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Light in their pores ; admitting that light moving with its utmost ve. locity hath no momentum. But fince the light, which is moved where it illuminates, so that the illumination proceeds above 10,000,000 of miles in a minute, or above 800,000 times swifter than founds, is not found * to impel the most opaque poised or light bodies, in the line of illumination ; the theory of illumination by emitted light is not reconcileable to the phænomena of light, by any argument from the smallness of its parts, and from the smallness of their momentum ; which subjects we hall speak of in due time.
“ The quantity of tallow which burns from a candle of the ordinary fize, in a second of time, is not equal in bulk to the sixth part of a drop of melted tallow. And the illumination caused by the combustion of this finall portion of tallow, may be seen even through air, in every part which the pupil measures of a space of two or three Thousand Lquare miles. Reafon revolts at the supposition that so much light could be crowded in so finall a compass in cold callow †; we know no powers by which it can be projected, except those of the mutual re. pulfions of its parts; we cannot from any analogy infer, that such a minute portion of gross matter can concentrate and combine with so great a quanuity of the repellent matter of light; and unless we admit hypothesis as a proof of this hypothesis of the motion of light, the fuppofitions of the ingenious Mr. Canton, Mr. Melville, and others, concerning the wide intervals between the parts of light, relatively to the bulk of them, does not give any colour of truth to the notion of this emission of light from burning bodies.
“ If we suppole the moved light, which illuminates in all spaces between the lun and visible bodies celestial and tes restrial, to be projected from the subítance of the sun; and that it moves from the fun to us in about eight minutes; all these spaces must be replenished in the fort eight minutes of sun-shine, with as much light as is jufficient for strong illumination, which is all that is at any time found equally diffused in these spaces; and every minute thereatier, light will be concentrated or accumulated in them. But as light can be concentrated by convex lenticular glasses and concave mirrors, belides other means to be noticed hereafter; and as we are well aflured that a concentra. tion of light to the thousandth part of the space in which it is usually diffused, is capable of exciting great heat and combustion in bodies exposed to it, wherever the sun illuminates freely; the concentration of light, resulting from the presumed continual emition of it, must make the sun-line of every hour much hotter than that of the
* It is peculiarly remarkable in this treatise, that the author supposes every thing loft, or not in being, which is not found. Now, though, with what we have o ce polefjed, it is much the fame thing, whether it be loft or not to be found, this is not the case, with things not yet potlessed or discovered. Reru.
+ Especially as the callow obscures the wick, blackens the cover of the Candlestick or the cicling, and proves, that our light is manufactured of the Daterials of darkness. Rev.
former hour; and in a certain time, from the first action of the fun, which is to eight minutes, as the area of a burning glass is to the area of its real focus; this our world must be red hot and faming : * and the fun must be waiting daily, and in time incapable of emitting light, or of illuminating our earth, by reason of the dispendium of light from every face of it, as well as from that presented to us at any instant. 1?" These consequences do not follow the action of the sun, but are totally repugnant to our experience during thousands of years; and the premises from which they flow are erroneous; however ingeniously the Rev. Dr. Horsley and others endeavour to extenuate the waste of the fun, which must be the consequence of the supposed emisiior. of light; unless the sun hath a power of drawing back light, as fast as he is lipposed to emit light: for the falling of comets into the fun, is a mere conjecture t. And, concerning that power, it were fufficient to remark, that the existence of it is not demonstrated. But we may further obferve, that it is altogether improbable. For the cause of illumination is not sensibly weaker in any one face of the fun than in another; and the fun acts inceflan:!y in illuminating the spaces and budies around it;' and it is impossible, by any known property of matter, or law of nature, for the fame body to exert, at the same instant and in the fame coast, a power of propelling any matier, and a con: trary power of drawing back the same matter again as quickly as it was propelled, and from the greatest known distances : and it is impossible that iwo such powets should not counteract each other, and rather poise the matter on which they act, in a state of a relative reit, than cause a rapid motion of it, progressive in some parts, and retrograde in others, interceding these. It is moreover tü be considered, that if the light supposed to be emitted from the fun, were drawn back to the fun as fast as it is emitted, there could be no darkne's in the night ; because the current of light returring to the fun, must illuminate, as well as that proceeding from it, it illumination consist in the progressive motion of light.
" Those who imagine that the sun emits the light by which it ile luminates, muft allow that thoufands of stars, and of fires on this our earth, are continually emitting light in addition to that of the fun 1. And thus the error of the hypothesis is aggravated; since a multitude of bodies emitting light inceliantly into the same spaces, are not found
* Here our author falls into the common errour, of confounding light with fire; to which it relates merely by the fame association of ideas, whichi makes an elephant dance at the beat of a drum. The dumb beast is conftantly burnt in the toes to the found of the ta-too. Rev.
+ By no means. Dr. H. though right in the main of this argument, is . plainly no astronomer. The planets, it is now demonftrable, tend conitantly to an obliquity, by which they will in time become comets, and from their excentricity either will fall into the sun, or be totally burnt up and disipated by their approach to it. Dr. H. talks idly of the experience of thousands of years on a subject, which that of millions of years cannot determine: Rev.
1 This is something like the widow's weeping and the drunkard's waters, ing into the sea. Rev.
to increase the quantity of light therein, nor to suffer any waste of their fubitance *
“ The philosophers who affirm that illumination is caused by a progreflive motion of Light; when they consider how many suns, it the bixed stars be funs, and how many millions of fires, are continually emitting Light, by their hypothelis; and when they recollect that alt mailes of gro's ma ter are pervious to Light; must acknowledge that all known places are replenished with Light, and no place can be vond of Light in any instint of time; and our atmosphere eliecially, which is open to the sun and stars and fires, can never be deficient of Light. And when they fur her observe how Light, condensed in bodies placed in the focus of a burning-glass, expands from the bodies, when the inStrument of condensation is removed, nearly in the fame manner as other elattic fluids expand when the condenfing power ceases; they will perceive, that by reafon or this expansibility of Light, all places of which we have any knowledge must contain Light in quantities which are nearly as the spaces, excepi in the vicinity of bodies which attract or concentrate Light; as we shall Thew hereafter. Now it the parts of Light by which we fee che fun, were enlisted from the sun, they muf dart swiftly 800,000,000 of miles through a space already supplied abundantly with Light, without impinging on the parts of it; and they must likewise move ravidly many leagues through the denser parts of our atmosphere, without clashing with the grof'er atoms of air io as to be stopped by them; and when an ethereal Huid diffused in all kno.vn space is affumed, they mutt moreover move through this ether without impediment, and with the foregoing velocity.. But we have no experiinental evidence of such motion; it was originally supposed, only for the sake of certain phænomeņa, which may, in the present riper state of natural knowledge. be explained without it, as we hhall few; and fince there is no fimilitude ot it in nature, and it hath no conforinity with the established laws of motion, it is to common understanding incredible.
" If the air were equally dense at every altitude of the atmosphere: and if the parts of air and Light, and of the ether if you will, were all disposed so as to be equally diltant from each other; it were pullible that Light might move rapidly and progrellively between theie parts, in certain directions without touching them. But this cannot happen in all the directions in which we have sense of rectilinear illumination : neither can it happen in air whose parts are unequally distant in any two different altitudes, and are perpetually changing their places and arrangement relatively to each other, when they are agitated by winds and orher causes. And as we lee again it the wind, or athwart the wind, as well as with it, and through still air in any one direction as well as in another, allowance being made for the turbid state and greater denfity of the air near the earth; and through water and glass of any
thick ness, in which the motion of Light is not stifled by means to be confdered bereafter, as well in one direction as in another; although the pores of these fluids, and especially of incompressible solid bodies cannot be continued, in all poisible directions, uninterrupted by the folid
One would think this representation of light, as a body, would have cured our author of this hypothetis likewise. Rev.
matter of the fluids and bodies, so as to give passage to the Light in a ftraight courle, however it may fall on them; Light is not moved from the sun and burning bodies with a continued progressive rapid motion."
Our author brings some collateral arguments to disprove the projection and progressive motion of light from the luminous body, which are apt and pertinent. "' 1f,” fays he,
“ it were true that the parts of Light, which illuminate when we see a number of stars or candles, are emitted with great velocity from these bodies; these parts being moved in different directions towards each other, ought, either by impinging or by repelling each other when they approach, to be turned out of the rectilinear way between each luminary and the eye, and ought to take a course very different from that of the experienced illumination. According to the hypothesis, the torrent of Light from the sun is inuch denser than that from a tar; and when the sun shines obliquely on our hori. zon, the weaker torrent of Light flowing from the stars, ought to fall in with, or be turned out of its way by, the denser and stronger torrent from the sun; so that stars near the zenith should be invisible to a man looking towards them from the bottom of a deep pit. But these conclufons resulting from the hypothesis are false; and therefore the hye pothesis is erroneous."
Again, “ The phænomena of reflections of Light further shew, that illumination doth not consist in a continued progreffive motion of its parts. For a mirror placed in the fun-beam admitted into a dark chamber, so as to reflect the illumination, in the line of incident illumination as nearly as is possible, does not weaken or featter the illumination in the beam of Light; which it ought to do, if the incident and reflected Light were moved progressively in contrary directions : and the illumination obliquely incident on a broad mirror, is not found to disturb the reflected illumination, as it ought, according to the laws of motion, if the Light of the incident and reflected beams which cross each other, were moved rapidly and progressively; and as it ought, whether the parts of this Light impinge on each other, or shun each other by virtue of a repulsive power; and for the like reasons the phenomena of illumination reflected regularly to focus, by concave mirsors, in a direction oblique and contrary to that in which the Light is supposed to be moved from the fun to the concave mirrors ; plainly new that Light is not moved in continued progreffion."
And again, “ if we consider the Light contained in illuminated spaces, we cannot from any analogy presume that illumination is continued by any progressive motion of Light. When a stone is thrown into stagnant water, the motion of the parts of water contiguous to the stone, is communicated to the circumambient water; and the extreme circular waves which reach the banks, do not consist of the parts first impelled, but of the water which was contiguous to the banks; and if the stone be small relatively to the water, the motion of the water is manifestly undulatory, where it is not sensibly progreffive. A person immersed in water, can hear the collision of two stones in the water, at a great dittance, as the learned Dr. Franklin observes. In this case the whole water hath no progretlite motion, although motion is propa
gated in it to the greatest distances at which the experiment has been made,
ti When we find motion thus excited in immense quantities of a gross and heavy fluid, by forces incapable of giving sensible progresfive motion to very small portions of it; and when we consider that the matter expanding from burning bodies doth not appear to have any considerable momentum; we find some reason from analogy to presume that the motion of Light is a propagated motion, but none to believe that Light is propelled freely through Light with progressive motion.
** When a bell is runy, motion is fenlibly propagated in air to the distance of many miles, without any continued progressive motion of the air; and the force neceilary to generate this motion is incomparably less than the force which would be necessary to propel such a body of air through air with the velocity of found.' Between our sense of the vibrations of air, and our sense of the motion of Light; between ftillness or quiescence of air, and darkness or quiefcence of Light; between found and illumination; between the swiftness of found in a gross and heavy Auid considerably dense, and the greater swiftness of illumination in a rare fluid, whole parts do not gravitate sensibly; between the force necessary to cause found, and the force neceisary to caufe illumination; we perceive fome analogy. But we find none in elastic Auids to countenance the hypothesis of the projection and rapid progreffive motion of Light through Light, or the continued propul. fion of one body of Light by another.
" Air propelled from the bellows, or by the explofion of gunpowder; and all other blalts of air against air, cause eddies and devious motions of the air; and the projected air lotes its motion in the ambient air in a sinall time and space. So Light, if it be an elastic fluid as we shall presently endeavour to Jbew, being projected from the luciferous parts of the sun, or froin the fire in a light-house; would in a short time and space bend out of the right course of illumination and spend its motion in the circumjacent light."
Having now come to the end, of what is published, of this Essay, we shall take leave of the present publication with obferving, that the author, having greatly excited our curiofity about the phænomenon of light, hath left us much in the dark about the manner in which he intends to account for it. He seems neither to fide with Sir Isaac Newton, in allowing it to consist of minute particles necessarily projected and progreffively propelled from the luminous body ; nor with those who consider it merely as motion, propogated through an elastic medium. To the latter, indeed, he appears, from his allusion to the propagation of found, the most to incline; but he inight as well call the coinmon atmosphere sound, or the matter of found, as call the medium, through which light is propagated, light, or the matter of light.-We would recoinmend to Dr. Higgins, in the prosecution of this treatise to consider, that more depends on logical precision of terins, even in natural philosophy, than he perhaps is aware of, and that it is to little