B sure of the column of fluid A B; and the similar pressure at C is the excess of the atmospheric pressure above the vertical pressure of the column of fluid B C; but the latter excess is evidently the greater, and hence the liquid in the vessel is necessarily forced upwards through the tube from C to B; and thus the vessel is drained of its contents. By placing a stopcock on the tube above A, the stream can be checked, and permitted to flow at pleasure. There are instances of towns being supplied with water by means of large sy Fig. 26. phons of this kind. In these cases the syphon is brought over a rising ground from a lake or fountain at some distance. Certain kinds of springs are accounted for on the principle of the syphon; they act from the combined effects of a vacuum and atmospheric pressure. APPLICATION OF HEAT TO WATER. The pressure of the atmosphere affects the boiling of water. At the common pressure of about 15 lbs. to the square inch, water will boil, or attain the vaporific point, at 212 degrees Fahrenheit. If we remove the atmospheric pressure by an air-pump, as is done in the boiling of sugar, we can produce the phenomenon of boiling at a much lower temperature. At the summit of Mont Blanc, where the atmospheric pressure is light, water is found to boil at 187 degrees. Steam produced from boiling water is a transparent, colourless, and invisible substance, like air. If we could look into the boiler of a steam-engine, we should see nothing but the water in a state of ebullition. The | white cloudy-looking matter which is emitted in the form of vapour, is moisture produced by the partial condensation of the steam in the atmosphere-taking the form of vapour is a step towards becoming liquid again. A cubic inch of water produces exactly a cubic foot, or 1728 cubic inches, of steam, at 212 degrees of temperature; in other words, when water is transformed into steam, it occupies 1728 times its former bulk. In this expanded condition steam is of a less specific gravity than air. Its density is expressed by 0-625, that of air being 1. The elastic force of steam in the process of heating -that is, the force with which it seeks to expand-differs at different temperatures. At first the force is inconsiderable, but it rapidly increases as the temperature is raised. At a temperature of 212 degrees, the elastic force is 15 lbs. on the square inch of the containing vessel, or equal to the external pressure of the atmosphere; at 250 degrees, it is 30 lbs.; at 272 degrees, it is 45 lbs.; and at 290 degrees, it is 66 lbs. BUOYANT PROPERTY OF AERIFORM FLUIDS. The atmosphere, as has been stated, possesses the property of buoying up bodies which, bulk for bulk, are fighter than itself. The law governing buoyancy in liquids is precisely the same as that governing buoyancy in aëriform fluids, and may here be repeated in reference to air. 1st, Any solid body immersed in a fluid displaces exactly its own bulk of fluid, and the force with which the body is buoyed up is equal to the weight of the fluid which is displaced. This refers to bodies of less density than air. 2d, Any solid body of a greater density than air, when wholly immersed in that fluid, loses exactly as much of its weight as the weight of an equal bulk of air-that is, of the air which it displaces. The support afforded to bodies in the atmospheric fluid by its resistance is very evident from many appearances in nature, as the support of vapours or clouds, the rising of smoke and fine particles of dust, and the flying of birds; in art, it is exemplified by the flying of a boy's paper kite, the rising of soap-bubbles, and its buoyant property by the floating of balloons. The flight of birds is not accomplished altogether by the buoyant property in the air. These animals support themselves by striking their wings against the fluid through which they are passing; and this friction, along with the property of buoyancy in the atmosphere, sustains them at any height to which they are pleased to ascend. Birds do not generally fly above half a mile in height, and seldom above a few hundred yards. At considerable elevations the air is so specifically light, as to be unsuitable for their easy support. Those which rise to the higher regions of the atmosphere, as for instance the eagle, are provided with large wings, which enable them to support themselves in the comparatively thin fluid in which they move. The light heated air which escapes from a fire, ascends, and is buoyed up by the more dense air beneath. Hydrogen or any other gas of a less specific gravity than air, in the same manner ascends and floats in the atmosphere at the height at which it finds air of its own specific gravity. On the same principle, if heated air or any light gas be enclosed in a large silk bag, it will ascend in the atmosphere till it reach a region of air which is incapable of supporting it. Thus, a soap-bubble enclosing warm air readily ascends to the ceiling of an apartment. If the bubble be made with cold water, it will sink instead of rising. A balloon is a bag made of fine varnished silk, and of such a magnitude that the difference betwixt the weight of its contents and that of the displaced air is sufficient to support the weight of the silk and the other parts of the apparatus. Balloons were originally made to rise by being filled with heated air from a fire hung beneath them; but this dangerous and inconvenient practice was in course of time superseded by the use of hydrogen gas, one of the lightest airs which can be prepared. Hydrogen gas has latterly been succeeded by carbureted hydrogen, which, though not so light, is more easily obtained, being the gas with which towns are now generally lighted. Employing a moderately pure and light gas, the contents of a balloon may be estimated to weigh only an eighth of the weight of the atmosphere, bulk for bulk; and hence, after adding another eighth for weight of apparatus, it will ascend with a force of six-eighths; in other words, if the gas and apparatus weigh two pounds, the balloon will lift from the ground a weight of other six pounds. The force with which a balloon will ascend is therefore to be calculated by measuring its capacity in cubic feet, and comparing the result with an equal bulk of atmospheric air: the difference of weight is the buoyant force of the balloon. Of aërostation, or the art of moving through the air in balloons, great expectations were originally entertained; but the experience of half a century has proved that it is of no practical value. Its only use is the exhibition of an interesting principle in pneumatics. A balloon constructed in the best known manner, and moving upwards with a powerful force, is subject to the following drawbacks:-As the balloon ascends, its contents expand in consequence of the increasing rarefaction of the atmosphere; if, therefore, it has been entirely filled when on the ground, a portion of the gas must be allowed to escape as it rises, otherwise it will burst. Discharges of ballast are also required in consequence of the absorption of moisture from clouds; and there being no means of recovering the lost ballast, the balloon, on the return of heat, rapidly rises in the air, its contents expanding in the ascent, and rendering further liberations of gas necessary to prevent explosion. These alternations continuing to operate more or less frequently, it is evident that they must soon put an end to the buoyant power, however great originally,and, along with the contending effects of winds, forcibly terminate the excursion through the air. Printed and published by W. and R. CHAMBERS, Edinburgh. Sold also by W. S. Orr and Co., London. CHAMBERS'S INFORMATION FOR THE CONDUCTED BY WILLIAM AND ROBERT CHAMBERS, EDITORS OF CHAMBERS'S NUMBER 54. NEW AND IMPROVED SERIES. PRICE lid. OPTICS-LIGHT-ACOUSTICS. OPTICS-LIGHT. THE term Optics is derived from a Greek word which signifies seeing, and applies to that branch of natural philosophy which treats of the phenomena of light and vision. Of the precise character of light, there are various theories, but none which admits of actual demonstration or proof. By some it has been described as consisting of very minute particles, which are thrown off from what are called luminous bodies in all directions, and with immense velocity; while others consider it as the effect of an undulation or vibration produced by luminous bodies in the thin and elastic medium which is interposed between them and the seat of our vision; this vibration producing an effect upon our organs which we recognise as light, in a manner analogous to the impression of sound on the ear, caused by vibrations of the atmosphere. This theory is called the undulatory theory of light; and the former theory, in which light is supposed to consist of material particles, is called the theory of emission. Whatever may be the cause or absolute nature of light, we know it is a remarkable property of luminous bodies, that it enables us to see the luminous objects themselves, as well as others, and that its absence produces darkness. All visible bodies may be divided into two classes self-luminous and non-luminous. Under the first head are comprised all those bodies which possess in themselves the property of exciting the sensation of light or vision, such as the heavenly luminaries, terrestrial flames of all kinds, phosphorescent bodies, and those bodies which shine by being heated or by friction. Under the second class we recognise such bodies as have not of themselves the power of throwing off particles or undulations of light, but which possess the property of reflecting the light which is cast upon them from self-luminous bodies. A non-luminous body may thus, by reflection, receive light from another nonluminous body, and communicate it to a third, and so on. All reflected light, however, is inferior in point of brilliancy to that which comes direct from a self-luminous body. Light proceeds in a straight direction from the luminous body which produces it, towards the part or situation against which it is permitted to act. In consequence of this directness, a shadow or darkened spot is observable behind any opaque object presented to the light. During night, we are in the earth's shadow, and this shadow reaches so far beyond us into space, that when the moon plunges into it in her course, she undergoes an eclipse. The direct shining of the sun, or any other luminous body, is in the form of rays, or thin ethereal lines, each acting independently of the other; no such separation of parts, however, is observable, in common circumstances, in consequence of the diffusive properties of our atmosphere. Seeing is simply the reception of the direct or reflected ray from an object by our eye. Until the rays of the sun reach the spot on which we are placed, we are neither conscious of light nor of the presence of the sun as an object. In the same manner, a candle being lighted and exposed in the open country in a dark night, all who are able to see it are within the influence of its rays; but beyond a given distance these rays are too weak to produce vision, and all who are in this remote situation cannot see the sinallest appearance of the candle. It will therefore be understood, that the seeing of any luminous object is equivalent to being within the influence of rays of sufficient intensity proceeding from it. The number of rays which proceed from even a common candie, is so vast as to be beyond our imagination to conceive; for if such a light is visible within a sphere of four miles, it follows, that if the whole of that space were surrounded with eyes, each eye would receive the impression of a ray of light. In proportion as light advances from its seat of production, it diminishes in intensity. The ratio of diminution is agreeable to that which governs physical forces, that is, the intensity of the light will diminish as the square of the distance increases, or at the rate of 1, 4, 16, &c. But in proportion as we lose in intensity we gain in volume; the light is the weaker the farther it is from the candle, but it is filling a wider space. Anciently, it was believed that light was propagated from the sun, and other luminous bodies, instantane- Preliminary to any further exposition of the nature ously; but the observations of modern inquirers have and action of light, we offer the following definitions of shown that this was an erroneous hypothesis, and that terms. Any parcel of rays, passing from a point, is called light, like sound, requires a certain time to pass from a pencil of rays. By an optical medium is meant any pelone part of space to another, though the velocity of its lucid or transparent body, as, for example, air, water, motion is truly astonishing, as has been manifested in or glass, which suffers light to pass through it. Parallel various ways. Astronomers have proved, by observ-rays are such as move always at the same distance from ing the eclipses of Jupiter's satellites, when that planet is nearest and when it is farthest from the carth, that light moves from the sun to the earth, a distance of 95,000,000 miles, in seven and a half minutes, or about 200,000 miles during a single vibration of a pendulum. So prodigiously great is this velocity, that, as far as any common observation is concerned, light may be each other. If rays continually recede from each other, they are said to diverge; if they continually approach each other, they are said to converge. The point at which converging rays meet is called the focus; the point towards which they tend, but which they are prevented from coming to by some obstacle, is called the imaginary focus. When rays, after passing through one medium, on entering another medium of different | tory way, take an upright empty vessel into a darkened density, are bent out of their former course, and made to change their direction, they are said to be refracted; when they strike against a surface, and are sent back again from the surface, they are said to be reflected. A lens is a glass ground into such a form as to collect or disperse the rays of light which pass through it. These are of different shapes, and thence receive different names. The following figures individually represent sections of the variously shaped lenses and other glasses used in optics. A is a triangular stalk of pure glass, of which we have here a cross sectional or end view, and which is called a prism. Each side of the prism is smooth. B is a section of a piece of plane glass, with sides parallel to each other. C is a sphere or ball of glass, and consequently is convex on all parts of its surface. D is a piece of glass convex or bulging on its two sides, and is called a double convex lens. It is this kind of lens which is used for magnifying objects, in spectacles, telescopes, and other instruments. E is a plano-convex lens, flat on one side and convex on the other. F is a double concave lens, or glass hollowed on each side. G is a plano-concave lens, or planed on one side and concave on the other. H is a meniscus, or lens convex on one side and concave on the other, both surfaces meeting, and of which we have an example in watch-glasses. I is an example of the concavo-convex lens, in which the surfaces disagree, or do not meet when continued. In all these lenses, an imaginary line, represented by MG N, and passing through the centres of the surfaces, is called the axis. Thus, the line said to pass through the centre of any lens, in a direction perpendicular to its surface, is called its axis. In treatises on optics, it is customary to divide the subject into two sections, under the heads Dioptrics and Catoptries. The term dioptrics is compounded of two Greek words signifying to see through, and refers to the transmission of rays of light through transparent bodies, as well as the laws by which they are produced. Catoptrics is a term also from the Greek, and signifies to see from or against; it refers to the reflection of light from surfaces, and the formation of images by means of mirrors and other objects. REFRACTION OF LIGHT. Refraction, as already mentioned, is the bending of rays of light from the course they formerly pursued. If the rays, after passing through a medium, enter another of a different density, perpendicular to its surface, they are not refracted, but proceed through this medium in their original direction. For instance, if the sun's rays were to strike upon the surface of a river at right angles, or perpendicularly, to its surface, they would go straight to the bottom, and the line they observed in the air would be continued in the water. But if they enter obliquely to the surface of a medium either denser or more rare than what they moved in before, they are made to change their direction in passing through that medium; in other words, they are refracted. room, which admits but a single beam of light obliquely through a hole in a window-shutter. Let the empty vessel stand on the floor, a few feet in advance of the window which admits the light, and let it be so arranged that, as the beam of light descends towards the floor, it just passes over the top of the side of the vessel next the window, and strikes the bottom on the side farthest from the window. Let the spot where it falls be marked. Now, on filling the vessel with water, the ray, instead of striking the original spot, will fall considerably nearer the side towards the window. And if we add a quantity of salt to the vessel of water, so as to form a dense solution, the point where the ray strikes the bottom will move still nearer to the window. In like manner, if we draw off the salt water, and supply its place with alcohol, the beam of light will be still more highly refracted; and oil will refract yet more than alcohol. The property of refraction may also be observable in the following experiment. Let the annexed oblong figure represent a vessel half filled with water, and R the ray of light which may be expected to pass through it to the bottom at d. The direction of the ray is perfectly straight until it enters the water at j, when, instead of proceeding in a straight line to d, it is bent from its course and compelled to strike the bottom of the vessel at e. If oil instead of water had been used, the ray would have been still more bent, and have reached the bottom at f. If the ray had been sent directly downwards, as from i to the surface of the water at j, it would not have been refracted, but have proceeded straight to the bottom at k. The following simple experiment is well known:Take an empty basin and place it on a table, then lay a shilling at the bottom of the basin, in such a position that the eye of the observer will not see it. Now, fill the basin with water, and the shilling, though lying unmoved, will come completely into sight. The expla nation of this phenomenon is, that the ray of light producing vision in the eye is bent on emerging from the water, and has all the effect of conveying our sight round a corner. The refractive power of water is also observable when we thrust a straight stick or instrument into it, on aiming at any object. We see that the stick seems to be bent, and fails in reaching the point which we desired it should. On this account, the aim by a person not directly over a fish must be made at a point apparently below it, otherwise the weapon will miss by flying too high. Persons who spear salmon in rivers require to calculate upon this refractive power in taking their aim. With regard to the refractive power of transparent substances or media, the general rule, with certain limitations, is, that it is in proportion to the densities of the bodies. It increases, for instance, from the most perfect vacuum which can be formed, through The mode of the refraction depends on the compara- air, fresh water, salt water, glass, and so on. But tive density or rarity of the respective media. If the those substances which contain the most inflammable medium which the rays enter be denser, they move matter, have the greatest refractive power. It was through it in a direction nearer to the perpendicular from the great refractive powers of the diamond and drawn to its surface. On the contrary, when light water, that Newton, with admirable sagacity, predicted passes out of a denser into a rarer medium, it moves in that they contained inflammable principles. This fact a direction farther from the perpendicular. This re- future discoveries in chemistry verified. Tables of the fraction is greater or less, that is, the rays are more or refractive powers of substances most interesting in less bent, or turned aside from their course, as the optics will be found in Brewster's Optics. From these second medium through which they pass is more or it would appear that substances which contain fluoric less dense than the first. To prove this in a satisfac-acid have the least refractive power, as inflammable ones have the greatest. With regard to the cause of refraction, on the theory of emission, the refracting medium would attract the particles of light, and increase their velocity during their transmission, and would alter the direction of their motion, thus causing refraction; but the intensity of the attractive force would require to be different for light of different colours; and on the undulatory theory, the ether within the refracting medium would be condensed by the attraction of its particles on the ether, and the velocity of transmission of the wave of light through this condensed ether would be less than in free space, and, from this cause, the direction of the motion would be altered, or refraction would take place; and from the different lengths of the waves of different colours, the velocity of their transmission would be different, thus causing different degrees of refrangibility according to the difference of colour. The refraction of rays of light is observable in the case of common window-glass. The two sides of a pane not being perfectly parallel to each other, bodies seen through it appear as if distorted; and as the obliquities in the glass are very various, the distortions are equally grotesque and numerous. Some windows are purposely ground on the surface, to produce universal and minute refraction; and thus so great a confusion is introduced among the rays, that objects are not distinguishable through the glass. When the obliquities on the surface of one side of a piece of glass stand distinct from each other, so as to admit of refraction in a clear and distinguishable manner, then each obliquity affords a separate view of an object on the opposite side, and thus an object seems to be multiplied as many times as there are obliquities. The refraction of light is observable on a great scale in relation to our atmosphere. The rays of the sun, on reaching the confines of the atmospheric fluid which envelops the earth, enter a medium of greater density than that which they have previously been pursuing, and consequently are refracted or bent. One obvious effect of this is, that we never see the sun in the actual position which he occupies. He is always less or more, in relation to our eyes, what the shilling is said to be in the above experiment with the basin of water. This is peculiarly the case in the morning, when his earliest rays reach our eyes; entering a denser medium, these rays bend round to meet our vision, and we actually see the body of the sun a few minutes before he has risen above the horizon-like the shilling in the basin, we see him round a corner. In proportion as the sun approaches the zenith, the refraction diminishes; and as he recedes towards setting, it increases. So considerable is it in the hazy atmosphere of the evening, that we retain a sight of the sun's disk after it has sunk. The same phenomena occur in relation to the other heavenly luminaries. From these explanations it will appear that the directness of our vision is at all times liable to be disturbed by atmospheric conditions. So long as the atmosphere betwixt our person and the object we are looking at is of the same density, we may be said to see in a straight line to the object. But if by any cause a portion of that atmosphere is rendered less or more dense, the line of vision is at once refracted or bent from its course. A thorough comprehension of this simple truth in science has banished a mass of superstition. It has been found that, by means of powerful refraction, objects at a great distance, and round the back of a hill, or considerably beneath the horizon, are brought into sight. In some countries, this phenomenon is called the mirage. The following is one of the most interesting and best authenticated cases of mirage. In a voyage performed by Captain Scoresby in 1822, he was able to recognise his father's ship, when below the horizon, from the inverted image of it which appeared in the air. "It was," says he "so well defined, that I could distinguish by a telescope every sail, the general rig of the ship, and its particular character; insomuch that I confidently pronounced it to be my father's ship the Fame, which it afterwards proved to be; though, on comparing notes with my father, I found that our relative position at the time gave our distance from one another very nearly thirty miles, being about seventeen miles beyond the horizon, and some leagues beyond the limit of direct vision. I was so struck by the peculiarity of the circumstance, that I mentioned it to the officer of the watch, stating my full conviction that the Fame was then cruising in the neighbouring inlet." A curious phenomenon of this kind was seen by Dr Vince, on the 6th of August 1806, at 7 P.M. To an observer at Ramsgate, the tops of the four turrets of Dover Castle are usually seen over a hill between Ramsgate and Dover. Dr Vince, however, when at Ramsgate, saw the whole of Dover Castle, as if it had been brought over and placed on the Ramsgate side of the hill. The image of the castle was so strong and well defined, that the hill itself did not appear through the image. In the sandy plains of Egypt, the mirage is seen to great advantage. These plains are often interrupted by small eminences, upon which the inhabitants have built their villages, in order to escape the inundations of the Nile. In the morning and evening, objects are seen in their natural form and position; but when the surface of the sandy ground is heated by the sun, the land seems terminated at a particular distance by a general inundation: the villages which are beyond it appear like so many islands in a great lake, and between each village an inverted image of it is seen. That the phenomena of the mirage are produced by variations in the refractive power of the atmosphere, can be proved by experiment. 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 have the 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. 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 observed through a telescope, and might be seen and recognised at a distance at which it would not otherwise have been visible. If the refractive power, on the contrary, varied so as to contract the object in both directions, the image of it would be diminished as if seen through a concave lens. In order to represent artificially the effects of the mirage, Dr Wollaston suggested the viewing of an object through a stratum of spirit of wine lying above water in a crystal jar, or a stratum of water lying above one of syrup. These substances, by their gradual incorporation, produce a refractive power diminishing from the spirit of wine to the water, or from the syrup to the water; so that, by looking through the mixed or intermediate stratum at a word or object held behind the bottle which contains the fluids, an inverted image will be seen. The same effect, it has been shown, may be produced by looking along the side of a red-hot poker at a word or object ten or twelve feet distant. At a distance less than three-eighths of an inch from the line of the poker, an inverted image is seen, and within and without that an erect image. The method employed by Sir David Brewster to illustrate these phenomena consists in holding a heated iron above a mass of water bounded by parallel plates of glass; as the heat descends slowly through the fluid, we have a regular variation of density, which gradually diminishes from the bottom to the surface. If we now withdraw the heated iron, and put a cold body in its place, or even allow the air to act alone, the superficial stratum of water will give out its heat, so as to produce a decrease of density from the surface to a certain depth below it. Through the medium thus constituted, the phenomena of the mirage may be seen in the finest manner. Double Refraction of Light.-In the preceding part of this section, we have considered a single ray of light, reflected or transmitted through the substance of a transparent body, as leaving it in the same way in which it came into contact with it, namely, in a single pencil or ray. But there are a great many bodies which have the power of breaking the pencil of light incident upon their surfaces into two separate parts or pencils, more or less inclined to one another, according to the nature and state of the body, and according to the direction of the incident pencil. This is called double refraction, and the bodies which produce it are called doubly refracting bodies or crystals. They are very numerous, and include all salts and crystallised minerals not having the primitive forms of the cube, the regular octohedron, and the rhomboidal dodecahedron. Of all known bodies, the Iceland spar, or rhomboidal carbonate of lime, shows the fact with the greatest certainty; and as it is a mineral easily procured, it has been generally used in experiments upon this subject. Its crystals are of a rhomboidal form, having six acute solid angles, and two obtuse. Double refraction of light is employed to advantage in some kinds of light-houses; and those who wish to investigate its nature and properties may be referred to advanced treatises on the subject. With respect to the polarisation of light-which is the separation of a ray of light into two rays, having different properties from each other, among which pro perties is that of producing colour in a variety of ways, although the original ray may be common or white light-we must also refer to works of higher scope than the present. COLOUR BY REFRACTION. One of the most remarkable phenomena attending refraction is, that the rays of light, which seem to us to be white, may be separated into rays of various colours. It will be obvious that light has the effect of representing colours, where no colour substantially exists, by noticing the glancing and varied hues on irregular surfaces of glass, ice, or other crystallised substances. The proper method of analysing the rays of light, and discovering into what colours they may be resolved, is to procure a prism, and perform the following experiment in a darkened chamber:-In the window shutter E of a darkened room, make a small hole H, through which admit a beam of the sun's light S, which, when nothing is interposed, will proceed in a straight line to P, and form a luminous white spot. If we now interpose a prism B A C, whose refracting angle is B a C, so that the beam of light may fall on its surface C A, and emerge at the same angle from its second surface BA in the direction g G, and if we receive the refracted beam on the opposite wall, or on a white screen M N, "we should expect," says Sir David Brewster," from the principles already laid down, that the white beam which previously fell upon P would suffer only a change in its direction, and fall somewhere upon M N, forming there a round white spot exactly similar to that at P. But this is not the case. Instead of a white spot, there will be formed upon the screen M N an oblong image K L of the sun, containing seven colours, viz., red, orange, yellow, green, blue, o, and violet, the whole beam of light diverging from its emergence out of the prism at g, and being bounded by the lines g K, g L. This lengthened image of the sun is called the solar spectrum, or the prismatic spectrum. If the aperture H is small, and the distance g G considerable, the colours of the spectrum will be very bright. The lowest portion of it at L is a brilliant red. This red shades off by imperceptible gradations into orange, the orange into yellow, the yellow into green, the green into blue, the blue into a pure indigo, and the indigo into a violet. No lines are seen across the spectrum thus produced; and it is extremely diffi cult for the sharpest eye to point out the boundary of the different colours. Sir Isaac Newton, however, by many trials, found the lengths of the colours to be as follow, in the kind of glass of which his prism was made:-Red, 45; orange, 27; yellow, 40; green, 60; blue, 60; indigo, 48; violet, 80-Total length, 360." These colours are not equally brilliant. At the lower end L of the spectrum, the red is comparatively faint, but grows brighter as it approaches the orange. The light increases gradually to the middle of the yellow, where it is brightest; and from this it gradually declines to the upper or violet end K of the spectrum, where it is extremely faint. From the phenomena which we have now described, Sir Isaac Newton concluded that the beam of white light is compounded of light of seven different colours, and that for each of these different kinds of light, the glass of which his prism was made had different indices, that is, measures of refraction; the index of refraction for the red light being the least, and that of the violet the greatest. By means of a second prism placed behind a hole in the screen M N, opposite the centre of each coloured space, Sir Isaac Newton refracted the light a second time. In this case it was not drawn out into an oblong image as before, and was not refracted into any other colour than that which formerly belonged to each par ticular ray. Hence this great philosopher concluded that the light of each particular colour possessed the same index of refraction; and he termed such light homogeneous, that is, of the same kind, or simple; white light being regarded as heterogeneous, that is, of different kinds, or compound. By various experiments, Sir Isaac proved that all the colours, when again combined, formed or recom posed white light. Indeed, the doctrine may be illustrated by mixing together in proper proportions seven colours as like those of the spectrum as can possibly be got. By their union a greyish white is formed, for powders of the exact tint as those of the spectrum cannot be obtained. It may also be proved in this manner-Let a circle of paper be divided into sections of the same size, and coloured like the spaces in the spec trum, and placed upon a humming-top, which is made to revolve rapidly; the effect of all the colours when combined is to produce a greyish white. "All transparent substances, in bending light," observes Dr Arnott, "produce more or less of the sepa ration of colour; but it is an important fact, that the quality of merely bending a beam, or of refraction, and that of dividing it into coloured beams, or of dispersion, are distinct qualities, and not having the same proportion to each other in different substances. Newton, from not discovering this, concluded that a perfect telescope of refraction could never be made; he supposed that the bent light would always become coloured, and so render the object indistinct. We now know, however, that, by combining two or more media, we may obtain bending of light without dispersion-thus, by opposing a glass which bends five degrees and disperses one degree, to another glass which bends three degrees and disperses one, the opposing dispersions will just counterbalance or neutralise each other, while the twa degrees of excess of bending will remain to be applied to use." It having been found, by the experiments of Newton and others, that none of the seven colours of the solar spectrum could be broken by the prism into new co |