empty honey-comb, were not only very diftinct, and of fimilar figures, PHYSICS. but confiderably large, and prodigiously deep; fo that their diftinct fhadows and fides were very confpicuous, and eafily to be reckoned, and might have been well diftinguifhed, had they been ten times lefs than they were: whence we may make fome eftimate, what a ftrange inequality, and what a multitude of little fhades there may really be in a scarce fenfible part of a phyfical fuperficies, tho' they remain invifible to the naked eye. There are alfo feveral experiments which confirm the fame thing in other like fubftances: fo that every fenfible part of an opake body, may be conceiv'd to confift of a multitude of corpufcles fingly infenfible. But in giving these furfaces a difpofition to alter the light reflected thence to the eye, after the manner requifite to make the object appear colour'd, the figures of thefe particles have a great, tho' not the only share. 'Tis true indeed, that the protuberant particles may be of very various figures, fpherical, elliptical, conical, &c. according to the nature whereof, and the fituation of the lucid body, the light must be variously affected; as, after one manner from furfaces confifting of fpherical, after another from thofe made up of conical or cylindrical particles; fome being fitted to reflect more of the incident rays of light, others lefs; and fome towards one part, others towards another. But befides this difference of figure, many other things may greatly concur to vary the forms of afperity, whereon colours have fo great a dependance: for allowing the figure of the particles, the fuperficial ones may be bigger in one body, and lefs in another; and confequently fitted to allay the light falling on them with greater fhades. The protuberant particles may also be set at a greater or a smaller distance from one another in different parts; and how thefe qualities may ferve to produce colour, we may guefs from what happens in the agitation of water; for if the bubbles that are thereby made, be large and few, the water will scarce acquire a fenfible colour; but if it be reduced to a froth, which confifts of bubbles very minute, and contiguous to each other, it exhibits a very manifeft whitenefs. Befides, it is not neceffary that the fuperficial particles which exhibit one colour, should be all of the fame fhape; but different figures may be mixed upon the furface of the opake body: as when the corpufcles that make a blue colour, and those that make a yellow, come to be accurately and skilfully united, they afford a green; which tho' it seem one fimple colour, yet in this cafe appears to be made by the union of corpufcles of very different kinds. Moreover, the figure and magnitude of the little depreffions, cavities, furrows, or pores, intercepted betwixt these protuberant corpufcles, are as well to be confider'd, as the fizes and fhapes of the corpufcles them Sir Ifaac Newton proves, that the cause of reflexion is not the impinging of light on the folid or impervious parts of bodies, as is commonly fuppofed; and makes it probable, that the reflexion of a ray is effected not by I a fingle point of the reflecting body, but by felves. PHYSICS. taking off the fcum, we perceiv'd the fmooth gloffy surface of the melted matter to be adorn'd with a very delightful colour, which almost immediately gave place to another vivid one, that was as immediately fucceeded by a third, and this by a fourth. Thus these wonderfully vivid colours fucceffively appear'd and vanish'd, till the metal cooling, put a ftop to this pleasant fight; tho' the colours which chanced to adorn the furface, when the lead began to cool, remain'd upon it, but were fo fuperficial, that how little foever we scraped off from the furface of the lead, all the colour came away, and difcovered only that which is natural to the metal. But unless lead be brought to a very high degree of fufion or fluidity, the phenomena will scarce appear. However, the fame colours did neither always, nor regularly, fucceed one another, as is ufual in fteel; but in the following diversify'd order, which paffed fo fuddenly, that I was scarce able to commit it to paper; blue, yellow, purple, blue, green, purple, blue, yellow, red, purple blue, yellow and blue, yellow, blue, purple, green mix'd, yellow, red, blue, green, yellow, red, purple, green. To the fame purpose I might add, that viewing a sphere of rock crystal, which was sawn afunder by a lapidary, and exposing the flat surfaces to the fun beams, the little particles that, notwithstanding their feeming smoothness in the shade, render'd their surfaces rough, so refracted and reflected the light, as to exceed the vivid colours of the rainbow; but in an interrupted order, fometimes on one part of the furface, fometimes on another, as it happen'd to be fituated with regard to the fun. And having caufed a fine-grain'd touchstone to be fawn afunder by the fame artificer, I obferved upon the new furfaces, tho' to the touch they feem'd smooth and polifh'd, the vivid colours, as above-mention'd, according as the furfaces were put into various pofitions, in respect of the fun and the eye; fo that, notwithstanding the great transparency of the cryftal, and the great opacity of the touch-ftone, their fuperficial corpufcles were found fit to exhibit the vivid colours we admire in the rain-bow. The atomifts of old, and fome learned men of late, have attempted to explain the variety of colours in opake bodies from the various figures of their fuperficial parts. The attempt is indeed ingenious, and the doctrine feems partly true; but I confefs other things appear to me neceffary to be taken in, as contributing to thofe different forms of afperity whereon the colours of opake bodies depend. But in order to prove it, we muft affume, that the furfaces of all fuch bodies, how smooth foever they may appear to our fight and touch, are only fo in a popular, or a physical fenfe. The truth hereof is evident from the ufe of microfcopes, which fhew us in many bodies, that feem fimooth to our naked eyes, little protuberancies rifing above that which may be conceiv'd to be the plain of the furface ; as also numerous depreffions beneath that level. And of this fort of cavities we have, by the help of an excellent magnifier, on the furface of a thin piece of cork, that appeared fmooth to the eye, obferv'd fixty in a row, within the compass of that glafs, tho' not above of an inch; and thefe too, which made that little piece of cork look almost like an empty honey-comb, were not only very diftinct, and of fimilar figures, PHYSICS. but confiderably large, and prodigiously deep; fo that their diftinct fhadows and fides were very confpicuous, and eafily to be reckoned, and might have been well diftinguifhed, had they been ten times lefs than they were: whence we may make fome eftimate, what a ftrange inequality, and what a multitude of little fhades there may really be in a fcarce fenfible part of a phyfical fuperficies, tho' they remain invifible to the naked eye. There are alfo feveral experiments which confirm the fame thing in other like fubftances: fo that every fenfible part of an opake body, may be conceiv'd to confift of a multitude of corpufcles fingly infenfible. But in giving these furfaces a difpofition to alter the light reflected thence to the eye, after the manner requifite to make the object appear colour'd, the figures of thefe particles have a great, tho' not the only share. 'Tis true indeed, that the protuberant particles may be of very various figures, spherical, elliptical, conical, &c. according to the nature whereof, and the fituation of the lucid body, the light muft be variously affected; as, after one manner from furfaces confifting of fpherical, after another from those made up of conical or cylindrical particles; fome being fitted to reflect more of the incident rays of light, others lefs; and fome towards one part, others towards another. But befides this difference of figure, many other things may greatly concur to vary the forms of afperity, whereon colours have fo great a dependance: for allowing the figure of the particles, the fuperficial ones may be bigger in one body, and lefs in another; and confequently fitted to allay the light falling on them with greater fhades. The protuberant particles may also be fet at a greater or a smaller distance from one another in different parts; and how thefe qualities may serve to produce colour, we may guess from what happens in the agitation of water; for if the bubbles that are thereby made, be large and few, the water will scarce acquire a fenfible colour; but if it be reduced to a froth, which confifts of bubbles very minute, and contiguous to each other, it exhibits a very manifeft whitenefs. Befides, it is not neceffary that the fuperficial particles which exhibit one colour, fhould be all of the fame fhape; but different figures may be mixed upon the furface of the opake body: as when the corpufcles that make a blue colour, and those that make a yellow, come to be accurately and skilfully united, they afford a green; which tho' it feem one fimple colour, yet in this cafe appears to be made by the union of corpufcles of very different kinds. Moreover, the figure and magnitude of the little depreffions, cavities, furrows, or pores, intercepted betwixt these protuberant corpufcles, are as well to be confider'd, as the fizes and fhapes of the corpufcles them Sir Ifaac Newton proves, that the cause of reflexion is not the impinging of light on the folid or impervious parts of bodies, as is commonly fuppofed; and makes it probable, that the reflexion of a ray is effected not by I a fingle point of the reflecting body, but by felves. PHYSICS. felves. For we may conceive the phyfical fuperficies of a body to be horizontally cut by a mathematical plain; and then, as fome parts of that phyfical fuperficies will be protuberant, or swell above the plain, fo others may be depreffed beneath it. Thus in many places of the earth's furface, there are hills, trees, &c. rais'd above the horizontal level of the valley; and rivers, wells, pits, and other cavities, depreffed beneath it. And that fuch protuberant and concave parts of a furface may remit the lights fo differently as greatly to vary a colour, we shall fee hereafter. But at prefent it may fuffice to fay, that the different degrees or kinds of afperity in the two flat fides of the fame piece of red marble, the one whereof is polished, and the other left rough, will so diversify the light reflected from the several different plains to the eye, thata painter would employ two different colours to represent them. The fituation alfo of the fuperficial particles is confiderable. This I diftinguish into the pofture of the fingle corpufcles, in refpect of the light, and of the eye; and the order of them in regard to one another: for a body may reflect the light otherwife, when its fuperficial particles are more erect upon the plain conceiv'd to pafs along their balis; and when the extremities of fuch particles are obverted to the eye, than when thofe particles lie fo inclined, that their fides are in great measure difcernible. Thus the colour of plufh or velvet will appear various, if you ftroak part of it one way, and part another; the pofture of the particular threads in regard to the light, or the eye, being thereby varied. And 'tis obfervable, that in a field of ripe corn, blown upon by the wind, there will appear waves of a colour different from that of the reft of the corn; because the wind, by depreffing fome of the ears more than others, caufes one to reflect more light from the lateral and ftrawy parts than another. Thus when dogs are fo enraged, as to erect the hairs upon their necks, and other parts of their bodies, thofe parts feem to acquire a colour different from that of the fame hairs, when, in their ufual pofture, they lay more inclined. And that the order wherein the superficial corpufcles are ranged is confiderable, we may guefs by the turning of water into froth, the beating of glafs, and the fcraping of horn; in which cafes the corpufcles that were before fo marshall'd, as to be tranfparent, become, by a difturbance of that order, difpofed to stop and reflect more light, and thereby to appear whitifh. There are alfo other ways wherein the order of the protuberant parts, in regard to the eye, may greatly contribute to the appearance of a particular colour: for I have often obferved, that when peafe have been planted or fet in parallel lines, and are fhot up about half a foot above the furface of the ground, by looking on the field where they grew along the lines they ftood in, much the greater part of the ground appeared of its own dirty colour; but if I view'd the field tranfverfly, it would appear very green; the upper parts of the peafe concealing the intercepted parts of the ground from the eye. And perhaps even the motion of the fmali parts of a vifible object may in fome cafes contribute, tho' it be not eafy to fay how, to the production or variation of a colour: for I have feveral times made a liquor, which, when well fettled in a clofe vial, is tranfparent PHYSICS. and colourlefs; but as foon as the glafs is unftopped, begins to fly away very plentifully in a white, opake fume: and there are other bodies, whofe fumes, when they fill a receiver, would make one fufpect it contain'd milk; and yet thefe fumes become a liquor that is not white, but tranfparent. And fuch white fumes I have feen afforded by unftopping a liquor, which is itfelf diaphanous and red; nor are these the only inftances of this kind that our experiments can fupply us with. And if the fuperficial corpufcles be of the groffer fort, and fo framed, that their differing fides may exhibit differing colours, then the motion or reft of thofe corpufcles may be confiderable, as to the colour of the fuperficies they compofe; be-. cause sometimes more, fometimes fewer of the fides difpofed to exhibit fuch a colour, may by this means become or continue more obverted to the eye than the rest, and compose a phyfical furface more or lefs fenfibly interrupted. Thus I remember, that in fome forts of plants, whofe leaves were thick fet by one another, and the two fides of each leaf of a fomewhat dif ferent colour, there would appear a notable difference in their colour, if look'd upon, first when the leaves being at reft, had their true upper fides obverted to the eye; and again, when a breath of wind paffing thorough them, made great numbers of the fides of the leaves, that are ufually hidden, become confpicuous. And tho' the little bodies above mentioned, may fingly and a-part feem almoft colourlefs; yet when many of them are placed by one another fo near, that the eye does not difcern an interruption in a fenfible space, they may exhibit a colour: as we fee, that tho' a flender thread of dy'd fcarlet filk, whilft look'd on fingly, feems almoft quite devoid of redness; yet when numbers of these threads are brought together into a skein, that colour then becomes glaring. And in changeable taffaties, where we fee differing colours arife and vanifh upon ruffling the fame piece of filk; I have often, with pleasure, obferved, by means of a convenient microscope, their component threads to pafs under and over each other, in almost innumerable points: and if I thus look'd upon any confiderable portion of the stuff that, for example, appeared red to the naked eye, I could plainly fee, that in fuch a pofition the red threads were confpicuous, and reflected a vivid light: and tho' I could also perceive that there were green ones, yet, by reafon of their difadvantageous pofition in the phyfical furface of the filk, they were in part hid by the more protuberant threads of the other colour; and, for the fame reafon, the reflection from as much of the green as was difcover'd, appear'd but dim and faint. And if, on the contrary, I look'd thro' the microfcope upon any part that appear'd green, I could plainly fee the red threads were lefs fully expofed to the eye, and obfcured by the green ones, which, therefore, made the predominant colour. And by obferving the texture of the filk, I could easily fo expose the threads either of the one colour or the other to my eye, as at pleasure to exhibit the appearance of red or green, or make thofe colours fucceed each other: fo that, obferving their fucceffion, I could mark how the predominant colour started up, when the threads that exhibited it, VOL. II. C came |