principle which may be no less satisfactorily explained than the minuteness of its particles. The propagation of light was, for a long period, supposed to be instantaneous; because no method that had been adopted had succeeded in marking the degree of its velocity. Astronomers had previously ascertained the fact, that the distance of the sun, the great source of light, from the earth, is about 95,000,000 of miles; and that a cannon ball, travelling with uniform velocity from the earth, would scarcely measure the distance in twenty-five years. The fixed stars had been previously determined to be still more remote; and yet the rays of light were known to be emitted from all these luminaries, and to arrive at the earth, comparatively, in a very short time. But, whilst the general fact of the rapidity of light might have been felt to be fully established by these familiar occurrences, it was evident that they could furnish no data by which to ascertain the degree of its velocity. Roemer, a Danish philosopher, who flourished in the seventeenth century; and Cassini, an astronomer born at Piedmont in 1635, discovered a rule by which to determine this velocity. They observed the eclipses of the satellites of Jupiter, and discovered that they are visible to us about eight minutes sooner (according to Dr. Bradley, eight minutes and thirteen seconds), than they ought to be when the earth is placed between the sun and this planet. On the other hand, it was remarked that, when the sun was between the planet Jupiter and the earth, the appearance of these eclipses was first observed about eight minutes later than it ought to have been, from accurate calculations previously made. Hence it was infered that light occupies about eight minutes in traversing the distance between the sun and earth, which we have already stated has been estimated at about 95,000,000 of miles. 31. The above will be better understood by referring to plate I, OPTICS, fig. 1. Let A be the sun, and BCDE the annual orbit of the earth round the sun; let the small circle at F represent the planet Jupiter, GHI the orbit of the nearest satellite, which, entering the shadow of Jupiter at G, comes out at the point H. When the earth is at B, in its annual progress round the sun, and the satellite is observed to come out of the shadow, the same emersion will be again perceived in 42 hours. Suppose the earth to remain constantly at B, then, in 424 hours multiplied by 50 (or 42 × 50 = 2125 hours), there would be fifty distinct emersions perceptible. Let the earth, however, be removed further from Jupiter towards C; then, if the doctrine of Descartes be not true, or, in other terms, if a portion, of time be requisite for the transmission of light, the emersion of the satellite must be obscured as much later than 42 hours × 50 as the time may amount to—that is, occupied in the transmission of light over the space K C, which is the difference of the spaces CG and K G. In fact, the time consumed by the passage of light over the space KBC must be added to 424 hours X 50, making 42 × 50 2125 hours + eight minutes; or, according to Dr. Bradley's computation, eight minutes and thirteen seconds. 32. To afford a more simple illustration of the prodigious velocity of light, we may refer to many facts of daily occurrence. For instance, we perceive the flash of a gun; but, perhaps, a second or two may elapse before we hear the report produced by its discharge. Of the same nature is the flash of lightning which illuminates the celestial hemisphere, and may be succeeded by the peal of thunder after the lapse of several minutes. Hence, we infer, that, whatever may be the rate at which sound travels, the rapidity of light is far greater. 33. The rectilinear propagation of light was known to the ancients; and in modern times has been abundantly confirmed by experiments. Every luminous body emits continually small particles of matter from its surface, which proceed in straight lines, and in every direction, until they meet with some resisting medium. When this, therefore, occurs-when the rays of light fall on the superficies of a dense and opaque substance, through which it is impossible for them to pass, they do not travel round it, but are altogether stayed in their course; and must, either by turning back again, form what philosophers call the angle of incidence, or be completely absorbed. No contrivance of man has hitherto been able to cause them to move in a curvilinear direction. 34. We have already stated the immense velocity with which light is emitted from the sun's body, and it will be easy to conceive that it cannot suffer any change in velocity or direction, till it meets with some ponderable matter. In approaching any planetary body, such as our earth, we have reason to believe that they are mutually attracted. Rays falling perpendicularly upon the atmosphere are equally attracted on every side, and come in a straight line to the earth; while those rays which fall obliquely are bent out of their original direction; and, since the atmosphere is not of uniform density, such oblique rays will come to the earth in curved lines. If our atmosphere were of uniform density, the refraction would not be altered; but the oblique rays falling upon its surface would be reflected in a very great degree; a circumstance which would deprive us of much of the sun's light. No doubt a great quantity of light becomes extinguished in its passage through the acrial medium, as we may justly learn from the difference of intensity in the light at different altitudes of the sun but how much must this loss of light appear, when we recur to the statement already made, namely, that the whole effects of the sun's light would be lost by passing through 679 feet of sea-water, and that the same effect would take place by its passage through 3,110,310 feet of air! 35. The following is a table from M. Bouguer, showing the intensity of the sun's light at different altitudes, and the thickness of air it has to penetrate at each angle. state and color of the surface; and 3dly, Upon the quantity of the angle of incidence. Under all these circumstances, however, the angle at which the ray is reflected is equal to the angle of incidence. The same laws, therefore, which govern the collision between perfectly elastic bodies and absolutely hard surfaces, may be applied to the reflection of light. Of the different bodies which reflect light, metals possess this power in the greatest degree, and perhaps in proportion to their density and hardness. Smooth or polished surfaces reflect more light than rough ones. 39. Of colored surfaces the lightest colors reflect the most; hence the whitest metals make the best reflectors. The order will, therefore, in all probability, be as follows, beginning with the best reflectors-white, yellow, red, blue, black. The two extremes are very striking in the well known experiment of two pieces of cloth, one white and the other black, laid on the surface of snow in the sun. The black piece very soon sinks into the snow, from absorbing a greater quantity of light, which causes the heat. The white piece reflects a greater portion, and is longer in becoming heated. With regard to the quantity of reflection, as affected by the angle of the incidence, it is found that opaque bodies are more heated as the rays strike their surfaces more perpendicularly, and the quantity of light which enters transparent bodies is the same. In both instances, therefore, more light enters the bodies, and less is reflected. In the first instance the light which is not reflected becomes extinguished, producing heat; in the second it is transmitted, still retaining the property of light. Hence, therefore, we ought to conclude that the reflection will be inversely as the angle of incidence, supposing the angle to be formed by the ray and the surface of the medium. 40. M. Bouguer has informed us that the light reflected from a surface of mercury, when the angle of incidence was 11° 30', was only equal to one-fourth of the whole, and he thinks it probable that no substance reflects more. It is certain, however, that polished silver reflects much more. The same philosopher observes that the metallic reflectors change less in their power of reflection with the angle of incidence. He made the following experiment with polished black marble:-At an angle of 3° 35' with the reflecting surface, 6 were reflected, the whole being unity; at 15° of incidence 156 were reflected; at 30°, 051; and at 80, 023. The rest of course became extinguished, and would heat the marble. 41. A similar diminution of the reflective power, with the angle of incidence, is observed in transparent bodies by the same author. The following table gives the results with water and plate glass :— 42. The reflections in this instance are partly made from the upper, and the rest from the under surface. The remainder of the 1000 parts are transmitted, with the exception of a few which are in all probability extinguished. 43. That, under certain circumstances, the rays of light are extinguished, even in transparent bodies, is rendered highly probable by the above enquirer. M. Bouguer tells us that, if our atmosphere were 518,385 toises in height, we should have no light from the sun, even in his meridian splendor. It has been estimated that, of the horizontal sun-beams passing through about 200 miles of air, th part only reaches apparently a rectilinear angle at that place. Thus, if a small hole be made in the shutter of a window of a darkened room, and the light of the sun be permitted to pass through it, the image of the sun, or white spot which is formed upon a screen placed to receive that light in the room, will be found to be larger than it ought to be if right lined rays proceeded from the various points of the sun's surface, and passed through the hole to the screen; hence it appears that they are bent at the hole; for otherwise the image would be smaller than experience shows it to be. 46. This property of light was not discovered till about the middle of the seventeenth century. The person who first made the discovery was F. Grimaldi; at least he first published an account of it in his treatise De Lumine, Coloribus, et Iride, printed in 1666. Dr. Hooke, however, laid claim to the same discovery, though he did not publish his observations till six years after Grimaldi. be 47. Dr. Hooke, having made his room completely dark, admitted into it a beam of the sun's light by a very small hole in a brass plate fixed in the window shutter. This beam, spreading itself, formed a cone, the apex of which was in the hole, and the base was on a paper, so placed as to receive it at a distance. In this image of the sun, thus painted on the paper, observed that the middle was much brighter than the edges, and that there was a kind of dark penumbra about it, of about a sixteenth part of the diameter of the circle; which penumbra, he says, must be ascribed to a property of light, which he promised to explain.-Having observed this, at the distance of about two inches from the former, he let in another cone of light; and, receiving the bases of them at such a distance from the holes as that the circles intersected each other, he observed that there was not only a penumbra, or darker ring, encompassing the lighter circle, but a manifest dark line, or circle, which appeared even where the limb of the one interfered with that of the other. 48. The shadows of all bodies, metals, stones, glass, wood, horn, ice, &c., in this light were bordered with three parallel fringes, or bands of colored light, of which that which was contiguous to the shadow was the broadest and most luminous, while that which was the most remote was the narrowest, and so faint as not easily to be visible. It was difficult to distinguish these colors unless when the light fell very obliquely upon a smooth paper, or some other smooth white body, so as to make them appear much broader than they would otherwise have done; but in these circumstances the colors were plainly visible, ard in the following order The first or innermost fringe was violet, and deep blue next the shadow, light blue, green, and yellow in the middle, and red without. The second fringe was almost contiguous to the first, and the third to the second; and both were blue within and yellow and red without; but their colors were very faint, especially those of the third. The colors, therefore, proceeded in the following order from the shadow: violet, indigo, pale blue, green, yellow, red; blue, yellow, red; pale blue, pale yellow, and red. The shadows made by scratches and bubbles in polished plates of glass, were bordered with the like fringes of colored light. By looking on the sun through a feather, or black riband, held close to the eye, several rainbows will appear, the shadows which the fibres or threads cast on the retina being bordered with the like fringes of colors. Measuring these fringes and their intervals, with the greatest accuracy, he found the former to be in the progression of the numbers, 1, √ √, and their intervals to be in the same progression with them, that is, the fringes and their intervals together to be in continual progression of the numbers 1, √ √ }, √ √ }, or thereabouts. And these proportions held the same very nearly at all distances from the hair, the dark intervals of the fringes being as broad in proportion to the breadth of the fringes at their first appearance as afterwards, at great distances from the hair, though not so dark and distinct. 49. In Sir Isaac's observations we find a very remarkable and curious appearance, pretty similar to the one noticed by Dr. Hooke. The sun shining into his darkened chamber, through a hole three-fourths of an inch broad, he placed at the distance of two or three feet from the hole, a sheet of pasteboard, black on both sides; in the middle of it he made a hole about one-fourth of an inch square, for the light to pass through; behind the hole he fastened to the pasteboard the blade of a sharp knife, to intercept some part of the light which passed through the hole. The planes of the pasteboard and blade of the knife were parallel to one another, and perpendicular to the rays; and when they were so placed that none of the light fell on the pasteboard, but all of it passed through the hole to the knife, and there part of it fell upon the blade of the knife, and part of it passed by its edge, he let that part of the light which passed by fall on a white paper, two or three feet beyond the knife, and there saw two streams of faint light shoot out both ways from the beam of light into the shadow, like the tails of comets. But because the sun's direct light, by its brightness upon the paper, obscured these faint streams, so that he could scarce see them, he made a little hole in the midst of the paper for that light to pass through and fall on a black cloth behind it: and then he saw the two streams plainly. They were like one another, and pretty nearly equal in length, breadth, and quantity of light. Their light, at that end which was next to the sun's direct light, was pretty strong for the space of about onefourth of an inch, or half an inch, and decreased gradually till it became insensible. The whole length of each of these streams, measured upon the paper, at three feet from the knife, was about six or eight inches; so that it subtended an angle, at the edge of the knife, of about 10° or 12°, or at most 14°. Yet sometimes he thought he saw it shoot 3° or 4° farther; but with a light so very faint that he could hardly perceive it. This light he suspected might, in part, arise from some other cause than the two streams; for, placing his eye in that light, beyond the end of that stream which was behind the knife, and looking towards the knife, he could see a line of light upon its edge; and that not only when his eye was in the line of the streams, but also when it was out of that line, either towards the point of the knife or towards the handle. This line of light appeared contiguous to the edge of the knife, and was narrower than the light of the innermost fringe, and narrowest when his eye was farthest from the direct light; and therefore seemed to pass between the light of that fringe and the edge of the knife; and that which passed nearest the edge seemed to be most bent, though not all of it. He then placed another knife by the former, so that their edges might be parallel, and look towards one another, and that the beam of light might fall upon both the knives and some part of it pass between their edges. In this situation he observed that, when the distance of their edges was about the 400th part of an inch, the stream divided in the middle, and left a shadow between the two parts. This shadow was so black and dark that all the light which passed between the knives seemed to be bent and turned aside to the one hand or the other; and, as the knives still approached one another, the shadow grew broader, and the streams shorter next to it, till, upon the contact of the knives, all the light vanished. 50. From this experiment he concludes that the light which is least bent, and which goes to the inward ends of the streams, passes by the edges of the knives at the greatest distance; and this distance, when the shadow began to appear between the streams, was about the 800th part of an inch; and the light which passed by the edges of the knives, at distances still less and less, was more and more faint, and went to those parts of the streams which were farther from the direct light; because, when the knives approached one another till they touched, those parts of the streams vanished last which were farthest from the direct light. In the experiment of one knife only, the colored fringes did not appear; but, on account of the breadth of the hole in the window, became so broad as to run into one another, and, by joining, to make one continued light in the beginning of the streams; but in the last experiment, as the knives approached one another, a little before the shadow appeared between the two streams, the fringes began to appear on the inner ends of the streams, on either side of the direct light; three on one side, made by the edge of one knife, and three on the other side, made by the edge of the other knife. They were most distinct when the knives were placed at the greatest distance from the hole in the window, and became still more so by making the hole less; so that he could sometimes see a faint trace of a fourth fringe beyond the three abovementioned: and, as the knives approached one another, the fringes grew more distinct and larger, till they vanished; the outermost vanishing first, and the innermost last. After they were all vanished, and the line of light which was in the middle between them was grown very broad, extending itself on both sides into the streams of light described before, the above-mentioned shadow began to appear in the middle of this light, and to divide it along the middle into two lines of light, and increased till all the light vanished. This enlargement of the fringes was so great, that the rays which went to the innermost fringe seemed to be bent about twenty times more when the fringe was ready to vanish, than when one of the knives was taken away. 51. From both these experiments, compared together, Sir Isaac concludes that the light of he first fringe passed by the edge of the knife at a distance greater than the 800th part of an inch; that the light of the second passed by the edge of the knife at a greater distance than the light of the first fringe, and that of the third at a greater distance than that of the second; and that the light, of which the streams above-mentioned consisted, passed by the edges of the knives at less distances than that of any of the fringes. 52. He then got the edges of two knives ground truly straight, and attaching them to a board, so that their edges might look to wards one another, and, meeting near their points, contain a rectilinear angle, he fastened their handles together, to make the angle invariable. The distance of the edges of the knives from one another, four inches from the angular point, where the edges of the knives met, was the eighth part of an inch; so that the angle contained by their edges was about 1° 54'. The knives being thus fixed together, he placed them in a beam of the sun's light let into his darkened chamber, through a hole the forty-second part of an inch wide, ten or thirteen feet from the hole; and he let the light which passed between their edges fail very obliquely on a smooth white ruler, half an inch or an inch from the knives; and there he saw the fringes made by the two edges of the knives run along the edges of the shadows of the knives, in lines parallel to those edges, without growing sensibly broader, till they met in angles equal to the angle contained by the edges of the knives; and where they met and joined they ended, without crossing one another. But if the ruler was held at a much greater distance from the knives, the fringes, where they were farther from the place of their meeting, were a little narrower, and they became something broader as they approached nearer to one another, and after they met they crossed one another, and then became much broader than before. From these observations he concluded that the distances at which the light composing the fringes passed by the knives were not in creased or altered by the approach of the knives, but that the angles, in which the rays were there bent, were much increased by that approach; and that the knife which was nearest to any ray determined which way the ray should be bent, but that the other knife increased the bending. 53. When the rays fell very obliquely upon the ruler, at the distance of a third part of an inch from the knives, the dark line between the first and second fringe of the shadow of one knife, and the dark line between the first and second fringe of the shadow of the other knife, met one another, at the distance of the fifth part of an inch from the end of the light which passed between the knives, where their edges met one another; so that the distance of the edges of the knives, at the meeting of the dark lines, was the 160th part of an inch; and one half of that light passed by the edge of one knife, at a distance not greater than the 320th part of an inch, and, falling upon the paper, made the fringes or the shadow of that knife; while the other half passed by the edge of the other knife, at a distance not greater than the 320th part of an inch, and, falling upon the paper, made the fringes of the shadow of the other knife. But, if the paper was held at a distance from the knives greater than the third part of an inch, the dark lines abovementioned met at a greater distance than the fifth part of an inch from the end of the light which passed between the knives, at the meeting of their edges; so that the light which fell upon the paper where those dark lines met passed between the knives, where their edges were farther distant than the 160th part of an inch. For at another time, when the two knives were eight feet and five inches from the little hole in the window, the light which fell upon the paper where the above-mentioned dark lines met passed between the knives, where the distance between their edges was, as in the following table, at the distances from the paper there noted :54. Distances of the pa- | Distances between the per from the knives in edges of the knives in inches. millesimal parts of an inch. 56. When the fringes of the shadows of the knives fell perpendicularly upon the paper, at a great distance from the knives, they were in the form of hyperbolas, their dimensions being as follows:-Let CA, CB, fig. 2, represent lines drawn upon the paper, parallel to the edges of the knives; and between which all the light would fall if it suffered no inflection. DE is a right line drawn through C, making the angles ACD, BCE, equal to one another, and terminating all the light which falls upon the paper, from the point where the edges of the knives meet. Then eis, fkt, and glv, will be three hyperbolical lines, representing the boundaries of the shadow of one of the knives, the dark line between the first and second fringes of that shadow, and the dark line between the second and third fringes of the same shadow. Also rip, yk q, and zlr, will be three other hyperbolical lines, representing the boundaries of the shadow of the other knife, the dark line between the first and second fringes of that shadow, and the dark line between the second and third fringes of the same shadow. These three hyperbolas are similar, and equal to the former three, and cross them in the points i, k, and l; so that |