LIGHT

, that principle by which objects are made perceptible to our sense of seeing; or the sensation occasioned in the mind by the view of luminous objects.

The nature of Light has been a subject of speculation from the first dawnings of philosophy. Some of the earliest philosophers doubted whether objects became visible by means of any thing proceeding from them, or from the eye of the spectator. But this opinion was qualified by Empedocles and Plato, who maintained, that vision was occasioned by particles continually flying off from the surfaces of bodies, which meet with others proceeding from the eye; while the effect was ascribed by Pythagoras solely to the particles proceeding from the external objects, and entering the pupil of the eye. But Aristotle defines Light to be the act of a transparent body, considered as such: and he observes that Light is not fire, nor yet any matter radiating from the luminous body, and transmitted through the transparent one.

The Cartesians have refined considerably on this notion; and hold that Light, as it exists in the luminous body, is only a power or faculty of exciting in us a very clear and vivid sensation; or that it is an invisible fluid present at all times and in all places, but requiring to be set in motion, by a body ignited or otherwise properly qualified to make objects visible to us.

Father Malbranche explains the nature of Light from a supposed analogy between it and sound.— Thus he supposes all the parts of a luminous body are in a rapid motion, which, by very quick pulses, is constantly compressing the subtle matter between the luminous body and the eye, and excites vibrations of pression. As these vibrations are greater, the body appears more luminous; and as they are quicker or slower, the body is of this or that colour.

But the Newtonians maintain, that Light is not a fluid per se, but consists of a great number of very small particles, thrown off from the luminous body by a repulsive power with an immense velocity, and in all directions. And these particles, it is also held, are emitted in right lines: which rectilinear motion they preserve till they are turned out of their path by some of the following causes, viz, by the attraction of some other body near which they pass, which is called inflection; or by paffing obliquely through a medium of different density, which is called refraction; or by being turned aside by the opposition of some intervening body, which is called reflection; or, lastly, by being totally stopped by some substance into which they penetrate, and which is called their extinction. A succession of these particles following one another, in an exact straight line, is called a ray of Light; and this ray, in whatever manner its direction may be changed, whether by refraction, reflection, or inflection, always preserves a rectilinear course till it be again changed; neither is it possible to make it move in the arch of a circle, ellipsis, or other curve. For the above properties of the rays of Light, see the several words, Refraction, Reflection, &c.

The velocity of the particles and rays of Light is truly astonishing, amounting to near 2 hundred thousand miles in a second of time, which is near a million times greater than the velocity of a cannon-ball. And this amazing motion of Light has been manifested in various ways, and sirst, from the eclipses of Jupiter's satellites. It was first observed by Roemer, that the eclipses of those satellites happen sometimes sooner, and sometimes later, than the times given by the tables of them; and that the observation was before or after the computed time, according as the earth was nearer to, or farther from Jupiter, than the mean distance. Hence Roemer and Cassini both concluded that this circumstance depended on the distance of Jupiter from the earth; and that, to account for it, they must suppose that the Light was aboút 14 minutes in crossing the earth's orbit. This conclusion however was afterward abandoned and attacked by Cassini himself. But Roemer's opinion sound an able advocate in Dr. Halley; who removed Cassini's difficulty, and left Roemer's conclusion in its full force. Yet, in a memoir presented to the Academy in 1707, M. Maraldi endeavoured to strengthen Cassini's arguments; when Roemer's doctrine found a new defender in Mr. Pound. See Philos. Trans. number 136, also Abridg. vol. 1, pa. 409 and 422, and Groves, Phys. Elem. number 2636. It has since been found, by repeated experiments, that when the earth is exactly between Jupiter and the sun, his satellites are seen e<*>lipsed about 8 1/4 minutes sooner than they could be according to the tables; but when the earth is nearly in the opposite point of its orbit, these eclipses happen about 8 1/4 minutes later than the tables prcdict them. Hence| then it is certain that the motion of Light is not instantaneous, but that it takes up about 16 1/2 minutes of time to pass over a space cqual to the diameter of the earth's orbit, which is at least 190 millions of miles in length, or at the rate of near 200,000 miles per second, as above-mentioned. Hence therefore Light takes up about 8 1/4 minutes in passing from the sun to the earth; so that, if he should be annihilated, we would see him for 8<*> minutes after that event should happen; and if he were again created, we should not see him till 8 1/4 minutes afterwards. Hence also it is casy to know the time in which Light travels to the earth, from the moon, or any of the other planets, or even from the sixed stars when their distances shall be known; these distances however are so im mensely great, that from the nearest of them, supposed to be Sirius, the dog-star, Light takes up many years to travel to the earth: and it is even suspected that there are many stars whose Light have not yet arrived at us since their creation. And this, by-the-bye, may perhaps sometimes account for the appearance of new stars in the heavens.

It may be just observed that Galileo first conceived the notion of measuring the velocity of Light; and a description of his contrivance for this purpose, is in his Treatise on Mechanics, pa. 39. He had two men with Lights covered; the one was to observe when the other uncovered his Light, and to exhibit his own the moment he perceived it. This rude experiment was tried at the drstance of a mile, but without success, as may naturally be imagined: and the members of the Academy Del Cimento repeated the experiment, and placed their observers, to as little purpofe, at the distance of 2 miles.

But our excellent astronomer, Dr. Bradley, afterwards found nearly the same velocity of Light as Roemer, from his accurate observations, and most ingenious theory, to account for some apparent motions in the fixed stars; for an account of which, see Aberration of Light. By a long series of these observations, he found the difference between the true and apparent place of several fixed stars, for different times of the year; which difference could no otherwise be accounted for, than from the progressive motion of the rays of Light. From the mean quantity of this difference he ingeniously found, that the ratio of the velosity of Light to the velocity of the earth in its orbit, was as 10313 to 1, or that Light moves 10313 times faster than the earth moves in its orbit about the sun; and as this l<*>tter motion is at the rate of 18 11/12 miles per second nearly, it follows that the former, or the velocity of Light, is at the rate of about 195000 miles in a second; a motion according to which it will require just 8′ 7″ to move from the sun to the earth, or about 95 millions of miles.

It was also inferred, from the foregoing principles, that Light proceeds with the same velocity from all the stars. And hence it follows, if we suppose that all the stars are not equally distant from us, as many arguments prove, that the motion of Light, all the way it passes through the immense space above our atmosphere, is equable or uniform. And since the different methods of determining the velocity of Light thus agree in the result, it is reasonable to conclude that, in the same medium, Light is propagated with the same velocity after it has been reflected, as before.

For an account of Mr. Melville's hypothesis of the different velocities of differently coloured rays, see Colour.

To the doctrine concerning the materiality of Light, and its amazing velocity, several objections have been made; of which the moit considerable is, That as rays of Light are continually passing in different directions from every visible point, they must necessarily interfere with each other in such a manner, as entirely to confound all distinct perception of objects, if not quite to destroy the whole sense of seeing: not to mention the continual waste of substance which a constant emission of particles must occasion in the luminous body, and thereby since the creation must have greatly diminished the matter in the sun and stars, as well as increased the bulk of the earth and planets by the vast quantity of particles of Light absorbed by them in so long a period of time.

But it has been replied, that if Light were not a body, but consisted in mere pression or pulsion, it could never be propagated in right lines, but would be continually inflected ad umbram. Thus Sir I. Newton: “A pressure on a fluid medium, i. e. a motion propagated by such a medium, beyond any obstacle, which impedes any part of its motion, cannot be propagated in right lines, but will be always inflecting and diffusing itself every way, to the quiescent medium beyond that obstacle. The power of gravity tends downwards; but the pressure of water arising from it tends every way with an equable force, and is propagated with equal ease and equal strength, in curves, as in strait lines. Waves, on the surface os the water, gliding by the extremes of any very large obstacle, inflect and dilate themselves, still diffusing gradually into the quiescent water beyond that obstacle. The waves, pulses, or vibrations of the air, wherein sound consists, are manifestly inflected, though not so considerably as the waves of water; and sounds are propagated with equal ease, through crooked tubes, and through strait lines; but Light was never known to move in any curve, nor to inslect itself ad umbram.”

It must be acknowledged, however, that many philosophers, both English and Foreignérs, have recurred to the opinion, that Light consists of vibrations propagated from the luminous body, through a subtle etherial medium.

The ingenious Dr. Franklin, in a letter dated April 23, 1752, expresses his dissatisfaction with the doctrine, that Light consists of particles of matter continually driven off from the sun's surface, with so enormous a swiftness. “Must not, says he, the smallest portion conceivable, have, with such a motion, a force exceeding that of a 24 pounder discharged from a cannon? Must not the sun diminish exceedingly by such a waste of matter; and the planets, instead of drawing nearer to him, as some have feared, recede to greater distances through the lessened attraction? Yet these particles, with this amazing motion, will not drive before them, or remove, the least and slightest dust they meet with; and the sun appears to continue of his ancient dimensions, and his attendants move in their ancient orbits.” He therefore conjectures that all the phenomena of| Light may be more properly solved, by supposing all space filled with a subtle elastic fluid, which is not visible when at rest, but which, by its vibrations, affects that fine sense in the eye, as those of the air affect the grosser organs of the ear; and even that different degrees of the vibration of this medium may cause the appearances of different colours. Franklin's Exper. and Observ. 1769, pa. 264.

The celebrated Euler has also maintained the same hypothesis, in his Theoria Lucis & Colorum. In the summary of his arguments against the common opinion, recited in Acad. Berl. 1752, pa. 271, besides the objections above-mentioned, he doubts the possibility, that particles of matter, moving with the amazing velocity of Light, should penetrate transparent substances with so much ease. In whatever manner they are transmitted, those bodies must have pores, disposed in right lines, and in all possible directions, to serve as canals for the passage of the rays: but such a structure must take away all solid matter from those bodies, and all coherence among their parts, if they do contain any solid matter.

Doctor Horsley, now Bp. of Rochester, has taken considerable pains to obviate the difficulties started by Dr. Franklin. Supposing that the diameter of each particle of Light does not exceed one millionth of one millionth of an inch, and that the density of each particle is even three times that of iron, that the Light of the sun reaches the earth in 7′, at the distance of 22919 of the earth's semidiameters, he calculates that the momentum or force of motion in each particle of Light coming from the sun, is less than that in an iron ball of a quarter of an inch diameter, moving at the rate of less than an inch in 12 thousand millions of millions of years. And hence he concludes, that a particle of matter, which probably is larger than any particle of Light, moving with the velocity of Light, has a force of motion, which, instead of exceeding the force of a 24 pounder discharged from a cannon, is almost infinitely less than that of the smallest shot discharged from a pocket pistol, or less than any that art can create. He also thinks it possible, that Light may be produced by a continual emission of matter from the sun, without any such waste of his substance as should sensibly contract his dimensions, or alter the motions of the planets, within any moderate length of time. In proof of this, he observes that, for the production of any of the phenomena of Light, it is not necessary that the emanation from the sun should be continual, in a strict mathematical sense, or without any interval; and likewise that part of the Light which issues from the sun, is continually returned to him by reflection from the planets, as well as other Light from the suns of other systems. He proceeds, by calculation, to shew that in 385,130,000 years, the sun would lose but the 13232d part of his matter, and consequently of the gravitation towards him, at any given distance; which is an alteration much too small to discover itself in the motion of the earth, or of any of the planets. He farther computes that the greatest stroke which the retina of a common eye sustains, when turned directly to the sun in a bright day, does not exceed that which would be given by an iron shot, a quarter of an inch diameter, and moving only at the rate of 16 1/6 inches in a year; whereas the o<*> dinary stroke is less than the 2084th part of this. See Philos. Trans. vol. 60 and 61.

In answer to the difficulty respecting the non-interference of the particles of Light with each other, Mr. Melville observes (Edinb. Ess. vol. 2), there is probably no physical point in the visible horizon, that does not send rays to every other point, unless where opaque bodies interpose. Light, in its passage from one system to another, often passes through torrents of Light issuing from other suns and systems, without ever interfering, or being diverted from its course, either by it, or by the particles of that elastic medium, which it has been supposed by some is diffused through all the mundane space. To account for this fact, he supposes that the particles of Light are incomparably rare, even when they are the most dense, or that their diameters are incomparably less than their distance from one another: which obviates the objection urged by Euler and others against the materiality of Light, from its influence in disturbing the freedom and perpetuity of the celestial motions. Boscovich and some others solve the difficulty concerning the non-interference of the particles of Light, by supposing that each particle is endued with an insuperable impulsive force; but in this case, their spheres of impulsion would be more likely to interfere, and on that account they be more liable to disturb one another.

M. Canton shews (Philos. Trans. vol. 58, p. 344), that the difficulty of the interference will vanish, if a very small portion of time be allowed between the emission of every particle and the next that follows in the same direction. Suppose, for instance, that a lucid point in the sun's surface emits 150 particles in a second of time, which, he observes, will be more than sufficient to give continual Light to the eye, without the least appearance of intermission; yet still the particles of such a ray, on account of their great velocity, will be more than 1000 miles behind each other, a space sufficient to allow others to pass in all directions without any perceptible interruption. And if we adopt the conclusions drawn from the experiments on the duration of the sensations excited by Light, by the chevalier D'Arcy, in the Acad. Scienc. 1765, who states it at the 7th part of a second, art interval of more than 20,000 miles may be admitted between every two successive particles.

The doctrine of the materiality of Light is farther confirmed by those experiments, which shew, that the colour and inward texture of some bodies are changed by being exposed to the Light.

Of the Momentum, or Force, of the Particles of Light. Some writers have attempted to prove the materiality of Light, by determining the momentum of their component particles, or by shewing that they have a force so as, by their impulse, to give motion to light bodies. M. Homberg, Ac. Par. 1708, Hist. pa. 25, imagined, that he could not only disperse pieces of amianthus, and other light substances, by the impulse of the solar rays, but also that by throwing them upon the end of a kind of lever, connected with the spring of a watch, he could make it move sensibly quicker; from which, and other experiments, he inferred the weight of the particles of Light. And Hartsoecker made preten-| sions of the same nature. But M. Du Fay and M. Mairan made other experiments of a more accurate kind, without the effects which the former had imagined, and which even proved that the effects mentioned by them were owing to currents of heated air produced by the burning glasses used in their experiments, or some other causes which they had overlooked.

However, Dr. Priestley informs us, that Mr. Michell endeavoured to ascertain the momentum of Light with still greater accuracy, and that his endeavours were not altogether without success. Having found that the instrument he used, acquired, from the impulse of the rays of light, a velocity of an inch in a second of time, be inferred that the quantity of matter contained in the rays falling upon the instrument in that time, amounted to no more than the 12 hundred millionth part of a grain. In the experiment, the Light was collected from a surface of about 3 square feet; and as this surface reflected only about the half of what fell upon it, the quantity of matter contained in the solar rays, incident upon a square foot and a half of surface, in a second of time, ought to be no more than the <*>2 hundred millionth part of a grain, or upon one square foot only, the 18 hundred millionth part of a grain. But as the density of the rays of Light at the surface of the sun, is 45000 times greater than at the earth, there ought to issue from a square foot of the sun's surface, in one second of time, the 40 thousandth part of a grain of matter; that is, a little more than 2 grains a day, or about 4,752,000 grains, which is about 670 pounds avoirdupois, in 6000 years, the time since the creation; a quantity which would have shortened the sun's semidiameter by no more than about 10 feet, if it be supposed of no greater denfity than water only.

The Expansion or Extension of any portion of Light, is inconceivable. Dr. Hook shews that it is as unlimited as the universe; which he proves from the immense distance of many of the fixed stars, which only become visible to the eye by the best telescopes. Nor, adds he, a<*> they only the great bodies of the sun or stars that are thus liable to disperse their Light through the vast expanse of the universe, but the smallest spark of a lucid body must do the same, even the smallest globule struck from a steel by a flint.

The Intensity of different Lights, or of the fame Light in different circumstances, affords a curious subject of speculation. M. Bouguer, Traité de Optique, found that when one Light is from 60 to 80 times less than another, its presence or absence will not be perceived by an ordinary eye; that the moon's Light, when she is 19° 16 high above the horizon, is but about 1/3 of her Light at 66° 11′ high; and when one limb just touched the horizon, her Light was but the 2000th part of her Light at 66° 11 high; and that hence Light is diminished in the proportion of 3 to 1 by traversing 7469 toises of dense air. He found also, that the centre of the sun's difc is considerably more luminous than the edges of it; whereas both the primary and secondary planets are more luminous at their edges than near their centres: That, farther, the Light of the sun is about 300,000 times greater than that of the moon; and therefore it is no wonder that philosophers have had so little success in their attempts to collect the Light of the moon with burning-glasses; for, should one of the largest of them even increase the Light 1000 times, it will still leave the Light of the moon in the focus of the glass, 300 times less than the intensity of the common Light of the sun.

Dr. Smith, in his Optics, vol. 1, pa. 29, thought he had proved that the Light of the full moon would be only the 90,900th part of the full day Light, if no rays were lost at the moon. But Mr. Robins, in his Tracts, vol. 2, pa. 225, shews that this is too great by one half. And Mr. Michell, by a more easy and accurate mode of computation, found that the density of the sun's Light on the surface of the moon is but the 45,000th part of the density at the sun; and that therefore, as the moon is nearly of the same apparent magnitude as the sun, if she reflected to us all the Light received on her surface, it would be only the 45,000th part of our day Light, or that which we receive from the sun. Admitting therefore, with M. Bouguer, that the moon Light is only the 300,000th part of the day or sun's Light, Mr. Michell concludes that the moon reflects no more than between the 6th and 7th part of what she receives.

Dr. Gravesande says, a lucid body is that which emits or gives fire a motion in right lines, and makes the difference between Light and heat to consist in this, that to produce the former, the fiery particles must enter the eye in a rectilinear motion, which is not required in the latter: on the contrary, an irregular motion seems more proper for it, as appears from the rays coming directly from the sun to the tops of mountains, which have not near that effect with those in the valley, agitated with an irregular motion, by several reflections.

Sir I. Newton observes, that bodies and Light act mutually on one another; bodies on Light, in emitting, reflecting, refracting, and inflecting it; and Light on bodies, by heating them, and putting their parts into a vibrating motion, in which heat principally consists. For all fixed bodies, he observes, when heated beyond a certain degree, do emit Light, and shine; which shining &c appears to be owing to the vibrating motion of their parts; and all bodies, abounding in earthy and sulphureous particles, if sufficiently agitated, emit Light, which way soever that agitation be effected. Thus, sea water shines in a storm; quicksilver, when shaken in vacuo; cats or horses, when rubbed in the dark; and wood, fish, and flesh, when putrefied.

Light proceeding from putrescent animal and vegetable substances, as well as from glow-worms, is mentioned by Aristotle. And Bartholin mentions four kinds of luminous insects, two of which have wings: but in hot climates it is said they are found in much greater numbers, and of different species. Columna observes, that their Light is not extinguished immediately on the death of the animal. The first distinct account that occurs of Light proceeding from putrescent animal flesh, is that which is given by Fabricius ab Aquapendente in 1592, de Visione &c, pa. 45. And Bartholin gives an account of a similar appearance, which happened at Montpelier in 1641, in his treatise De Luce Animalium.

Mr. Boyle speaks of a piece of shining rotten wood, which was extinguished in vacuo; but upon re-admitting the air, it revived again, and shone as before;| though he could not perceive that it was increased in condensed air. But in Birch's History of the Royal Soc. vol. 2, pa. 254, there is an account of the Light of a shining fish, which was rendered more vivid by putting the fish into a condensing engine. The fish called Whitings were those commonly used by Mr. Boyle in his experiments: though in a discourse read before the R. Soc. in 1681, it was asserted that, of all fishy subslances, the eggs of lobsters, after they had been boiled, shone the brightest. Birch's Hist. vol. 2, pa. 70. In 1672 Mr. Boyle accidentally observed Light issuing from flesh meat; and, among other remarks on this subject, he observes that extreme cold extinguishes the Light of shining wood; probably because extreme cold checks the putrefaction, which is the cause of the Light. The shell sish called Pholas, is remarkable for its luminous quality. The luminousness of the S<*>a has been also a subject of frequent observation. See Ignis fatuus, Phosphorus, and Putrefaction, &c.

Mr. Hawksbee, and many writers on the subject of electricity since his time, have produced a great variety of instances of the artificial production of Light, by the attrition of bodies naturally not luminous; as of amber rubbed on woollen cloth in vacuo; of glafs on woollen, of glass on glass, of oyster shells on woollen, and of woollen on woollen, all in vacuo. On the several experiments of this kind, he makes these following reflections: that different sorts of bodies afford Light of various kinds, different both in colour and in force; that the effects of an attrition are various, according to the different preparations and treatment of the bodies that are to endure it; and that bodies which have yielded a particular Light, may be brought by friction to yield no more of that Light.

M. Bernoulli found by experiment, that mercury amalgamated with tin, and rubbed on glass, produced a considerable Light in the air; that gold rubbed on glass, exhibited the same in a greater degree; but that the most exquisite Light of all was produced by the attrition of a diamond, this being equally vivid with that of a burning coal briskly agitated with the bellows. See Electricity, &c.

Of the Attraction of Light. That the particles of Light are attracted by those of other bodies, is evident from numerous experiments. This phenomenon was observed by Sir I. Newton, who found, by repeated trials, that the rays of Light, in their passage near the edges of bodies, are diverted out of the right lines, and always inslected or bent towards those bodies, whether they be opaque or transparent, as pieces of metals, the edges of knives, broken glasses, &c. See Inflection and Rays. The curious observations that had been made on this subject by Dr. Hook and Grimaldi, led Sir I. Newton to repeat and diversify their experiments, and to pursue them much farther than they had done. For a particular account of his experiment and observations, see his treatise on Optics, pa. 293 &c.

This action of bodies on Light is found to exert itself at a sensible distance, though it always increases as the distance is diminished; as appears very sensibly in the passage of a ray between the edges of two thin planes at different apertures; which is attended with this peculiar circumstance, that the attraction of one <*>dge is increased as the other is brought nearer it. The rays of Light, in their passage out of glass into a vacuum, are not only inflected towards the glass, but if they fall too obliquely, they will revert back again to the glass, and be totally reflected. Now the cause of this reflection cannot be attributed to any resistance of the vacuum, but must be entirely owing to some force or power in the glass, which attracts or draws back the rays as they were passing into the vacuum. And this appears farther from hence, that if you wet the back surface of the glass with water, oil, honey, or a solution of quicksilver, then the rays which would otherwise have been reflected, will pervade and pass through that liquor; which shews that the rays are not reflected till they come to that back surface of the glass, nor even till they begin to go out of it; for if, at their going out, they fall into any of the aforesaid mediums, they will not then be reflected, but will persist in their former course, the attraction of the glass being in this case counterbalanced by that of the liquor.

M. Maraldi prosecuted experiments similar to those of Sir I. Newton on inslected Light. And his observations chiesly respect the inflection of Light towards other bodies, by which their shadows are partially illuminated. Acad. Paris 1723, Mem. p. 159. See also Priestley's Hist. pa. 521 &c.

M. Mairan, without attempting the discovery of new facts, endeavoured to explain the old ones, by the hypothesis of an atmosphere surrounding all bodies; and consequently two reflections and refractions of Light that impinges upon them, one at the surface of the atmosphere, and the other at the surface of the body itself. This atmosphere he supposed to be of a variable density and refractive power, like the air.

M. Du Tour succeeded Mairan, and imagined that he could account for all the phenomena by the help of an atmosphere of an uniform density, but of a less refractive power than the air surrounding all bodies. Du Tour also varied the Newtonian experiments, and discovered more than three fringes in the colours produced by the inflection of light. He farther concludes that the refracting atmospheres, surrounding all kinds of bodies, are of the same size; for when he used a great variety of substances, and of different sizes too, he always found coloured streaks of the same dimensions. He also observes, that his hypothesis contradicts an observation of Sir I. Newton, viz, that those rays are the most inflected which pass the nearest to any body. Mem. de Math. & de Phys. vol. 5, pa. 650, or Priestley's Hist. pa. 531.

M. Le Cat found that objects sometimes appear magnisied by means of the inflection of Light. Looking at a distant steeple, when a wire, of a less diameter than the pupil of his eye, was held pretty near to it, and drawing it several times between that object and his eye, he was surprised to find that every time the wire passed before his eye, the steeple seemed to change its place, and some hills beyond the steeple seemed to have the same motion, just as if a lens had been drawn between them and his eye. This discovery led him to several others depending on the inflećtíon of the rays of Light. Thus, he magnified s<*> objects, as the head of a pin, by viewing them thro<*>gh a small hole in a card; so that the rays which fo<*>med the image must| necessarily pass so near the circumference of the hole, as to be attracted by it. He exhibited also other appearances of a similar nature. Traité des Sens, pa. 299. Priestley, ubi supra, pa. 537.

Reflection and Refraction of Light. From the mutual attraction between the particles of Light and other bodies, arise two other grand phenomena, besides the inflection of Light, which are called the reflection and refraction of Light. It is well known that the determination of bodies in motion, especially elastic ones, is changed by the interposition of other bodies in their way: thus also Light, impinging on the surfaces of bodies, should be turned out of its course, and beaten back or reflected, so as, like other striking bodies, to make the angle of its reflection equal to the angle of incidence. This, it is found by experience, Light does; and yet the cause of this effect is different from that just now assigned: for the rays of Light are not reflected by striking on the very parts of the reflecting bodies, but by some power equally diffused over the whole surface of the body, by which it acts on the Light, either attracting or repelling it, without contact: by which same power, in other circumstances, the rays are refracted; and by which also the rays are first emitted from the luminous body; as Newton abundantly proves by a great variety of arguments, See Reflection and Refraction.

That great author puts it past doubt, that all those rays which are reflected, do not really touch the body, though they approash it infinitely near; and that those which strike on the parts of solid bodies, adhere to them, and are as it were extinguished and lost. Since the reflection of the rays is ascribed to the action of the whole surface of the body without contact, if it be asked, how it happens that all the rays are not reflected from every surface; but that, while some are reflected, others pass through, and are refracted? the answer given by Newton is as follows:—Every ray of Light, in its passage through any refracting surface, is put into a certain transient constitution or state, which in the progress of the ray returns at equal intervals, and disposes the ray at every return to be easily transmitted through the next refracting surface, and between the returns to be easily reflected by it: which alteration of reflection and transmiffion it appears is propagated from every surface, and to all distances. What kind of action or disposition this is, and whether it consists in a circulating or vibrating motion of the ray, or the medium, or something else, he does not enquire; but allows those who are fond of hypotheses to suppose, that the rays of Light, by impinging on any reflecting or refracting surface, excite vibrations in the reflecting or refracting medium, and by that means agitate the solid parts of the body. These vibrations, thus produced in the medium, move faster than the rays, so as to overtake them; and when any ray is in that part of the vibration which conspires with its motion, its velocity is increased, and so it easily breaks through a refracting surface; but when it is in a contrary part of the vibration, which impedes its motion, it is easily reflected; and thus every ray is successively disposed to be easily reflected or transmitted by every vibration which meets it. These returns in the disposition of any ray to be reflected, he calls f<*>s of easy reflection; and the returns in the disposition to be transmitted, he calls fits of easy transinission; also the space between the returns, the interval of the fits. Hence then the reason why the surfaces of all thick transparent bodies reflect part of the Light incident upon them, and refract the rest, is that some rays at their incidence are in fits of easy reflection, and others of easy transmission. For the properties of reflected Light, fee Reflection, Mirror, &c.

Again, a ray of Light, passing out of one medium into another of different density, and in its passage making an oblique angle with the surface that separates the mediums, will be refracted, or turned out of its direction; because the rays are more strongly attracted by a denser than by a rarer medium. That these rays are not refracted by striking on the solid parts of bodies, but that this is effected without a real contact, and by the same force by which they are emitted and reflected, only exerting itself differently in different circumstances, is proved in a great measure by the same arguments by which it is demonstrated that reflection is performed without contact. See Refraction, Lens, Colour, Vision, &c.