that he defcribes for this purpose, he must have been under fome deception with respect to it.

72. B. Porta fays, that this effect may be produced by a plane mirror only, and that an ingenious perfon may fucceed in it; but his particular description of a method to produce this extraordinary appearance is by a plane mirror and a concave one combined. KIRCHER alfo fpeaks of the poffibility of exhibiting thefe pendulous images, and fuppofes that they are reflected from the dense air; and the most perfect and pleafing deception depending upon the images in the air, is one of which this writer gives a particular account in his Ars Magna Lucis et Umbra, p. 783. In this cafe the image is placed at the bottom of a hollow polished cylinder, by which means it appears like a real folid fubftance, fufpended within the mouth of the veffel. In this manner, he fays, he once exhibited a reprefentation of the afcenfion of Chrift; when the images were fo perfect, that the fpectators could not be perfuaded, but by attempting to handle them, that they were not real fubftances.

73. Among other amusing things that were either invented or improved by Kircher, was the method of throwing the appearance of letters, and other forms of things, into a darkened room from without, by means of a lens and a plane mirror. The figures or letters were written upon the face of the mirror, and inverted; and the focus of the lens was contrived to fall upon the fcreen or wall that received their images. In this manner, he fays, that with the light of the fun he could throw a plain and distinct image 500 feet.

72. KEPLER first discovered the true reafon of the apparent places of objects feen by reflecting mirrors, as it depends upon the angle which the rays of light, iffuing from the extreme part of an object, make with one another after fuch reflections. In plane mirrors, thefe rays are reflected with the fame degree of inclination to one another that they had before their incidence; but he hows that this inclination is changed in convex and concave mirrors.

75. Mr BOYLE made fome curious obfervations concerning the reflecting powers of differently coloured fubitances. Many learned men, he fays, imagined that fnow affects the eyes, not by a bor. rowed, but by a native light; but having placed a quantity of fnow in a room from which all foreign light was excluded, neither he nor any body elte was able to perceive it. To try whether white bodies reflect more light than others, he held a sheet of white paper in a fun-beam admitted into a darkened room, and obferved that it reflected much more light than a paper of any other colour, a confiderable part of the room being enlightened by it. To fhow that white bo. dies reflect the rays outwards, he adds, that common burning-glaties will not of a long time burn or difcolour white paper. When he was a boy, he fays, it fet him very early upon gueffing at the nature of whitenefs, elpecially as he obferved that the image of the fun was not fo well defined upon white paper as upon black; and as, when he put ink upon the paper, the moisture would be quickly dried up, and the paper, which he could not burn before, would prefently take fire.

76. To fatisfy himself still farther with respect to this fubject, he took a broad and large tile; and having made one half of its furface white and the other black, he expofed it to the fummer fun; and he found, that while the whited part remained cool, the part that was black was grown very hot. He fometimes left part of the tile of its native red; and, after expofing the whole to the fun, obferved, that this part grew hotter than the white, but was not fo hot as the black part. He alfo obferved, that rooms hung with black are not only darker than they would otherwise be, but warmer too. A virtuofo of unfuspected credit acquainted him, that, in a hot climate, he had feen eggs well roafted in a fhort time, by firft blackening the fhells and then expofing them to the fun.

77. We have already taken notice of the remarkable property of lignum nephriticum firft obferved by Kircher. See GUILANDINA, N° 3. Mr Boyle defcribes it to be a whitish kind of wood, brought from Mexico, which the natives call coatl or tlapazatli, and which had been thought to tinge water of a green colour only; but he found it to communicate all kinds of colours. If, fays he, an infufion of this wood be put into a glafs globe, and expofed to a strong light, it will be as colourlefs as pure water; but if it be carried into a place a little shaded, it will be a most beautiful green. In a place ftill more fhaded, it will incline to red; and in a very shady place, or in an opaque veffel, it will be green again. A cup of this remarkable wood was fent to Kircher by the procurator of his fociety at Mexico, and was prefented by him to the emperor as a great curiofity. It is called lignum nephriti cum, because the infufion of it was imagined to be of fervice in diseases of the kidneys and bladder, and the natives of the country where it grows make ufe of it for that purpose.

78. Mr BOYLE corrected several hafty observations of KIRCHER concerning the colours that appear in the infufion of lignum nephriticum, and he diverfified the experiments with it in a very pleafing manner. He firft diftinctly noted the two very different colours which this remarkable tinc ture exhibits by tranfmitted and reflected light. If, fays he, it be held directly between the light and the eye, it will appear tinged (excepting the very top of it, where a sky-coloured circle fometimes appears) almost of a golden colour, except the infution be too ftrong; in which cafe it will be dark or reddish, and requires to be diluted with water. But if it be held from the light, fo that the eye be between the light and the phial, it will appear of a deep lovely blue colour; as will alfo the drops, if any lie on the outfide of the glafs. When a little of this tincture was poured upon a fheet of white paper, and placed in a window where the fun could fhine upon it, he obferved, that if he turned his back upon the fun, the fhadow of his pen, or any fuch flender fubftance, projected upon the liquor, would not be all dark like other fhadows; but that part of it would be curiously coloured, the edge of it next the body being almost of a lively golden colour, and the more remote part blue.

79. Sufpecting that the tinging particles aboundTtz

ed

ed with falts, whofe texture, and the colour thence arifing, would probably be altered by acids, he poured into a fmall quantity of it a very little Spirit of vinegar, and found that the blue colour immediately vanished, while the golden one remained, on whichever fide it was viewed with refpect to the light. Upon this he imagined, that as the acid falts of the vinegar had been able to deprive the liquor of its blue colour, a fulphu reous falt, which is of contrary nature, would deftroy their effects; and having placed himself betwixt the window and the phial, and let fall into the fame liquor a few drops of oil of tartar per deliquium, he found that it was immediately reftored to its former blue colour, and exhibited all the fame phenomena which it had done at the first. . 80. Having brought a round long-necked phial, filled with this tincture, into a darkened room, into which a beam of the fun was admitted by a mall aperture; and holding the phial fometimes near the fun-beams, and fometimes partly in them and partly out of them, changing alfo the pofition of the glafs, and viewing it from feveral parts of the room, it exhibited a much greater variety of colours than it did in an enlightened room. Befides the ufual colours, it was red in fome places and green in others, and within were intermediate colours produced by the different degrees and odd mixtures of light and fade. Mr Boyle pbferved the difference between reflected and tranfmitted light alfo in gold, though no perfon explained the cause of these effects before Sir Ifaac Newton. He took a piece of leaf-gold, and holding it betwixt his eye and the light, obferved that it did not appear of a golden colour, but of a greenish blue. He alfo obferved the fame change of colour by candle-light; but the experiment did not fucceed with a leaf of filver.

81. The conftitution of the atmosphere and of the fea, we find, by obfervations made in later periods, to be fimilar to that of this infufion; for the blue rays, and others of a faint colour, do not penetrate fo far into them as the red, and others of a stronger colour: but what this conftitution is, which is common to them all, deferves to be inquired into. For almoft all other tinctures, and this of lignum nephriticum too, after fome change made in it by Mr Boyle, as well as all other femitranfparent coloured fubftances, as glafs, appear of the fame hue in all pofitions of the eye. To increase or diminish the quantity makes no difference, but to make the colour deeper or more dilute. 82. The firft diftinct account of the colours cxhibited by thin plates of various fubftances, are met with among the obfervations of Mr BOYLE. To fhow the chemists that colours may be made to appear to vanifh, where there is no acceffion or change either of the fulphureous, the faline, or the mercurial principle of bodies, he obferves, that all chemical effential oils, as well as good fpirit of wine, being fhaken till they rife in bubbles, appear of various colours, which immediately vanish when the bubbles burst; fo that a colourless liquor may be made to exhibit a variety of colours, and lofe them in a moment, without zny change in its effential principles. He then mentions the colours that appear in. bubbles of foap and water, and alfo in turpentine, He

fometimes got glafs blown fo thin as to exhibit fimilar colours; and obferves, that a feather, of a proper fhape and fize, and also a black ribbon, held at a proper diftance between his eye and the fun, fhowed a variety of little rainbows, as he calls them, with very vivid colours, none of which were conftantly to be seen in the fame objects.

83. A much greater number of observations were made on this fubject by Dr HOOKE. He promised, at a meeting of the fociety on the 7th of March 1672, to exhibit, at their next meeting, fomething which had neither reflection nor refrac tion, and yet was diaphanous. Accordingly he produced the famous coloured bubble of foap and water, of which such admirable use was afterwards made by Sir Isaac Newton, but which Dr Hooke and his contemporaries feem to have overlooked in Mr Boyle's treatife on colours, though it was published nine years before. By the help of a fmall glafs pipe, there were blown several small bubbles, out of a mixture of foap and water; where, at firft, they appeared white and clear; but, after fome time, the film of water growing thinner, there appeared upon it all the colours of the rainbow: First a pale yellow; then orange, red, purple, blue, green, &c. with the fame feries of colours repeated; in which it was farther obfervable, that the first and laft feries were very faint, and that the middlemoft was very bright. After these colours had paffed over the changes above mentioned, the film of the bubble began to appear white again; and presently, in feveral parts of this fecond white film, there appeared several holes, which by degrees grew very big, feveral of them running into one another. Dr Hooke fays it is ftrange, that though both the encompaffing and encompaffed air have furfaces, yet he could not obferve that they afforded either reflection or refraction, which all the other parts of the encompaffed air did. This experiment, he adds, at first fight, may appear very trivial; yet as to the finding out the nature and cause of reflection, refraction, colours, congruity and incongruity, and feveral other properties of bodies, he looked upon it as one of the moft inftructive. He adds, that that which gives one colour by reflection, gives another by trajection, like the tincture of lignum nephriticum.

84. Dr HOOKE was the first to obferve, if not to defcribe, the beautiful colours that appear in thin plates of mufcovy glafs. Thefe, he fays, are very beautiful to the naked eye, but much more when viewed with a microfcope. With this he could perceive that thefe colours were ranged in rings furrounding the white fpecks or flaws in this thin substance, that the order of the colours was the fame as in the rainbow, and that they were often repeated ten times, but they were difpofed as in the outer bow, and not the inner. Some of them were alfo much brighter and broader than others. He alfo obferved, that if there was a place where the colours were very broad, and confpicuous to the naked eye, they might be made, by preffing the place with the finger, to change places, and move from one part to another. Laftly, he obferved, that if great care be ufed, this fubftance may be split into plates of or of an inch in diameter, each of which wil

appear

having precifely the fame intenfity of light. It was alfo neceffary that the hole Q fhould be a little higher than P, in order that the two images fhould be at the fame height, and near one another: and that the screen GH be exactly vertical, in order that the direct and reflected beams may fall upon it with the fame inclination; fince, otherwife, though the two lights were perfectly equal, they would not illuminate the fcreen equally. This difpofition ferves to answer another im portant condition in thefe experiments; for the direct ray QS muft be of the fame length with the fum of the incident and reflected rays PO and OR, that the quantity of light introduced into the room may be fenfibly proportional to the fizes of the apertures.

92. Before we recite the refult of his experiments to measure the quantity of light loft by reflection, it is proper to mention fome which were made previous to them on the diminution of light by reflection, and the tranfmiffion of it to confiderable distances through the air, by BUFFON, when he was constructing his machine to burn at great distances. See BURNING-GLASS, 16. Receiving the light of the fun in a dark place, and comparing it with the fame light of the fun reflected by a mirror, he found, that at fmall diftances, as 4 or 5 feet, about one half was loft by reflection; as he judged by throwing two reflected beams upon the fame place, and comparing them with a beam of direct light; for then the intensity of them both seemed to be the fame. Having received the light at greater diftances, as at 100, 200, and 300 feet, he could hardly perceive that it loft any of its intenfity by being transmitted through fuch a space of air. He afterwards made the fame experiments with candles, thus: He placed himfelf opposite to a looking-glafs, with a book in his hand, in a room perfectly dark; and having one candle lighted in the next room, at the diftance of about 40 feet, he had it brought nearer to him by degrees, till he was just able to diftinguish the letters of the book, which was then 24 feet from the candle. He then received the light of the candle, reflected by the looking-glass, upon his book, carefully excluding all the light that was reflected from any thing elfe; and he found that the diftance of the book from the candle, including the distance from the book to the looking-glass (which was only half a foot) was in all 15 feet. He repeated the experiment feveral times, and always with nearly the fame refult; and therefore concluded, that the quantity of direct light is to that of reflected as 576 to 225; fo that the light of five candles reflected from a plane mirror is about equal to that of two candles. From thefe experiments it appeared, that more light was loft by reflection of the candles than of the fun, which M. Buffon thought was owing to this circumftance, that the light iffuing from the candles diverges, and therefore falls more obliquely upon the mirror than the light of the fun, the rays of which are nearly parallel.

93. The experiments and obfervations of Count Buffon are curious; though they fall far short of thofe of M. Bouguer. We fhall begin with thofe which he made to afcertain the difference in the quantity of light reflected by glafs and polifhed

metal. Ufing a smooth piece of glass one line in thickness, he found, that when it was placed at an angle of 15° with the incident rays, it reflected 628 parts of 1000 which fell upon it; at the fame time that a metallic mirror, which he tried in the fame circumstances, reflected only 561 of them. At a lefs angle of incidence much more light was reflected; so that at an angle of 3° the glafs reflected 700 parts, and the metal fomething lefs. Trying the reflection of bodies that were not polished, he found that a piece of white plafter, at an angle of 75° with the incident rays, reflected part of the light that it received from a candle 9 inches from it. White paper reflected in the fame proportion; but at 3 inches, they both reflected 150 parts of 1000 that were incident. Proceeding to make farther observations on reflected light, he premises the two following theorems: 94. I. When the luminous body is at an infinite diftance, and its light is received by a globe, the furface of which has a perfect polish, and absorbs no light, it reflects the light equally in all direc tions, provided it be received at à considerable dif. tance. He only excepts the place where the shadow of the globe falls: but this, he fays, is no more than a fingle point, with respect to the immenfity of the spherical furface which receives its light.

95. II. The quantity of light reflected in one certain direction will always be exactly the same, whether it be reflected by a very great number of fmall polifhed hemifpheres, by a less number of larger hemifpheres, or by a fingle hemisphere, provided they occupy the fame bafe, or cover the fame ground plan.

96. The ufe he proposes to make of thefe theorems is, to affift him in diftinguishing whether the light reflected from bodies be owing to the extinction of it within them, or whether the roughness or eminences which cover them have not the fame effect with the fmall polished hemifpheres above mentioned. He begins with obferving, that, of the light reflected from mercury, at leaft is loft, and that probably no fubftances reflect more than this. The rays were received at an angle of 11 degrees of incidence, that is measured from the furface of the reflecting body, and not from the perpendicular, which, he fays, is what we are from this place to understand whenever he mentions the angle of incidence.

97. The moft ftriking observations he made relate to the very great difference in the quantity of light reflected at different angles of incidence. In general, he fays, that reflection is ftronger at Imall angles of incidence, and weaker at large ones. The difference is exceffive when the rays ftrike the furface of transparent substances, with different degrees of obliquity; but it is almost as great in fome opaque fubftances, and it was always more or lefs fo in every thing that he tried. He found the greatest inequality in black marble; in which he was aftonished to find, that, with an angle of 3° 35' of incidence, though not perfectly polished, yet it reflected almost as well as quickfilver. Of 1000 rays which it received, it returned 600; but when the angle of incidence was 14° it reflected only 156; when it was 30°, it reflected 51; and when it was 80°, it reflected only 23. Similar experiments made with metallic mirrors

always

always gave the differences much less confiderable. The greatest was hardly ever an 8th or a 9th part of it, but they were always in the fame way.

98. The great difference between the quantity of light reflected from the furface of water, at different angles of incidence, is truly furprising; but our author obferves, that this difference was greater when the smallest inclinations were compared with those which were near to a right angle. In very fmall angles, water reflects nearly of the direct light. In ftill weather, on the brink of a lake oppofite to the fun, the reflected light is,, or fometimes a greater proportion: The direct light of the fun diminishes gradually as it approaches the horizon, while the reflected light at the fame time grows ftronger: fo that there is a certain elevation of the fun, in which the united force of the direct and reflected light will be the greateft poffible, and this he fays is 12° or 13°. On the other hand, the light reflected from water at great angles of incidence is extremely fmall. When the light was perpendicular, it reflected no more than the 37th part that quickfilver does in the fame circumftances; for it did not appear, from all his observations, that water reflects more than the 60th or 55th part of perpendicular light. When the angle of incidence was 50°, the light reflected from the surface of water was about the 321 part of that which mercury reflected; and as the reflection from water increases with the diminution of the angle of incidence, it was twice as ftrong in proportion at 39°; for it was then the 16th part of the quantity that mercury reflected. 99. Partly by obfervation and partly by calculation, he drew up the following table of the quantity of light reflected from the furface of water (which he pitched upon as the most commodious fluid), at different angles with the surface.

101. Upon pouring a quantity of water into a veffel containing quickfilver, there will be two images of any objects seen by reflection from them, one at the furface of the water, and the other at that of the quickfilver. In the largest angles of incidence, the image at the furface of the water will difappear, when it is about a 60th or 80th part less luminous than the image at the surface of the quick filver. Depreffing the eye, the image on the water will grow ftronger, and that on the quickfilver weaker in proportion; till at laft the latter will be incomparably weaker than the former, and at an angle of about 10° they will be equally luminous. According to the table, 3 of the incident rays are reflected from the water at this angle of 10 degrees. At the furface of the mercury they were reduced to 500; and of thefe, part being reflected back upon it from the under furface of the water, only 333 remained to make the image from the mercury.

102. It has been obferved by Mr EDWARDS and others (fee Phil. Trans. vol. 53, p. 229), that there is a remarkably ftrong reflection into water, with refpect to rays iffuing from the water; and perfons under water have seen images of things in the air in a manner peculiarly diftinct and beautiful: but this fact had not been obferved with a fufficient degree of attention, till M. BOUGUER did it. In this cafe, he says, that from the smallest angles of incidence to a certain number of degrees, the greateft part of the rays are reflected, perhaps in as great a proportion as at the surface of metallic mirrors, or of quickfilver; while the other part, which does not efcape into the air, is extinguished or absorbed; so that the furface of the transparent body appears opaque on the infide. If the angle of incidence be increased only a few degrees, the ftrong reflection ceases altogether, a great number of rays escape into the air, and very few are abforbed or extinguished. In proportion as the angle of incidence is farther increased, the quantity of the light reflected becomes lefs and lefs; and when it is near 90°, almost all the rays escape out of the transparent body, its furface lofing almoft all its power of reflection, and becoming almost as transparent as in other cafes, or when the light falls upon it from without. The property belonging to the furfaces of tranfparent bodies, of absorbing or extinguishing the rays of light, is truly remarkable, and had not been noticed by any perfon before M. Bouguer. It had been conjectured by Sir Ifaac Newton, that rays of light become extinct only by impinging upon the folid parts of bodies; but thefe obfervations of M. Bouguer fhow that the fact is quite otherwise, and that this effect is to be attributed, not to the folid parts of bodies, but to fome power lodged at the furfaces of bodies only, and therefore probably the fame with that which reflects, refracts, and inflects the light.

103. One of the above-mentioned obfervations, viz. all the light being reflected at certain angles of incidence, from air into denfer fubftances, had frequently been made, especially in glass prifms; fo that Sir Ifaac Newton made ufe of one of them, instead of a reflecting mirror, in the conftruction of his telescope. If a beam of light fall upon the air from within these prifms, at an angle

of