of 10°, 20°, or 30°, the effect will be nearly the 104. When our author proceeded to measure the quantity of light reflected by these internal furfaces at great angles of incidence, he found many difficulties, efpecially on account of the many alterations which the light underwent before it came to his eye: but at length, using a plate of cryftal, he found, that, at an angle of 75°, this internal reflection diminished the light 27 or 28 times; and as the external reflection at the fame angle diminished the light only 26 times, it follows that the internal reflection is a little ftronger than the other. The image reflected internally was always a little redder than an object which was feen directly through the plate of crystal. 105. Refuming his obfervations on the diminution of light, occafioned by the reflection of opaque bodies obliquely fituated, he compared it with the appearances of fimilar fubftances which reflected the light perpendicularly. Ufing pieces of filver made very white, he found, that, when one of them was placed at an angle of 75° with respect to the light, it reflected only 640 parts out of 1000. He then varied the angle, and alfo ufed white plafter and fine Dutch paper, and drew up the following table of the proportion of the light reflected from each of thofe fubftances at certain angles: them had their roughnefs equivalent to small hemifpheres, which would have difperfed the light equally in all directions; and, from the data in the preceding table, he deduces mathematically the number of the little planes that compofe thofe furfaces, and that are inclined to the general furface at the angles above-mentioned, fuppofing that the whole furface contains 1000 of them that are parallel to itself, fo as to reflect the light perpendi cularly, when the luminous body is fituated at right angles with respect to it. 108. His conclufions reduced to a table, correfponding to the preceding, are as follow: 109. These variations in the number of little planes, or furfaces, he expreffes in the form of a curve; and afterwards fhows, geometrically, what would be the effect if the bodies were enlightened in one direction, and viewed in another; upon which fubject he has feveral curious theorems and problems: as, the pofition of the eye being given, to find the angle at which the luminous body muft be placed, in order to its reflecting the moft light; or, the fituation of the luminous body being given, to find a proper fituation for the eye, in order to fee it the oft enlightened, &c. But it would carry us too far into geometry to follow him through all thefe difquifitions. 110. Our geometrician next proceeds to afcertain the quantity of furface occupied by the small planes of each particular inclination, from confidering the quantity of light reflected by each, allowing thofe that have a greater inclination to the common furface to take up proportionably lefs space than those which are parallel to it. And comparing the quantity of light that would be reflected by fmall planes thus difpofed, with the quantity of light that was actually reflected by the three fubftances above-mentioned, he found, that plafter, notwithstanding its extreme whitenefs, ab. forbs much light; for that, of 1000 rays that fall upon it, of which 166 or 167 ought to be reflected at an angle of 77°, only 67 are in fact returned; fo that 100 out of 167 were extinguished, that is, about three sths. With refpect to the planets, our author concludes, that of 300,000 rays which the MOON receives, 172,000 are abforbed, or perhaps 204,100. 111. Having confidered the surfaces of bodies as confifting of planes only, he thus explains himfelf:-Each small furface, separately taken, is extremely irregular, and fome of them are really concave, and others convex ; but, in reducing them to a middle state, they are to be confidered as planes. Nevertheless he confiders them as planes planes only with refpect to the reception of the rays; for as they are almost all curves, and as, befides this many of those, whose fituation is dif. ferent from others, contribute to the fame effect, the rays always iffue from an actual or imaginary focus, and after reflection always diverge from another. If it be asked, what becomes of thofe rays that are reflected from one afperity to another? he thows that very few of the rays can be in those circumstances; fince they muft fall upon planes which have more than 45° obliquity to the furface, of which there are very few in natural bodies. 112. Mr MELVILLE has made fome ingenious obfervations on the manner in which bodies are heated by light. He obferves, that, as each colorific particle of an opaque body muft be fomewhat moved by the reaction of the particles of light, when it is reflected backwards and forwards between the fame particles, it is manifeft that they muft likewife be agitated with a vibratory motion, and the time of a vibration will be equal to that which light takes up in moving from one particle of a body to another adjoining. This diftance, in the moft folid opaque bodies, cannot be fuppofed greater thanth of an inch, which space a particle of light defcribes in the 150000000000th of a second. With fo rapid a motion, therefore, may the internal parts of bodies be agitated by the influence of light, as to perform 125,000,000,000,000 vibrations, or more, in a fecond of time. 113. Since there is no reflection of light but at the furface of a medium, he obferves, that the greatest quantity of rays, though crowded into the smallest space, will not of themselves produce any heat. Hence it follows, that the portion of air which lies in the focus of the most potent fpeculum, is not at all affected by the paffage of light through it, but continues of the fame temperature with the ambient air; though any opaque body, or even any tranfparent body denfer than air, when put in the fame place, would be intenfely heated in an inftant. This confequence, evidently flowing from the most certain principles, not feeming to have been understood by many philofophers, he thought it proper to say something in explication of it. He obferves, that the eafieft way to be satisfied of the matter experimentally is, to hold a hair, or a piece of down, immediately above the focus of a lens or fpeculum, or to blow a ftream of smoke from a pipe horizontally over it; for if the air in the focus were hotter than the furrounding fluid, it would continually afcend upon account of its rarefaction, and thereby fenfibly agitate thofe flender bodies. Or a lens may be fo placed as to form its focus within a body of water, or fome other trenfparent fubftance, the heat of which may be examined from time to time with a thermometer; but care must be taken, in this experiment, to hold the lens as near as poffible to the tranfparent body, left the rays, by falling clofer than ordinary on its furface, fhould warm it more than the common fun-beams. 114. To apply these observations to the patural phenomena, he obferves, that the atmosphere is not much warmed by the paffage of the fun's ight through it, but chiefly by its contact with VOL. XVI. PART 1, 1 the heated furface of the globe. This, he thought, furnished one very fimple and plaufible reason, why it is coldeft in all climates on the tops of very high mountains; namely, because they are removed to the greatest distance from the general fur face of the, earth. For it is well known, that a fluid. heated by its contact with a folid body, decreafes in heat in an inverfe proportion to the dif tance from the body. He found, by repeated trials, that the heat of water in deep lakes decreafes regularly from the furface downwards. But to have this queftion fully determined, the temperature of the air in the valley and on the mountain's top must be obferved every hour, both night and day, and carefully compared together. From this doctrine he fuppofes, that the heat pro duced by a given number of rays, in an opaque body of given magnitude, must be greater when the rays are more inclined to one another, than when they are lefs fo; for the direction of the vibrations raised by the action of the light, whether in the colorific particles, or those of an inferior order, will more interfere with one another; whence the inteftine fhocks and collifions must in creafe. Befides, the colorific particles of opaque bodies being difpofed in various fituations, the rays will fall more directly on cach, the more they are inclined to one another. 115. The attempts of Abbe NOLLET to fire in flammabie fubftances by the power of the folar rays collected in the foci of burning mirrors, have a near relation to the prefent fubject. Confidering the great power of burning mirrors and lenfes, it will appear furprifing that this celebrated experimental philofopher fhould not be able to fire any liquid fubftance. But though he made the trial with all the care imaginable on the 19th Feb. 1757, he was not able to do it either with spirit of wine, olive-oil, oil of turpentine, or æther; and though he could fire fulphur, yet he could not fucceed with Spanish-wax, rofin, black pitch, or fuet. He both threw the focus of thefe mirrors upon the fubftances them felves, and alfo upon the fumes that rofe from them; but all the effect was, that the liquor boiled, and was difperf ed in vapours or very fmall drops, but would not take fire. When linen rags, and other folid fubftances, were moistened with any of thefe inflammable liquids, they would not take fire till the liquid was difperfed in a copious fume; fo that rags thus prepared were longer in burning than thofe that were dry. 116. M. BEAUME, who affifted M. Nollet in fome of thefe experiments, obferved farther, that the fame fubftances which were eafily fired by the flame of burning bodies, could not be fet on fire by the contact of the hottest bodies that did not actually flame. Neither æther nor fpirit of wine could be fired with a hot coal, or even red-hot iron, unlefs they were of a white heat. From thefe experiments our author concludes, that, fuppofing the electric matter to be the fame with fire or light, it must fire spirit of wine by means of fome other principle. The members of the academy Del Cimento had very early attempted to fire feveral of thefe fubftances, though with out fuccefs, The Abbe read an account of his experiments to the Royal Academy at Paris feve U u ral years before he attended to what had been done by the Italian philofophers. 117. By the help of optical principles, and ef-, pecially by obfervations on the reflection of light, Mr MELVILLE discovered that bodies which feem to touch one another are not always in actual con. tact. "It is common (fays he) to admire the volubility and luftre ́ of drops of rain that lie on the leaves of colewort, and fome other vegetables;" but no philofopher had explained this curious phe nomenon. Upon infpecting them narrowly, he found that the luftre of the drop is produced by a copious reflection of light from the flattened part of its furface contiguous to the plant; and that, when the drop rolls along a part which has been wetted, it immediately lofes all its luftre, the green plant being then feen clearly through it; whereas, in the other cafe, it is hardly to be difcerned. From thefe two obfervations, he concluded, that the drop does not really touch the plant, when it has the mercurial appearance, but is fufpended in the air at some distance from it by the force of a repulsive power; for there could not be any copious reflection of white light from its under furface, unless there were a real interval between it and the furface of the plant. If that furface were perfectly fmooth, the under furface of the drop would be fo likewife, and would therefore fhow an image of the illuminating body by reflection, like a piece of polished filver; but as it is confiderably rough and unequal, the under furface becomes rough likewife, and fo, by reflecting the light copiously in different directions, affumes the refplendent white colour of unpolished filver. It being thus proved by an optical argument, that the drop is not really in contact with the plant which fupports it, it may eafily be conceived whence its volubility arifes, and why it leaves no moisture where it rolls. r18. Baron ALEXANDER FUNK, vifiting fome filver mines in Sweden, obferved, that, in a clear day, it was as dark as pitch underground in the eye of a pit, at 60 or 70 fathoms deep; whereas, in a cloudy or rainy day, he could even fee to read at 106 fathoms deep. Inquiring of the miners, he was informed that this is always the cafe; and, reflecting upon it, he imagined that it arofe from this circumftance, that when the atmosphere is full of clouds, light is reflected from them into the pit in all directions, and that thereby a confiderable proportion of the rays are reflected perpendicularly upon the earth; whereas, when the atmofphere is clear, there are no opaque bodies to reflect the light in this manner, at least in a fufficient quantity; and rays from the fun itself car never fall perpendicularly in that country. The other was that of the ingenious Mr GREY, who makes fuch a figure in the hiftory of ELECTRICITY. This gentleman took a piece of stiff brown paper, and pricking a small hole in it, he held it at a little diftance before him; when, applying a nerdle to his eye, he was surprised to fee the point of it inverted. The nearer the needle was to the hole, the more it was magnified, but the Jefs diftinct; and if it was fo held, as that its image was near the edge of the hole, its point seemed crooked. From these appearances he concluded, that thefe fmall holes, or fomething in them, pro 119. THIS property of light was not difcovered till about the middle of the 17th century. The perfon who first made the discovery was F. GRIMALDI; at leaft he first published an account of it in his treatise De lumine, coloribus, et irides, printed in 1666. Dr Hooke, however, laid claim to the fame difcovery, though he did not publish his obfervations till fix years after Grimaldi. 120. Dr HOOKE having made his room completely dark, admitted into it a beam of the fun's light by a very small hole in a brass plate fixed in the window-fhutter. This beam spreading itself, formed a cone, the apex of which was in the hole, and the bafe was on a paper, fo placed as to receive it at fome diftance. In this image of the fun, thus painted on the paper, he obferved that the middle was much brighter than the edges, and that there was a kind of dark penumbra about it, of about a 16th part of the diameter of the circle; which penumbra, he fays, muft be afcribed to a property of light, which he promised to explain.Having obferved 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 interfected each other, he obferved that there was not only a penumbra, or darker ring, encompaffing the lighter circle, but a manifeft dark line, or circle, which appeared even where the limb of the one interfered with that of the other. This appearance is diftinctly represented, fig. 6. plate 249. 121. Comparing the diameter of this bafe with its diftance from the hole, he found it to be by no means the fame as it would have been, if it had been formed by ftraight lines drawn from the extremities of the fun's disk, but varied with the size of the holes, and the diftance of the paper. Struck with this appearance, he proceeded to make farther experiments concerning the nature of light thus tranfmitted. To give a juft idea of which, he held an opaque body BB, fig. 7. fo as to intercept the light that entered at a hole in the window fhutter O, and was received on the fcreen AP. In these circumftances, he obferved, that the shadow of the opaque body (which was a round piece of wood, not bright or polished) was all over fomewhat enlightened, but more efpecially towards the edge. Some persons who were prefent, imagining that this light within the fhadow might be produced by fome kind of reflection from the fide of this opaque body, on account of its roundness; and others fuppofing it might proceed from fome reflection from the fides of the hole in the piece of brafs, through which the light was admitted into the room; to obviate both these objections, he admitted the light through a hole burnt in a piece of pafte board, and intercepted it with a razor which had a very tharp edge; but fill the appearances we the very fame as before. -122. He diverfified this experiment, by placing the razor so as to divide the cone of light into two parts, the hole in the fhutter remaining as before, and and placing the paper fo as that none of the enlightened part of the circle fell upon it, but only the fhadow of the razor; and to his surprise he faw what he calls a very brisk and visible radiation ftriking down upon the paper, of the fame breadth with the diameter of the lucid circle. It always ftruck perpendicularly from the line of shadow, and, like the tail of a comet, extended more than 10 or probably 100 times the breadth of the remaining part of the circle: nay, as far as he could find, by many trials, the light from the edge ftruck downwards into the fhadow very near to a quadrant. Wherever there was part of the interpofed body higher than the reft, oppofite to it, the radiation of light into the fhadow was brighter, as in the figure; and wherever there was a notch or gap in it, there was a dark ftroke in the balf-enlightened fhadow. From all these appearances, he concluded, that they were to be afcrib. ed to a new property of light, whereby it is deflected from straight lines, contrary to what had been before afferted by optical writers. It does not appear, however, that the Dr ever profecuted this experiment; as all that we find of his on the fubject, after this time, are fome crude thoughts which he read at a meeting of the Royal Society; on the 18th of March 1675, confifting of 8 articles, viz. 1. There is a deflection of light, differing both from reflection and refraction, and feeming to depend on the unequal denfity of the conftituent parts of the ray, whereby the light is difperfed from the place of condenfation, and rarefied, or gradually diverged into a quadrant. 2. This deflection is made towards the fuperficies of the opaque body perpendicularly. 3. Thofe parts of the diverged radiations which are deflected by the greatest angle from the straight or direct radiations are the fainteft, and those that are deflected by the leaft angles are the strongest. 4. Rays cutting each other in one common foramen do not make the angles at the vertex equal. 5. Colours may be made without refraction. 6. The diameter of the fun cannot be truly taken with common fights. 7. The fame rays of light, falling upon the fame point of an object, will turn into all forts of colours, by the various inclinations of the object. 8. Colours begin to appear when two pulfes of light are blended fo well, and fo near together, that the fenfe takes them for one... 123. We proceed to the discoveries of F. GRIMALDI. Having introduced a ray of light, through a very small hole, (AB, fig. 8. Pl. 249.) into a darkened room, he observed that the light was diffufed in the form of a cone, the base of which was CD; and that if any opaque body, FE, was placed in this cone of light, at a confiderable diftance from the hole, and the fhadow was received upon a piece of white paper, the boundaries of it were not confined within GH, or the penumbra IL, occafioned by the light proceeding from different parts of the aperture, and of the difk of the fun, but extended to MN; at which he was furprised, fufpecting, and finding by calculation, that it was confiderably broader than it could have been made by rays paffing in right lines by the edges of the object. But the most remarkable circumftauce in this appearance was, that upon the lucid part of the bafe, CM and ND, streaks of coloured light were plainly diftinguished, each being terminated by blue on the fide next to the fhadow, and by red on the other; and though these coloured freaks depended, in fome measure, on the fize of the aperture AB, because they could not be made to appear if it was large, yet he found that they were not li mited, either by it, or by the diameter of the fun's difk. These coloured ftreaks were not all of the fame breadth, but grew narrower as they receded from the fhadow, and were each of them broader the farther the fhadow was received from the opaque body, and also the more obliquely the paper on which they were received was held with refpect to it. He never obferved more than three of thefe ftreaks. 124. To give a clearer idea of thefe coloured ftreaks, he drew the reprefentation of them, exhibited in fig. 9. in which NMO reprefents the broadeft and moft luminous ftreak, next to the dark fhadow X. In the space in which M is placed there was no diftinction of colour, but the space NN was blue, and the space OO, on the other fide of it, was red. The ad ftreak QPR was narrower than the former; and of the 3 parts of which it confifted, the space P had no particular colour, but QQ was a faint blue, and RR a faint red. The 3d freak, TSV, was exactly fimilar to the two others, but narrower than either of them, and the colours ftill fainter. These coloured ftreaks lay parallel to the fhadow of the opaque body; but when it was of an angular form, they did not make the fame acute angles, but were bent into a curve, the outermoft being rounder than those that were next the fhadow, as in fig. 10. If it was an inward angle, as DGH, the coloured ftreaks, parallel to each other, of the two fides croffed without obliterating one another; only the colours were thereby rendered either more intenfe or mixed. The light that formed these coloured ftreaks muft have been bent from the body; but this attentive obferver has likewife given an account of other appearances, which must have been produced by the light bending towards the body. For within the fhadow itfelf he fometimes perceived coloured ftreaks, fometimes more and fometimes fewer; but for this purpose a very ftrong light was requifite, and the opaque body was obliged to be long, and of a moderate breadth. A hair, for inftance, or a fine needle, did not answer fo well as a thin and narrow plate; and the streaks were moft diftinguishable when the fhadow was taken at the greatest distance; but the light grew fainter in proportion. The number of thefe ftreaks within the fhadow was greater in propor. tion to the breadth of the plate. They were two, and fometimes 4, if a thicker rod were made use of. These coloured ftreaks within the fhadow, like thofe on the outfide of it, were bent in an arch, round the acute angles of the fhadow, as in fig. 11. At this angle alfo, as at D, other fhorter lucid ftreaks were vilible, bent in the form of a plume, as they are drawn betwixt D and C. each bending round and meeting again in D. These angular ftreaks appeared, though the plate or rod was not wholly immersed in the beam of light, but the angle of it only; and there were more or fewer in number in proportion to the breadth of the rod of plate. If the plate or rod was very thin, the coU u 2 loured - loured streaks within the fhadow might be feen to bend round from the oppofite fides, and meet one another, as at B. A only represents a fection of the figure, and not a proper termination of the fhadow, and the ftreaks within each fide of it. The coloured freaks without the shadow bend round it in the fame manner. 125. To obtain the more fatisfactory proof that rays of light do not always proceed in ftraight lines, but really bend, in paffing by the edges of bodies, he diverfified the firft of the above mentioned experiments in the following manner. He admitted a beam of light, by a very fmall aperture, into a darkened room as before; and, at a great distance from it, he fixed a plate EF, fig. 12. plate 249. with a small aperture, GH, which admitted only a part of the beam of light, and found, that when the light tranfmitted through this plate was received at fome diftance upon a white paper, the bafe IK was confiderably larger than it could poffibly have been made by rays if fuing in right lines through the two apertures, as the other ftraight lines drawn clofe to their edges plainly demonftrate. 126. That thofe who repeat these experiments may not be disappointed, F. Grimaldi gives the following inftructions. The fun's light must be very intenfe, and the apertures through which it is tranfmitted very narrow, particularly the firft, CD, and the white paper, IK, on which it is received, must be at a confiderable diftance from the hole GH; otherwife it will not much exceed NO, which would be the breadth of the beam of light proceeding in ftraight lines. He generally made the aperture CD or parts of an ancient Roman foot, and the fecond aperture, GH, or; and the distances DG and GN were at leaft 12 fuch feet. The observation was made in fummer, when the atmosphere was free from all vapours, and about mid-day. He alfo made the fame experiment that has been recited from Dr Hooke. 127. DECHALES remarked, that if fmall fcratches be made in any piece of polished metal, and it be expofed to the beams of the fun in a darkened room, it will reflect the rays treaked with colours in the direction of the fcratches; as will appear if the reflected light be received upon white paper. That thefe colours are not produced by refraction, he says, is manifeft; for, if the fcratches be made upon glafs, the effect will be the fame; and in this cafe, if the light had beeen refracted at the furface of the glafs, it would have been tranfmitted through it. From these, and -other obfervations, he concludes that colour does not depend upon the refraction of light only, nor upon many other circumftances, which he enumerates and difcuffes, but upon the intensity of the light only. 128. A phenomenon of vifion, obferved by M. DE LA HIRE, requires to be mentioned here, becaufe the inflection of light feems to fupply the true folution of it, though the author thought otherwife. When we look at a candle, or any luminous body, with our eyes nearly fhut, rays of light are extended from it, in feveral directions, to a confiderable diftance, like the tails of comets. This appearance exercifled the fagacity of Defcartes and Rohault, as well as of our author; but all three feem to have been mistaken with refpect to it. DESCARTES afcribed this effect to certain wrinkles in the furface of the humours of the eye. ROHAULT fays, that when the eye-lids are nearly clofed, the edges of them act like convex lenfes. But our author fays, that the moifture on the furface of the eye, adhering partly to the eye itself, and partly to the edge of the eyelid, makes a concave mirror, and so disperses the rays at their entrance into the eye. But the true reason feems to be, that the light paffing among the eye lashes, in this fituation of the eye, is inflected by its near approach to them, and there. fore enters the eye in a great variety of directions. The two former of thefe opinions are objected to by our author. 129. The experiments of GRIMALDI and Dr HOOKE were not only repeated with the greateft care by Sir ISAAC NEWTON, but carried much farther than they had thought of. So little ufe had been made of Grimaldi's obfervations, that all philofophers before Newton had afcribed the broad fhadows, and even the fringes of light which he defcribed, to the ordinary refraction of the air. Sir Ifaac made in a piece of lead a small hole with a pin, the breadth of which was the 42d part of an inch. Through this hole he let into his darkened chamber a beam of the fun's light; and found that the shadows of hairs, and other flender fubftances placed in it, were confiderably broader than they would have been if the rays of light had paffed by those bodies in right lines. He therefore concluded that they muft have pafied as they are reprefented in fig. 1. Plate CCL. in which X reprefents a fection of the hair, and AD, BE, &c. rays of light paffing by at different diftances, and then falling upon the wall GQ. Since, when the paper which receives the ray is at a great diftance from the hair, the shadow is broad, it mult follow, that the hair acts upon the rays of light at fome confiderable diftance from it, the action being ftrongeft on thofe rays which are at the leaft diftance, and growing weaker and weaker on thofe which are farther off, as reprefented in this figure; and hence it happens, that the fhadow of the hair is much broader, in proportion to the distance of the paper from the hair, when it is nearer than when it is at a greater distance. He found, that it was not material whether the hair was furrounded with air, or with any other pellucid fubftance; for, upon repeating the experiment with the hair be tween wetted glaffes, he found the fhadow at the fame diftances was as big as before. Alfo the fhadows of fcratches made in polished plates of glass, and the veins in the glass, caft the like broad fhadows: fo that this breadth of fhadow muft proceed from fome other cause than the refraction of the air. 130. The fhadows of all bodies, metals, ftones, glafs, wood, horn, ice, &c. in this light were bordered with three parallel fringes, or bands of coloured light, of which that which was contiguous to the fhadow was the broadeft and moft luminous, while that which was the moft remote was the narroweft, and fo faint as not easily to be vifible. It was difficult to diftinguish theie colours, unles |