminous matter, not yet formed into any regular system, and which are known by the name of NebulÆ. What should hinder us from supposing that certain exterior portions of those masses form speculums of enormous size, as some parts of our atmosphere are sometimes found to do? Such specula may be conceived to be hundreds and even thousands of miles in diameter, and that they may form images of the most distant objects in the heavens, on a scale of immense magnitude and extent, and which may be reflected, in all their grandeur, to the eyes of intelligences at a vast distance. And, if the organs of vision of such beings, be far superior to ours in acuteness and penetrating power, they may thus be enabled to take a survey of an immense sphere of vision, and to descry magnificent objects at distances the most remote from the sphere they occupy. Whatever grounds there may be for such suppositions, it must be admitted, that all the knowledge we have hitherto acquired respecting the operation of light, and the splendid effects it is capable of producing, is small indeed, and limited to a narrow circle, compared with the immensity of its range, the infinite modifications it may undergo, and the wondrous scenes it may create in regions of creation CHAPTER V.SECT. I.—ON THE COLOURS OF LIGHT.We have hitherto considered light chiefly as a simple homogeneous substance, as if all its rays were white, and as if they were all refracted in the same manner by the different lenses on which they fall. Investigations however, into the nature of this wonderful fluid, have demonstrated that this is not the case, and that it is possessed of certain additional properties, of the utmost importance in the system of nature. Had every ray of light been a pure white, and incapable of being separated into any other colours, the scene of the universe would have exhibited a very different aspect from what we now behold. One uniform hue would have appeared over the whole face of nature, and one object could scarcely have been distinguished from another. The different shades of verdure which now diversify every landscape, the brilliant colouring of the flowery fields, and almost all the beauties and sublimities which adorn this lower creation would have been withdrawn. But it is now ascertained that every ray of white light is composed of an assemblage of colours, whence proceed that infinite variety of Many strange opinions and hypotheses were entertained respecting colours, by the ancients, and even by many modern writers, prior to the time of Sir Isaac Newton. The Pythagoreans called colour the superficies of bodies; Plato said that it was a flame issuing from them. According to Zeno it is the first configuration of matter, and according to Aristotle, it is that which moves bodies actually transparent. Among the moderns, Des Cartes imagined that the difference of colour proceeds from the prevalence of the direct or rotatory motions of the particles of light. Grimaldi, Dechales, and others, thought the differences of colour depended upon the quick or slow vibrations of a certain elastic medium filling the whole universe. Rohault imagined that the different colours were made by the rays of light entering the eye at different angles with respect to the optic axis; and Dr. Hook conceived that colour is caused by the sensation of the oblique or uneven pulse of light; and this being capable of no more than two varieties, he concluded that there could be no more than two primary colours. Such were some of the crude opinions which prevailed Before proceeding to describe the experiments by which the above results were obtained, it may be proper to give some idea of the form and effects of the Prism by which such experiments are made. This instrument is triangular and straight, and generally about three or four inches long. It is commonly made of white glass, as free as possible from veins and bubbles, and other similar defects, and is solid throughout. Its lateral faces, or sides, should be perfectly plane and of a fine polish. The angle formed by the two faces, one receiving the ray of light that is refracted in the instrument, and the other affording it an issue on its returning into the air, is called the refracting angle of the prism, as ACB, (fig. 31.) The manner in which Newton performed his experiments, and established the discovery to which we have alluded, is as follows. In the window-shutter EG, (fig. 31.) of a dark room, a hole F, was made, of about one third of an inch diameter, and behind it was placed a glass prism ACB, so that the beam of light, SF, proceeding directly from the sun was made to pass through the prism. Before the interposition of the prism, the beam proceeded in a straight line towards T, where it formed a round white spot; but being now bent out of its course by the prism, it formed an oblong image OP, upon the white pasteboard, or screen LM, containing the seven colours marked in the figure—the red being the least, and the violet the most refracted from the original direction of the solar beam, ST. This oblong image is called the prismatic spectrum. If the refracting angle of the prism ACB, be 64 figure 31. By making a hole in the screen LM opposite any one of the colours of the spectrum, so as to allow that colour alone to pass—and by letting the colour thus separated fall upon a second prism—Newton found that the light of each of the colours was alike refrangible, because the second prism could not separate them into an oblong image, or into any other colour. Hence he From what has been now stated, it is evident that, in proportion as any part of an optic glass bears a resemblance to the form of a prism, the component rays that pass through it must be necessarily separated, and will consequently paint or tinge the object with colours. The edges of every convex lens approach to this form, and it is on this account that the extremities of objects when viewed through them are found to be tinged figure 32. figure 33. This may be illustrated by experiment in the following manner. Take a card or slip of white The quantity of dispersion of the coloured rays in convex lenses depends upon the focal length of the glass; the space which the coloured images occupy being about the twenty-eighth part. Thus if the lens be twenty-eight inches focal distance, the space between Rr and Vv (fig 32) will be about one inch; if it be twenty-eight feet focus, the same space will be about one foot, and so on in proportion. Now, when such a succession of images formed by the different coloured rays, is viewed through an eye-glass, it will seem to form but one image, and consequently very indistinct, and tinged with various colours, and as the red figure Rr is largest, or seen under the greatest angle—the extreme parts of this confused image will be red, and a succession of the prismatic colours will be formed within this red fringe, as is generally found in common refracting-telescopes, constructed with a single object-glass. It is owing to this circumstance that the common refracting telescope cannot be much improved without having recourse to lenses of a very long focal distance; and hence, about 150 years ago, such telescopes were constructed of 80, and 100, and 120 feet in length. But still the image was not formed so It was originally remarked by Newton, and the fact has since been confirmed by the experiments of Sir W. Herschel, that the different-coloured rays have not the same illuminating power. The violet rays appear to have the least illuminating effect; the indigo more, and the effect increases in the order of the colours,—the green being very great; between the green and the yellow the greatest of all; the yellow the same as the green; but the red less than the yellow. Herschel also endeavoured to determine whether the power of the differently-coloured rays to heat bodies, varied with their power to illuminate them. He introduced a beam of light into a dark room, which was decomposed by a prism, and then exposed a very sensible thermometer to all the rays in succession, and observed the heights to which it rose in a given time. He found that their heating power increased from the violet to the red. The mercury in the thermometer rose higher when its bulb was placed in the Indigo than when it was placed in the violet, still higher in blue, and highest of all at red. Upon placing the bulb of the thermometer below the red, quite out of the spectrum, he was surprised to find that the mercury rose highest of all; and concluded that rays proceed from the sun, which have the power of HEATING, M. Ritter of Jena, Wollaston, Beckman and others, have found that the rays of the spectrum are possessed of certain chemical properties—that beyond the least brilliant extremity, namely, a little beyond the violet ray, there are invisible rays which act chemically, while they have neither the power of heating nor illuminating bodies. Muriate of silver exposed to the action of the red rays becomes blackish; a greater effect is produced by the yellow: a still greater by the violet, and the greatest of all by the invisible rays beyond the violet. When phosphorus is exposed to the action of the invisible rays beyond the red, it emits white fumes; but the invisible rays beyond the violet extinguish them. The influence of these rays is daily seen in the change produced upon vegetable colours, which fade, when frequently exposed to the direct influence of the sum. What object they are destined to accomplish in the general economy of nature, is not yet distinctly known; It has likewise been lately discovered that certain rays of the spectrum, particularly the violet, possesses the property of communicating the magnetic power. Dr. Morichini, of Rome, appears to have been the first who found that the violet rays of the spectrum had this property. The result of his experiments, however, was involved in doubt, till it was established by a series of experiments instituted by Mrs. Somerville, whose name is so well known in the scientific world. This lady having covered half of a sewing-needle, about an inch long, with paper, she exposed the other half for two hours, to the violet rays. The needle had then acquired North polarity. The indigo rays produced nearly the same effect; and the blue and green rays produced it in a still less degree. In the yellow, orange, red and invisible rays, no magnetic influence was exhibited, even though the experiment was continued for three successive days. The same effects were produced by enclosing the needle in blue or green glass, or wrapping it in blue and green ribbands one half of the needle being always covered with paper. One of the most curious discoveries of modern times, in reference to the solar spectrum, is that of Fraunhofer of Munich—one of the most distinguished artists and opticians on the Continent.13 From what has been stated in reference to the solar spectrum it will evidently appear, that white light is nothing else than a compound of all the prismatic colours; and this may be still farther illustrated by shewing, that the seven primary colours, when again put together, recompose white light. This may be rudely proved for the purpose of illustration, by mixing together seven different powders, having the colours and proportion of the spectrum; but the best mode, on the whole, is the following. Let two circles be drawn on a smooth round board, covered with white paper, as in fig. 34: Let the outermost be divided into 360 equal parts; then draw seven right lines as A,B,C, &c., from the center to the outermost circle, making the lines A and B include 80 degrees of that circle. The lines B and C, 40 degrees; C and D, 60; D and E, 60; E and F, 48; F and G, 27; G and A, 45. Then between these two circles paint the space AG red, inclining figure 34. That all the colours of light, when blended together in their proper proportions, produce a pure white is rendered certain by the following experiment. Take a large convex glass, and place it SECT. 2.—ON THE COLOURS OF NATURAL OBJECTS.From what has been stated above we may learn the true cause of those diversified hues exhibited by natural and artificial objects, and the variegated colouring which appears on the face of nature. It is owing to the surfaces of bodies being disposed to reflect one colour rather than another. When this disposition is such that the body reflects every kind of ray, in the mixed state in which it receives them, that body appears white to us—which, properly speaking, is no colour, but rather the assemblage of all colours. If the body has a fitness to reflect one sort of rays more abundantly than others, by absorbing all the others, it will appear of the colour belonging to that species of rays. Thus, the grass is green, because it absorbs all the rays except the green. It is these green rays only which the grass, the trees, the shrubs, and all the other verdant parts of the landscape reflect to our sight, and which make them appear green. In the same manner the different flowers reflect their respective colours; the rose, the red rays; the violet, the blue; the jonquil, the yellow; the marigold, the It is evident, then, that all the various assemblages of colours which we see in the objects around us, are not in the bodies themselves, but in the light which falls upon them. There is no colour inherent in the grass, the trees, the fruits, and the flowers, nor even in the most splendid and variegated dress that adorns a lady. All such objects are as destitute of colour, in themselves, as bodies which are placed in the centre of the earth, or as the chaotic materials out of which our globe was formed, before light was created. For where there is no light, there is no colour. Every object is black, or without colour, in the dark, and it only appears coloured as soon as light renders it visible. This is further evident from the following experiment. If we place a coloured The doctrine we are now illustrating is one which a great many persons, especially among the fair sex, find it difficult to admit. They cannot conceive it possible that there is no colour really inherent in their splendid attire, and no tints of beauty in their countenances. ‘What,’ said a certain lady, ‘are there no colours in my shawl, and in the ribbons that adorn my head-dress—and, are we all as black as negroes in the dark; I should almost shudder to think of it.’ Such persons, however, need be in no alarm at the idea; but may console themselves with the reflection, that, when they are stripped of all their coloured ornaments in the dark, they are certain that they Take a pint of common spirit, and pour it into a soup dish, and then set it on fire; as it begins to blaze, throw a handful of salt into the burning spirit, and keep stirring it with a spoon. Several handfuls may thus be successively thrown in, and then the spectators, standing around the flame, will see each other frightfully changed, their colours being altered into a ghastly blackness, in consequence of the nature of the light which falls upon them—which produces colours very different from those of the solar light. The following experiment, as described by Sir D. Brewster, illustrates the same principle. ‘Having obtained the means of illuminating any apartment with yellow light, let the exhibition be made in a room with furniture of various bright colours, and with oil or water coloured paintings on the wall. The party which is to witness the experiment should be dressed in a diversity of the gayest colours; and the brightest coloured flowers, and highly coloured drawings should be placed on the tables. The room being at first lighted with ordinary lights, the bright and gay colours of every thing that it contains will be finely displayed. If the white lights are now suddenly extinguished, and the yellow lamps lighted, the most appalling metamorphosis will be exhibited. The astonished individuals will no longer be able to recognise each other. All the furniture of the room, and all the objects it contains, will exhibit only one colour. The flowers ——Like the unnatural hue Which autumn paints upon the perished leaf, From such experiments as these we might conclude, that were the solar rays of a very different description from what they are now found to be, the colours which embellish the face of nature, and the whole scene of our sublunary creation would assume a new aspect, and appear very different from what we now behold around us in every landscape. We find that the stars display great diversity of colour; which is doubtless owing to the different kinds of light which are emitted from those bodies; and hence we may conclude, that the colouring thrown upon the various objects of the universe is different in every different system, and that thus, along with other arrangements, an infinite variety of colouring and of scenery is distributed throughout the immensity of creation. The atmosphere, in consequence of its different refractive and reflective powers, is the source of a variety of colours which frequently embellish and When a direct spectrum is thrown on colours darker than itself, it mixes with them: as the yellow spectrum of the setting sun, thrown on the green grass, becomes a greener yellow. But when a direct spectrum is thrown on colours brighter than itself, it becomes instantly changed into the reverse spectrum, which mixes with those brighter colours. Thus the yellow spectrum of the setting sun thrown on the luminous sky, becomes blue, and changes with the colour or brightness of the clouds on which it appears. The red part of light being capable of struggling through thick and resisting mediums which intercept all other colours—is likewise the cause why the sun appears red when seen through a fog,—why distant light, though transmitted through blue or green glass, appears red—why lamps at a distance, seen through the smoke of a long street, are red, while those that are near, are white. To the same cause it is owing that a 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. Hence, Dr. Halley informs us that, when he was in a diving bell, at the bottom of the sea, his hand always appeared red in the water. The blue rays, as already noticed, being unable to resist the obstructions they meet with in their course through the atmosphere, are either reflected or absorbed in their passage. It is to this cause, that most philosophers ascribe the blue colour of the sky, the faintness and obscurity of distant objects, and the bright azure which tinges the mountains of a distant landscape. SECT. 3.—PHENOMENA OF THE RAINBOW.Since the rays of light are found to be decomposed by refracting surfaces, and reflected in an infinite variety of modes and shades of colour, we need not be surprised at the changes produced in any scene or object by the intervention of another, and by the numerous modifications of which the primary colours of nature are susceptible. The vivid colours which gild the rising and the setting sun, must necessarily differ from those which adorn its noon-day splendour. Variety of atmospheric scenery will thus necessarily be produced, greater than the most lively fancy can well imagine. The clouds will sometimes assume the most fantastic forms, and at other times will be irradiated with beams of light, or, covered with the darkest hues, will assume a lowering aspect, prognostive of the thunder’s roar and the lightning’s flash—all in accordance with the different rays that are reflected to our eyes, or the quantity absorbed by the vapours which float in the atmosphere. Light, which embellishes with so much magnificence a pure and serene sky, by means of innumerable bright starry orbs which are spread over it, sometimes, in a dark and cloudy sky, exhibits an ornament which, by its pomp, splendour and variety of colours, attracts the attention of every eye that has an opportunity of beholding it. At certain times, when there is a shower either around us, or at a distance from us in an opposite quarter to that of the sun, a species of arch or bow is seen in the sky, adorned with all the seven primary colours of light. This phenomenon, As the full elucidation of this subject involves a variety of optical and mathematical investigations, We have just now alluded to an experiment with a glass globe:—If, then, we take either a solid glass globe, or a hollow globe filled with water, and suspend it so high in the solar rays above the eye, that the spectator, with his back to the sun, can see the globe red;—if it be lowered slowly, he will see it orange, then yellow, then green, then blue, then indigo, and then violet; so that the drop at different heights, shall present to the eye the seven primitive colours in succession. In this case, the globe, from its form, will act in some measure like a prism, and the ray will be separated into its component parts. The following figure will more particularly illustrate this point. Suppose A (fig. 35.) to represent a drop of rain—which may be considered as a globe of glass in miniature, and will produce the same effect on the rays of light—and let Sd represent a ray from the sun falling upon the upper part of the drop at D. At the point of entering the drop, it will suffer a refraction, and instead of going forward to C, it will be bent to N. From N a part of the light will be reflected to Q—some part of it will, of course, pass through the drop. By the obliquity with which it falls on the side of the drop at Q, that part becomes a kind of prism, and separates the ray into its primitive colours. It is found by computation that, after a ray has suffered two refractions and one reflection, as here represented, the least refrangible part of it, namely the red ray, will make an angle with the incident solar ray of 42° 2´, as Sfq; and the figure 35. It appears, then, that the first or primary bow is formed by two refractions and one reflection; but there is frequently a second bow, on the outside of the other, which is considerably fainter. This is produced by drops of rain above the drop we have supposed at A. If B (fig. 35.) represent one of these drops, the ray to be sent to the eye enters the drop near the bottom, and suffers two refractions and two reflections, by which means the colours become reversed, that is, the violet is lowest in the exterior bow, and the red is lowest in the interior one, and the other colours are reversed accordingly. The ray T is refracted at R: a part of it is reflected from S to T, and at T it suffers another reflection from T to U. At the points S and T part of the ray passes through the drop on account of its transparency, towards W and X, and therefore we say that part only of the ray is reflected. By these losses and reflections the exterior bow becomes faint and ill-defined in comparison of the interior or primary bow. In this case the upper part of the secondary bow will not be seen when the sun is above 54° 10´ above the horizon; and the lower part of the bow will not be seen when the sun is 60° 58´ above the horizon. figure 36. For the further illustrations of this subject, we may introduce the following section of a bow, (fig. 36.) and, in order to prevent confusion in attempting to represent all the different colours—let us suppose only three drops of rain, and three different colours, as shown in the figure. The spectator O being in the centre of the two bows, here represented,—the planes of which must be considered as perpendicular to his view—the drops A,B, and C produce part of the interior bow by two refractions and one reflection as stated above, The rainbow assumes a semicircular appearance, because it is only at certain angles that the refracted rays are visible to our eyes, as is evident from the experiment of the glass globe formerly alluded to, which will refract the rays only in a certain position. We have already stated that the red rays make an angle of 42° 2´, and the violet an angle of 40° 17´. Now, if a line be drawn horizontally from the spectator’s eye, it is evident that angles formed with this line, of a certain dimension, in every direction, will produce a circle, as will appear by attaching a cord of a given length to a certain point, round which it may turn as round its axis; and, in every point will describe an angle with the horizontal line of a certain and determinate extent. Sometimes it happens that three or more bows are visible, though with different degrees of distinctness. I have more than once observed this phenomenon, particularly in Edinburgh, in the month of August, 1825, when three rainbows If there were no ground to intercept the rain and the view of the observer, the rainbow would form a complete circle, the centre of which is diametrically opposite to the sun. Such circles are sometimes seen in the spray of the sea or of a cascade, or from the tops of lofty mountains, when the showers happen in the vales below. Rainbows of various descriptions are frequently observed rising amidst the spray and exhalations of waterfalls, and among the waves of the sea whose tops are blown by the wind into small drops. There is one regularly seen, when the sun is shining, and the spectator in a proper position, at the fall of Staubbach, in the bosom of the Alps; one near Schaffhausen; one at the cascade of Lauffen; and one at the cataract of Niagara in North America. A still more beautiful one is said to be seen at Terni, where the whole current of the river Velino, rushing from a steep precipice of nearly 200 feet high, presents to the spectator below, a variegated circle, over-arching the fall, and two other bows suddenly reflected on the right and left. Don Ulloa, in the account of his journeys in South America, relates that circular rainbows are frequently seen on the mountains above Quito in Peru. It is said that a rainbow was once seen near London, caused by the exhalations of that city, after the sun had been below the horizon more than twenty minutes.14 A naval The following is a summary view of the principal facts which have been ascertained respecting the rainbow:—1. The rainbow can only be seen when it rains, and in that point of the heavens which is opposite to the sun. 2. Both the primary and secondary bows are variegated with all the prismatic colours—the red being the highest colour in the primary, or brightest bow, and the violet the highest in the exterior. 3. The primary rainbow can never be a greater arc than a semicircle; The appearance of the rainbow may be produced by artificial means, at any time when the sun is shining and not too highly elevated above the horizon. This is effected by means of artificial fountains or Jet d’eaus, which are intended to throw up streams of water to a great height. These streams, when they spread very wide, and blend together in their upper parts, form, when falling, a shower of artificial rain. If, then, when the fountain is playing, we move between it and the sun, at a proper distance from the fountain, till our shadow point directly towards it, and look at the shower,—we shall observe the colours of the rainbow, strong and vivid; and, what is particularly worthy of notice, the bow appears, notwithstanding the nearness of the shower, to be as large, and as far off, as the rainbow which we see in a natural shower of rain. The same experiment may be made by candle-light, and with any instrument that will form an artificial shower. Lunar Rainbows.—A lunar bow is sometimes formed at night by the rays of the moon striking on a rain-cloud, especially when she is about the full. But such a phenomenon is very rare. Aristotle is said to have considered himself the first who had seen a lunar rainbow. For more than a hundred years prior to the middle of the last century, The rainbow is an object which has engaged universal attention, and its beautiful colours and form have excited universal admiration. The poets have embellished their writings with many beautiful allusions to this splendid meteor; and the playful school-boy, while viewing the ‘bright enchantment,’ has frequently run ‘to catch the falling glory.’ When its arch rests on the opposite sides of a narrow valley, or on the summits of two adjacent mountains, its appearance is both beautiful and grand. In all probability, its figure first suggested the idea of arches, which are now found of so much utility in forming aqueducts and bridges, and for adorning the architecture of palaces and temples. It is scarcely possible seriously to contemplate this splendid phenomenon, without feeling admiration and gratitude towards that wise and beneficent Being, whose hands have bent it into so graceful and majestic a form, and decked it with all the pride of colours. “Look upon the rainbow,” says the son of Sirach,15 and praise Him that made it: very beautiful it is in the brightness thereof. It compasseth the heaven about with a glorious circle, and the hands of the Most High have bended it." To this grand etherial bow, the inspired writers frequently allude as one of the emblems of the majesty and splendour of the Almighty. In the prophecies of Ezekiel, the throne of Deity is represented as adorned with a brightness “like the appearance of the bow that is in the cloud in the day of rain—the appearance of the likeness of the glory of Jehovah.” And, in the visions recorded in the Book of the Revelations, where the Most High is represented as sitting upon a throne; “there was a rainbow round about the throne, in sight like ——On the broad sky is seen “A dewy cloud, and in the cloud a bow Conspicuous, with seven listed colours gay Betokening peace with God and covenant new.— He gives a promise never to destroy The earth again by flood, nor let the sea Surpass his bounds, nor rain to drown the world.” SECT. 4.—REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS.Colour is one of the properties of light which constitutes, chiefly, the beauty and sublimity of the universe. It is colour, in all its diversified shades, which presents to our view that almost infinite variety of aspect which appears on the scene of nature, which gives delight to the eye and the imagination, and which adds a fresh pleasure to every new landscape we behold. Every flower which decks our fields and gardens is compounded of different hues; every plain is covered with shrubs and trees of different degrees of verdure; and almost every mountain is clothed with herbs and grass of different shade from those which appear on the hills and landscape with which it is surrounded. In the country, during summer, nature is every day, and almost every hour, varying her appearance, by the multitude and variety of her hues and decorations, so that the eye wanders with pleasure over objects continually diversified, and extending as far as the sight can reach. In the flowers with which every landscape is adorned, what a lovely assemblage of colours, and what a wonderful art in the disposition of their shades! Here, a light pencil seems to have laid on the delicate tints; there, they are blended according to the nicest rules of art. Although green is the general colour which prevails over the scene of sublunary nature, yet it is diversified by a thousand different shades, so that every species of tree, To the same cause, likewise, are to be ascribed those beautiful and diversified appearances, which frequently adorn the face of the sky,—the yellow, orange and ruby hues which embellish the firmament at the rising of the sun, and when he is about to descend below the western horizon; and those aerial landscapes, so frequently beheld in tropical climes, where rivers, castles and mountains, are depicted rolling over each other along the circle of the horizon. The clouds, especially in some countries, reflect almost every colour in nature. Sometimes they wear the modest blush of the rose; sometimes they appear like stripes of deep vermillion, and sometimes as large brilliant masses tinged with various hues; now they are white as ivory, and now as yellow as native gold. In some Let us now consider for a moment, what would be the aspect of nature, if, instead of the beautiful variety of embellishments which now appear on every landscape, and on the concave of the sky,—one uniform colour had been thrown over the scenery of the universe. Let us conceive the whole of terrestrial nature to be covered with snow, so that not an object on earth should appear with any other hue, and that the vast expanse of the firmament presented precisely the same uniform aspect. What would be the consequence? The light of the sun would be strongly reflected The vault of heaven, too, would wear a uniform aspect. Neither planets nor comets would be visible to any eye, nor those millions of stars which now shine forth with so much brilliancy, and diversify the nocturnal sky. For, it is by the contrast produced by the deep azure of the heavens and the white radiance of the stars, that those bodies are rendered visible. Were they depicted Such would have been the aspect of creation, and the inconveniences to which we should have been subjected, had the Creator afforded us light without that intermixture of colours which now appears over all nature, and which serves to discriminate one object from another. Even our In short, without colour, we could have had no books nor writings: we could neither have corresponded with our friends by letters, nor have known any thing with certainty, of the events which happened in former ages. No written revelation of the will of God, and of his character, such as we now enjoy, could have been handed down to us from remote periods and generations. The discoveries of science, and the improvements of art, would have remained unrecorded. Universal ignorance would have prevailed throughout the world, and the human mind have remained in a state of demoralization and debasement. All these, and many other inconveniences and evils would have inevitably followed, had not God painted the rays of light with a diversity of colours, And hence we may learn, that the most important scenes and events in the universe, may depend upon the existence of a single principle in nature, and even upon the most minute circumstances, which we may be apt to overlook, in the arrangements of the material world. In the existing state of things in the visible creation, we cannot but admire the Wisdom and Beneficence of the Deity, in thus enabling us to distinguish objects by so easy and expeditious a mode as that of colour, which in a moment, discriminates every object and its several relations. We rise in the morning to our respective employments, and our food, our drink, our tools, our books, and whatever is requisite for our comfort, are at once discriminated. Without the least hesitation or uncertainty, and without any perplexing process of reasoning, we can lay our hands on whatever articles we require. Colour clothes every object Since colours, then, are of so much value and importance, they may be reckoned as holding a rank among the noblest natural gifts of the Creator. As they are of such essential service to the inhabitants of our globe, there can be no doubt that they serve similar or analogous purposes On the whole, the subject of colours, when seriously considered, is calculated to excite us to the adoration of the goodness and intelligence of that Almighty Being whose wisdom planned all | ||||||||||||||||||||||||||||||||||
|
