er; and the vifibility of a star depends very much upon the difference between its own light and that of the ground upon which it is feen. A fixed ftar will be very nearly equally visible with telescopes of very different apertures, provided the magnify. ing power remains the fame. If a comet, or any other heavenly body be viewed through this equa torial telescope, properly rectified, it is feen im mediately by the help of the fame machinery what is its true place in the heavens. Other aftronomical problems may also be solved by it, with great eafe and certainty. M. PINUS proposes to bend the tubes of long telescopes at right angles, fixing a plane mirror in the angle, in order to make them more commodious for viewing objects near the zenith of the obferver; and he gives particular in structions how to make them in this form, efpecially when they are furnished with micrometers. A little plane fpeculum is fometimes placed betwist the laft eye-glafs and the eye in the reflecting telescopes, at an angle of 45°, for the fame purpose.

200. The invention of MICROSCOPES was not much later than that of telescopes; and, according to Borellus, we are indebted for them to Z. JANSEN, and his fon. These two inftruments, though different in their application, are very fimilar; as both affift us in the discovery of objects that we must otherwise have remained unacquaint ed with, by enlarging the angle which they fub tead at the eye.

201. The JANSENS, however, have not always enjoyed that reputation to which they feem to be entitled, with refpect either to the telescope or the microscope. The discovery of the latter has been confidered as more uncertain than that of the former. Many fay, that microfcopes were first used in Germany about 1621. Others affirm, that this inftrument was the contrivance of CORNELIUS DREBELL, no philofopher, but a man of ingenuity, who alfo invented the thermometer. Accord ing to Borellius, Zacharias Jansen and his fon prefented the first microscopes they had conftructed to prince Maurice, and Albert archduke of Auftria. William Borelli, in a letter to his brother Peter, fays, that when he was ambassador in England, in 1619, Cornellius Drebell showed him a microfcope, which he faid was the fame the archduke had given him, and had been made by Janfen himself. This inftrument was fix feet long, confifting of a tube of gilt copper, an inch in diameter, fupport ed by 3 brass pillars in the fhape of dolphins, on a bafe of ebony, on which the small objects were placed.

202. This microscope was evidently a compound one, or rather fomething betwixt a telescope and a microscope, what we fhould now, perhaps, call a MEGALASCOPE. It appears from Jamblicus and Plutarch, quoted by Dr Rogers, that the ancients gave fuch inftruments as they used for magnifying fmall objects the name of dioptra. As fpectacles were certainly in ufe long before the invention of telescopes, one can hardly help concluding, that lenfes must have been made fmaller, and more convex, for the purpose of magnifying minute objects. At what time lenfes were made fo fmall as we now ufe them for magnifying in fingle microfcopes, we have not found. But as this muft have been done

gradually, the only object of inquiry is the invention of the double or compound microfcope'; and this is clearly given, by Borellus to Zacharias Janfen, the inventor of the telescope, or his fon.

203. The invention of compound microscopes is alfo claimed by FONTANA, who claimed that of telescopes; though he did not publish any account of this invention till 1646, notwithstanding he pretended to have made the difcovery in 1618. Euftace Divini made microfcopes with two common object-glaffes, and two plano-convex eyeglaffes joined together on their convex fides fo as to meet in a point. The tube in which they were inclofed was as big as a man's leg, and the eyeglaffes almoft as broad as the palm of a man's hand. Mr Oldenburg, fecretary to the royal fociety, received an account of this inftrument from Rome, and read it at a meeting, Aug. 6, 1668. About this period HARTSOCKER improved tingle microscopes by ufing small globules of glass, made by melting them in the flame of a candle, inftead of the lenses which had before been used for that purpose. By these he first discovered the animalcula in femine mafculino, which gave rife to a new fyftem of generation. A microscope of this kind, confifting of a globule of one 10th of an inch in di ameter, M. Huygens demonftrated to magnify 100 times; and as it is easy to make them of lefs than half a line in diameter, they may be made to magnify 300 times. Were it not for the difficulty of applying objects to thefe magnifiers, the want of light, and the fmall field of diftinct vifion, they would certainly have been the moft perfect of all microscopes.

204. But no man diftinguished himself so much by microscopical discoveries as the famous M. LEEUWENHOEK, though he used only fingle lenfes with fhort foci, preferring diftin&tnefs of vifion to a large magnifying power. M. Leeuwenhoek's microscopes were all fingle ones, each of them confifting of a small double convex glafs, fet in a focket between two filver plates rivetted together, and pierced with a small hole; and the object was placed on the point of a needle, fo contrived as to be placed at any diftance from the lens. If the objects were folid, he faftened them with glue; and if they were fluid, or required to be spread upon glafs, he placed them on a small piece of Mufcovy talc, or glass blown very thin, which he afterwards glued to his needle. He had, however, a different apparatus for viewing the circulation of the blood, which he could fix to the fame microfcopes. The greatest part of his microscopes M. Leeuwenhoek bequeathed to the Royal Society.. They were contained in a fmall Indian cabinet, in the drawers of which were 13 little boxes, in each of which were two microscopes, neatly fitted up in filver; and both the glafs and the apparatus were made with his own hands. The glafs of all thefe lenfes is exceedingly clear, but none of them magnifies fo much as thofe globules which are often ufed in other microfcopes; but Mr Folkes, who examined them, thought that they showed objects with much greater diftin&tnefs, which M. Leeuwenhoek principally valued. His difcoveries, however, are to be afcribed not fo much to the goodnefs of his glaffes, as to his great judgment, acquired by long experience, in ufing them. He

alfe

alfo excelled in his manner of preparing objects for being viewed to the most advantage.

205. Mr BAKER, who alfo examined M. Leeuwenhoek's microscopes, and made a report concerning them to the Royal Society, found that the greatest magnifier among them enlarged the diameter of an object about 160 times, but that all the rest fell much fhort of that power; fo he concluded that M. Leeuwenhoek must have had other microscopes of a much greater magnifying power for many of his discoveries. And it appears, he fays, by many circumftances, that he had fuch microfcopes. It appears from M. Leeuwenhoek's writings, that he was not unacquainted with the method of viewing opaque objects by a small concave reflecting mirror, which was afterwards improved by M. LIEBERKHUN. For, after defcribing his apparatus for viewing eels in glafs tubes, he adds, that he had an inftrument to which he fcrew ed a microscope fet in brass, upon which microscope he fastened a little difh of brafs, probably to affift his eye to fee objects better; for he fays, he had polished the brafs round his microscope as bright as he could, that the light, while he was viewing objects, might be reflected from it as much as poffible. This microscope, with its difh, is conftructed upon principles fimilar to thofe which are the foundation of our fingle microscope by reflection. See MICROSCOPE, 2, 3.

206. In 1702, Mr WILSON made feveral ingenious improvements in the method of using single magnifiers, for the purpose of viewing transparent objects; and his microscope, which is alfo a neceffary part of the folar microscope, is in very general ufe. See MICROSCOPE, § 7.

207. In 1710, Mr ADAMS gave to the Royal Society the following account of his method of making fmall globules for large magnifiers. He took a piece of fine window glass, and cut it with a diamond into as many lengths as he thought proper, not exceeding of an inch in breadth; then, holding one of them between the fore finger and thumb of each hand over a very fine flame, till the glass began to foften, he drew it out till it was as fine as a hair, and broke; then putting each of the ends into the pureft part of the flame, he had two globules, which he could make larger or lefs at pleasure. If they were held long in the flame, they would have spots in them, fo that he drew them out presently after they became round. The stem he broke off as near to the globule as he could, and lodging the remainder between the plates, in which holes were drilled exactly round. the microscope performed to admiration. Through thefe magnifiers the fame thread of very fine muflin appeared 3 or 4 times bigger than it did in the largeft of Mr Wilfon's magnifiers.

208. The ingenious Mr GREY hit upon a very eafy expedient to make very good temporary microfcopes, at a very little expence. They confift of nothing but very small drops of water, taken up with a point of a pin, and put into a small hole made in a piece of metal. Thefe globules of water do not indeed magnify fo much as thofe which re made of glass of the fame fize, because the refractive power of water is not fo great; but the fame purpose will be answered nearly as well by making them fomewhat fmaller. Mr Grey,

obferving that small heterogeneous particles, inclofed in the glass of which microscopes are made, were much magnified when those glasses were looked through, thought of making his microscopes of water that contained living animalcula, to fee how they would look in this new fituation; and he found his scheme to answer beyond his utmoft expectation, fo that he could not even account for their being magnified fo much as they were; for it was much more than they would have been magnified if they had been placed beyond the globule, in the proper place for viewing objects. But MONTUCLA obferves, that, when any object is inclosed within this small transparent globule, the hinder part of it acts like a concave mirror, provided they be fituated between that furface and the focus; and that they are thus magnified above 34 times more than they would have been in the usual way.

209. After the happy execution of the reflecting telescope, it was natural to expect that attempts would alfo be made to render à fimilar service to microfcopes. Accordingly we find two plans of this kind. The firft was that of Dr ROBERT BARKER. His inftrument differs in nothing from the reflecting telescope, excepting the diftance of the two fpeculums, to adapt it to those pencils of rays which enter the microfcope diverging; whereas they come to the telescope from very distant objects nearly parallel to each other. This microscope is not so easy to manage as the common fort. For vifion by reflecting, as it is much more perfect, fo it is far more difficult than that by refraction. Nor is this microscope so useful for any but very small or tranfparent objects. For the object, being between the fpeculum and image, would, if it were large and opaque, prevent a due reflection.

210. Dr SMITH invented a double reflecting microscope, of which a theoretical and practical account is given in the remarks on the ad volume of his Syftem of Optics. As it is conftructed on principles effentially different from all others, and, in the opinion of the ableft judges, is incomparably fuperior to them all, we infert the following practical defcription. Pl. CCLI, fig. 2. is a fection of this microfcope, where ABC and abc are two fpecula, the former concave, and the latter convex, inclofed within the tube DEFG. The fpeculum ABC, is perforated like the fpeculum of a Gregorian telescope; and the object to be magnified is fo placed between the centre and principal focus of that speculum, that the rays flowing from it to ABC are reflected towards an image pq. But before they are united in that image they are received by the convex fpeculum abc, and thence reflected through the hole BC in the vertex of the concave to a fecond image, to be viewed through an eye-glafs . The object may either be fituated between the two fpecula, or, which is perhaps better, between the principal focus and vertex e of the convex fpeculum abc, a small hole being made in its vertex for the incident rays to pass through. When the microscope is used, let the object be included between two little round plates of Mufcovy glafs, fixed in a hole of an oblong brafs plate mn, intended to slide close to the back fide of the convex fpeculum; which muft therefore be ground

flat

flat on that fide, and so thin that the object may come precisely to its computed diftance from the vertex of the fpeculum. The flider must be kept tight to the back of the metal by a gentle fpring. The diftance of the object being thus determined once for all, diftinct vifion to different eyes, and through different eye-glaffes, must be procured by a gentle motion of the little tubes in which thefe glaffes are fixed. Thefe tubes must be made in the ufual form of those that belong to Sir Ifaac Newton's reflecting telescope (fee TELESCOPE), having a fmall hole in the middle of each plate, at the ends of the tube, fituated exactly in each focus of the glass: the ufe of thefe holes and plates is to limit the visible area, and hinder any fraggling rays from entering the eye. To the tube of the eye-glafs is faftened the arm g, on which the adjusting fcrew turns. A fimilar arm u is attached to the fixed tube X, in which the neck of the fcrew turns; and by turning the buttony, the eye tube is moved farther from or nearer to the object, by which means different forts of eyes obtain diftinct vifion. The rays which flow from the object directly through the hole in the concave speculum and through the eye glass, by mixing with the reflected rays, would dilute the image on the retina, and therefore must be intercepted. The little hole in the convex fpeculum is ground conical as in the figure; and a conical folid P, of which the base is larger than the ori. fice in the back of the concave fpeculum, fupported on the flender pillar PQ, is fo placed as to intercept all the direct rays from the eye glafs. All the tubes are ftrongly blacked on their infides, and fo is the conical folid, to hinder all reflection of rays from these objects upon the convex fpeculum. The little bafe, too, of the folid thould be made concave, that whatever light it may still reflect, may be thrown back upon the object; and its back fide being conical and blacked all over, will either absorb or laterally disperse any ftrag gling rays which the concave fpeculum may fcatter upon it, and fo prevent their coming to the eyeglafs. Notwithstanding the interpofition of this conical folid, yet when the eye glass is taken out, diftant objects may be diftinctly seen through the microscope, by rays reflected from the metals, and diverging upon the eye from an image behind the convex fpeculum. But this mixture of foreign rays with those of the object, which is common to all kinds of microfcopes in viewing transparent objects, is usually prevented by placing before the object a thick double convex lens L, to collect the fky-light exactly upon the object. This lens fhould be just fo broad as to fubtend the oppofite angle to that which the concave speculum fubtends at the object. The annular frame of the lens muft be very narrow, and connected to the microscope by two or three flender wires or blades, whofe planes produced may pass through the object, and intercept from it as little sky-light as poffible.

211. In 1738 or 1739, M. LIEBERKHUN made two capital improvements in microscopes, by the invention of the folar microscope, and the microscope for opaque objects. When he was in England in winter 1739, he showed an apparatus of his own making, for each of thefe purposes, to several gentlemen of the Royal Society, as well as to fome

opticians, particularly Mr Cuff in Fleet-ftreet, who took great pains to improve them. (See MICROSCOPE, 7 and 8.) The microscope for opaque objects remedies the inconvenience of having the dark fide of an object next the eye. For by means of a concave fpeculum of filver, highly polished, in the centre of which a magnifying lens is placed, the object is fo ftrongly illuminated, that it may be examined with all imaginable eafe and pleasure. A convenient apparatus of this kind, with 4 different fpeculums and magnifiers of different powers, was brought to perfection by Mr Cuff. This improvement induced M. EPINUS to attend to the fubject; and he produced a very valuable improvement in this inftrument. For by throwing the light upon the fore fide of any object by means of a mirror, before it is tranfmitted through the object-lens, all kinds of objects are equally well reprefented by it.

212. M. EULER proposed a scheme to introduce vifion by reflected light into the magic lantern and folar microscope, by which many inconveniences to which thofe inftruments are fubject might be avoided. But more perfect inftruments are' described under the article MICROSCOPE. Several improvements were made in the apparatus to the folar microscope, by M. ZEIHER, as well as by Meffrs MARTIN, ADAMS, &c. See MICROSCOPE, § 4 and 5.

213. The fmalleft globules, and confequently the greatest magnifiers, for microscopes, that have yet been executed, were made by T. DI TORRE of Naples, who, in 1765, fent 4 of them to the Royal Society. The largeft was only two Paris points in diameter, and it was faid to magnify the diameter of an object 640 times. The ad was the fize of one Paris point, and the 3d was only half of a Paris point, or the 144th part of an inch, in diameter, and was faid to magnify the diameter of an object 2560 times. One of these globules was wanting when they came into the hands of Mr BAKER, to whofe examination they were referred by the Royal Society. This gentleman, fo famous for his skill in microscopes, and his extraordinary expertnefs in managing them, was not able to make any use of these.

214. The construction of a telescope with fix eye-glaffes led M. Euler to a fimilar conftruction of microscopes, by introducing into them fix lenfes, one of which admits of fo fmall an aperture, as to ferve, inftead of a diaphragm, to exclude all foreign light, though, as he fays, it neither leffens the field of view, nor the brightness of objects.

215. The improvement of all dioptric inftruments is greatly impeded by inequalities in the fubftance of the glass of which they are made: but though many attempts have been made to make glass without that imperfection, none of them have been hitherto quite effectual. M. A. D. MERKLEIN, having found fome glafs which had been melted when a building was on fire, and which proved to make excellent object-glaffes for telefcopes, concluded that its peculiar goodness arofe from its not having been disturbed when it was in a fluid ftate; and therefore he proposed to take the metal out of the furnace in iron veffels, of the fame form that was wanted for the glafs; and after it had been perfectly fluid in those vessels, to

let

PART I.

THEORY OF OPTICS.

DIVISIONS of the SCIENCE.

216. THIS part of the science comprehends all that hath been difcovered concerning the various motions of the rays of light, either through different mediums, or when reflected from different fubftances in the fame medium. It contains alfo the rationale of every thing which hath been discovered with regard to vifion; the optical deceptions to which we are liable; and, in fhort, ought to give the reafon of all the known optical phenomena. The fcience is commonly divided into three parts, viz. dioptrics, which contain the laws of refraction, and the phenomena depending upon them; catoptrics, which contain the laws of reflection, and the phenomena which depend on them; and chromatics, which treat of the phenomena of colour. But this definition is of no ufe in a treatife of Optics, as most of the phenomena depend both on refraction and reflection, colour itfelf not excepted. For this reafon, though we have given detached articles under the words DIOPTRICS, CATOPTRICS, and CHROMATICS, we have referved to this place the explanation of the laws of reflection and refraction, by which all optical pheno mena may be accounted for.

SECT. I. Of the PROPERTIES of LIGHT in ge

'neral.

217. UNDER the artice, LIGHT we have given fome account of the controverfies concerning its nature. The opinions of philofophers may, in ge neral, be arranged under thefe two: 1. That the phenomena of vifion and illumination are pro duced by the undulations of an elaftic fluid, much in the fame manner, as found is produced by the undulations of air. This opinion was first offered to the public by DESCARTES, and afterwards by Mr Huygens, and has lately been revived by Mr Euler, who has endeavoured to explain the pheno mena upon mechanical principles.-2d, That the phenomena of vifion are produced by the motion and action of matter emitted from the fhining body with immenfe velocity, moving uniformly in ftraight lines, and acted on by other bodies, fo as to be reflected, refracted, or inflected, in various ways, by means of forces which act on it in the fame manner as on other inert matter.

218. SIR ISAAC NEWTON has shown, in the most incontrovertible manner, the total diffimilarity between the phenomena of vifion and the legitimate confequences of the undulations of an elaftic fluid. All Mr EULER's ingenious and laborious difcuf. fions have not removed, Newton's objections in the fmaileft degree. Sir Ifaac adopts the vulgar opinion, therefore, making light of the difficulties objected to it, because none of them are inconfiftent with the established principles of mechanics, and are merely difficulties of conception to our limited faculties. We need not defpair of being able to decide, by experiment, which of these opinions is neareft to the truth; because there are phenomena

the two hypothefes. Here we shall only give fome account of the legitimate confequences of the vulgar opinion as modified by Sir Isaac Newton, viz.. that light confifts of small particles emitted with very great velocity, and attracted or repelled by other bodies at very fmali diftances.

219. Every visible body emits or reflects inconceivably small particles of matter from each point of its furface, which iffue from it continually (net unlike fparks from a coal) in ftraight lines, and in all directions. These particles entering the eye, and friking upon the retina (a nerve expanded on the back part of the eye to receive their impulses), excite in our minds the idea of light. And as they differ in fubstance, denfity, velocity, or magnitude, they produce in us the ideas of different colours; as will be explained in its proper place.

220. That the particles which conftitute light are exceedingly fmall, appears from hence, viz. that if a hole be made through a piece of paper with a needle, rays of light from every object on the farther fide of it are capable of paffing through it at once without the leaft confution; for any one of those objects may as clearly be feen through it, as if no rays paffed through it from any of the reft. Further, if a candle is lighted, and there be no obftacle in the way to obftruct the progrefs of its rays, it will fill all the fpace within two miles of it every way with luminous particles, before it has loft the leaft fenfible part of its fubftance thereby. That thefe particles proceed from every point of is clear from hence, that wherever a fpectator is the furface of a vifible body, and in all directions, placed with regard to the body, every point of that part of the furface which is turned towards him is vifible to him. That they proceed from the body in right lines, we are affured, because juft fo many and no more will be intercepted in their paflage to any place by an interpofed object, as that object ought to intercept, fuppofing them to come in fuch lines.

221. The VELOCITY with which they proceed from the surface of the visible body is no lets furpriting than their minuteness: the method whereby philofophers eftimate their swiftnefs, is by obfervations made on the eclipses of Jupiter's fatellites; which eclipfes to us appear about 7 minutes fooner than they ought to do by calculation, when the earth is placed between the fun and him, that is, when we are nearest to him; and as much later when the fun is between him and us, at which time we are fartheft from him: from whence it is concluded, that they require about 7 minutes to pass over a space equal to the diftance between the fun and us, which is about 95,000,000 of miles.

222. A ftream of thefe particles iffuing from the furface of a vifible body in one and the fame direction, is called A RAY OF LIGHT. As rays proceed from a vifible body in all directions, they neceffarily become thinner and thinner, continually fpreading themselves as they país along into a larger space, and that in proportion to the fquares of their diftances from the body; that is, at the diftance of 2 fpaces, they are 4 times thinner than they are at one; at the diftance of 3 spaces, 9 times thinner, and fo on: the reafon of which is, because they spread themfelves in a twofold

manner

manner, viz. upwards and downwards, as well as `fidewife.

223. The particles of light are fubject to the laws of attraction of cohefion, like other small bodies; for if a ray of light be made to pafs by the edge of a knife, it will be diverted from its natural course, and be inflected towards the edge of the knife. The like inflection happens to a ray when it enters obliquely into a denfer or rarer fubftance than that in which it was before, in which cafe it is faid to be refracted; the laws of which refraction are the fubject of the following fection.

SECT. II. Of REFRACTION.

224. LIGHT, when proceeding from a luminous body, without being reflected from any opaque fubftance, or inflected by paffing very near one, is invariably found to proceed in ftraight lines, without the leaft deviation. But if it hap. pens to pass obliquely from one medium to another, it always leaves the direction it had before, and affumes a new one; and this change of course is called its refra&ion. After having taken this new direction, it then proceeds invariably in a ftraight line till it meets with a different medium, when it is again turned out of its courfe. It must be obferved, however, that though we may thus cause the rays of light make any number of angles in their course, it is impoffible for us to make them defcribe a curve, except in one fingle cafe, namely, where they pass through a medium, the denfity of which uniformly either increases or decreases. This is the cafe with the light of the celeftial bodies, which paffes downwards through our atmosphere, and likewife with that which is reflected upwards through it by terrestrial objects. In both these cases, it describes a curve of the hyperbolic kind; but at all other times it proceeds in ftraight lines, or in what may be taken for ftraight lines without any fenfible error.

I. Of the CAUSE of REFRACTION, and the LAW by WHICH it is PERFORMED.

225. THE phenomena of refraction are explained by an attractive power in the medium through which light paffes, in the following manner: All bodies being endowed with an attractive force, which is extended to fome diftance beyond their furfaces; when a ray of light paffes out of a rarer into a denfer medium (if this latter has a greater attractive force than the former, as is commonly the cafe), the ray, juft before its entrance, will begin to be attracted towards the denfer medium; and this attraction will continue to act upon it, ill fome time after it has entered the medium; and therefore, if a ray approaches a denfer medium in a direction perpendicular to its furface, its velocity will be continually accelerated during its paffage through the space in which that attraction exerts itfelf; and therefore, after it has paffed that fpace, it will move on, till it arrives at the oppofite fide of the medium, with a greater degree of velocity than it had before it entered. So that in this cafe its velocity only will be altered. Whereas, if a ray enters a deufer medium obliquely, it will not only have its velocity augmented thereby, but its direction will become lefs oblique to the furface. Just as when a ftone is thrown downVol. XVI. PART II.

wards obliquely from a precipice, it falls to the furface of the ground in a direction nearer to a perpendicular one than that with which it was thrown from the hand. Hence a ray of light, in paffing out of a rarer into a denfer medium, is refracted towards the perpendicular; i. e. fuppofing a line drawn perpendicularly to the furface of the medium, through the point where the ray enters, and extended both ways, the ray, in paffing through the furface, is refracted or bent towards the perpendicular line; or, which is the fame thing, the line which it defcribes by its motion after it has passed through the furface, makes a lefs angle with the perpendicular than the line it described before. All which may be illustrated thus:

226. Suppose first, that the ray passes out of a vacuum into the denfer medium ABCD (fig. 3. Pl. 251.) and that the attractive force of each particle in the medium is extended from its refpec tive centre to a distance equal to that which is between the lines AB and EF, or AB and GH; and let KL be the path described by a ray of light in its progress towards the denfer medium; this ray, when it arrives at L, will enter the attractive forces of thofe particles which lie in AB, the furface of the denfer medium, and will therefore cease to proceed any longer in the right line KLM, but will be diverted from its course by being attracted towards the line AB, and will begin to defcribe the curve LN, paffing through the surface AB in fome new direction, as oQ; thereby making a lefs angle with a line, as PR, drawn perpendicu larly through the point N, than it would have done had it proceeded in its firft direction KLM.

227. Whereas we have fuppofed the attractive force of each particle to be extended through a fpace equal to the distance between AB and EF, it is evident that the ray, after it has entered the farface, will fill be attracted downwards, till it has arrived at the line EF; for, till that time, there will not be so many particles above it which will attract it upwards, as below, that will attrac it downwards. So that after it has entered the furface at N, in the direction oQ, it will not proceed in that direction, but will continue to describe a curve, as NS; after which it will proceed straight on towards the oppofite fide of the medium, being attracted equally every way; and therefore will at laft proceed in the direction xST, still nearer the perpendicular PR than before. Now, if we fuppofe ABZY not to be a vacuum, but a rarer medium than the other, the cafe will still be. the fame; but the ray will not be fo much refracted from its rectilineal courfe, becaufe the attrac. tion of the particles of the upper medium being in a contrary direction to that of the attraction of thofe in the lower one, the attraction of the denfer medium will in fome measure be destroyed by that of the rarer. On the contrary, when a ray paffes out of a denfer into a rarer medium, if its direction be perpendicular to the furface of the medium, it will only lose fomewhat of its velocity, in paffing through the spaces of attraction of that medium (that is, the fpace wherein it is attracted more one way than it is another.) If its direction be oblique, it will continually recede from the perpendicular during its paffage, and by these

Z z

means