hand in the production of colours, appears from light paffing thro' drops of PHYSICS. water, which exhibits a rain-bow; thro' prifmatic glaffes, and many other transparent bodies. And I find, that in a darkned room, where the light is permitted to enter but at one hole, the little floating particles of duft, commonly called motes, being viewed by an eye placed on one fide of the beam of light, appear in certain pofitions adorned with very vivid colours, like thofe of the rain-bow, or rather like thofe of very minute fparkling fragments of diamonds; and that as foon as their motion has brought them to an inconvenient pofition in regard to the light and the eye, they are only vifible, without affording any lively colours: which feems to argue, that this duft, or minute fragments of several forts of bodies reputed opake, and only crumbled into duft, do not barely reflect the rays that fall upon them, but reflect them to the eye refracted.

We may alfo obferve, that feveral bodies, which pafs for opake, appear in great measure tranfparent, when reduced into thin parts, and held against a ftrong light. This I have not only taken notice of in pieces of ivory reduced to thickish leaves, in many confiderable thick fhells of fish, and in fhavings of wood; but have alfo found, that a piece of very thick deal purpofely interpos'd betwixt my eye and the clear day-light, appear'd quite thorough of a lovely red. And in the darkned room above-mentioned, bodies held against the hole at which the light enter'd, appear'd far lefs opake than they would elsewhere have done: fo that I could eafily and plainly fee, thro' the whole thickness of my hand, the motions of a body placed at a very small distance beyond it. And even in minerals, the opacity is not always fo exceeding great, if the body be made thin; for white marble, tho' of a pretty thickness, being within a due diftance placed betwixt the eye and a convenient light, will fuffer the motions of one's finger to be well difcern'd through it; and fo will thick pieces of many common flints. But, above all, that inftance afforded us by Muscovy glafs is remarkable; fer tho' plates of this mineral of a moderate thicknefs often appear opake, yet if one of these be dexterously fplit into the thinneft leaves 'tis made up of, it will yield fuch a number of them, as fcarce any thing but experience could have induced me to believe. Thefe leaves afford the moft tranfparent fort of folid bodies that, for ought I have observed, are yet known: and a fingle plate of it will be fo

bodies, have different degrees both of reflexibility and refrangibility; and that the mixture of all the coloured rays, into which the fun's light is thrown by the prifm, makes white light; and therefore, that all homogeneal light, that is, light whofe rays are all alike refrangible, have a peculiar colour anfwering to their degree of refrangibility; that this is unalterable by any farther refle&ions or refractions whatever; that the fun's light is compofed of all the primary colours to

gether; and that compound colours pro-
ceed from a mixture of the primary.
Hence it follows, that no colours can
arife from any new modifications of
light. But Sir Ifaac is of opinion, that
the permanent colours of bodies are ow-
ing to the difpofition of them to reflect
fome one fort of rays, as red, blue, c.
more plentifully than others. Hence
red bodies appear most beautiful in red
light, blue bodies in blue light, &c.

far

PHYSICS. far from being opake, that 'tis fcarce fo much as visible. And multitudes of bodies there are, whofe fragments feem opake to the naked eye, which yet, when included in good microfcopes, appear transparent. On the other hand, there may, perhaps, be fome bodies, whofe minute particles, even in an excellent microfcope, will not appear diaphanous. For upon viewing, by the help of a good glafs, mercury precipitated per fe, the little granules that made up the powder, appeared like little fragments of coral beheld at a diftance, with the naked eye. Filings of feel likewife, and copper, tho' viewed in an excellent microfcope, on a fair day, fhow'd like pretty large fragments of thofe metals, and appear'd confiderably bright on fome of their furfaces; yet I was not fatisfy'd that I perceived reflections from the inner parts of any of them. Nay, looking thro' my beft microfcope upon the red calx of lead, neither I, nor any to whom I fhew'd it, could difcern it to be other than opake, tho' the day was clear, and the object ftrongly enlighten'd. And even calcined vitriol, tho' deeply red, appeared in the fame microfcope like coarse brick-duft. Nor would I be forward to determine how far, or in what cafes the tranfparency or femi-diaphaneity of the fuperficial corpufcles of larger bodies may have an intereft in the production of their colours; efpecially fince even in feveral white fubftances, as beaten glafs, fnow, and froth, where it feems manifeft that the fuperficial parts are fingly diaphanous, we fee no fuch variety of colours as is ufual from the refraction of light in those bodies, when by their bignefs, fhape, &c. they are properly qualified to exhibit the colours of the rain-bow and prifmatic glafs f.

† For a clear account of opacity and tranfparency, let us hear the excellent Sir If. Newton. The opacity of bodies proceeds (according to that great Philofopher) from the multitude of reflections caused in their internal parts, by a discontinuity of them; for "between the parts of opake "and coloured bodies are many spaces, "either empty, or replenished with me"diums of other denfities.-And that this "difcontinuity of parts is the principal "caufe of the opacity of bodies, appears "from confidering, that opake fubftances "become tranfparent by filling their pores "with any fubftance of nearly equal den"fity with their parts. Thus paper dipt in water or oil, the Oculus mundi ftone fteeped in water, linen cloth oil'd or "varnish'd, and many other substances "foaked in fuch liquors as will inti"mately pervade their little pores, be

come, by that means, more transparent "than otherwife: fo, on the contrary, "the moft tranfparent fubftances may,

By

"by evacuating their pores, or feparating "their parts, be render'd fufficiently o"pake; as Salts, or wet paper, or the "Oculus mundi ftone, by being dried; "horn, by being scraped; glafs, by being "reduced to powder, or otherwife, "flaw'd; turpentine, by being stirred "about with water, till they mix imper"fely; and water, by being form'd "into many fmall bubbles, either alone, "in the form of froth, or by fhaking it "together with oil of turpentine, or oil "olive, or with fome other convenient "liquor, wherewith it will not perfectly

incorporate. And to the increase of "the opacity of thefe bodies, it condu"ces fomething, that the reflexions of "very thin, tranfparent fubftances are "confiderably ftronger than those made by the fame fubftances of a greater "thickness." Newton. Optic. p. 223, 224.

Sir Ifaac Newton farther obferves, that the parts of bodies, and their interftices, muft not be less than of fome definite

By what has hitherto been delivered we may be affifted to judge of PHYSICS. that famous controverfy anciently held, betwixt the atomifts on one fide, Whether object and moft other philofophers on the other fide; the former denying bodies are coloured in to be coloured in the dark, and the latter maintaining colour to be an in- the dark? herent quality, as well as figure, hardness, weight, &c. For, if colour be only light modify'd, how can it fubfift in the dark? But if colour be confidered as a certain conftant difpofition of the fuperficial parts of the object, to disorder the light they reflect after a determinate manner; as this modifying difpofition perfeveres in the object, whether it be enlighten'd or not, there feems no juft reafon to deny, that, in this sense, bodies retain their colour as well in the night as in the day. Or it may, upon this fuppofition, be otherwife faid, that bodies are potentially coloured in the dark, and actually coloured in the light.

But 'tis of greater moment in the inquiry into the nature of colours, Whether emphato decide that controverfy, whether thofe of the rain-bow; thofe often tical colours be real or imaginafeen in clouds before the rifing, or after the fetting of the fun; and, "? in a word, whether thofe other colours, ufually called emphatical, ought to be accounted true: for it being generally granted, and may eafily be proved, that emphatical colours are light modify'd chiefly by refractions,

bigness to render them opake and coloured; "for the opakeft bodies, if their parts be fubtily divided, become per"fectly transparent. Thus water, falt, 66 glafs, ftones, and fuch like fubftances,

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are tranfparent; for, upon divers con"fiderations, they feem to be as full of pores or interftíces between their parts as other bodies are; but yet their parts " and interftices feem to be too small to "cause reflexions in their common furfa"ces." Newton. Optic. p. 225.

"The transparent part of bodies, fays "Sir Ifaac Newton, according to their feve"ral fizes, reflect rays of one colour, and "tranfmit thofe of another, on the fame "grounds that thin plates or bubbles do "reflect or tranfmit those rays;" and this Sir Ifaac takes to be the ground of all their colours: " for if, fays he, a thinned "or plated body, which being of an even "thickness, appears all over of one uni"form colour, thould be flit into threads, " or broken into fragments, I fee no rea"fon why every thread or fragment "fhould not keep its colour; and, by con"fequence, why a heap of thofe threads "or fragments, should not conftitute a "mafs or powder of the fame colour "which the plate exhibited before it was

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with

"broken. And the parts of all natural
"bodies being like fo many fragments of
"a plate, muft, on the fame grounds, ex-
"hibit the fame colour. Now that they
"do fo, will appear by the affinity of
"their properties. The finely coloured
"feathers of fome birds, and particularly
"those of peacocks tails, do, in the very
"fame part of the feather, appear of fe-
"veral colours in feveral pofitions of the

eye, after the very fame manner that
"thin plates do. And therefore their co-
"lours arife from the thinness of the
"transparent parts of the feathers; that
"is, from the flenderness of the very fine
"hairs, or Capillaments, which grow out
"of the fides of the groffer lateral branches
"or fibres of those feathers. And to the
"fame purpose it is, that the webs of
"fome fpiders, by being fpun very fine,
"have appeared coloured; and that the
"coloured fibres of fome filks, by varying
"the position of the cye, do vary

their "colour. Alfo the colours of filks, cloths, "and other fubftances which water or "oil can intimately penetrate, become

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more faint and obfcure, by being im"merfed in thofe liquors, and recover "their vigor again, by being dried after "the manner of thin bodies. Leaf-gold,

fome

PHYSICS. With the concurrence fometimes of reflections, and perhaps other accidents depending on them both; if these emphatical colours be refolved genuine, it will feem to follow, that colours, or at leaft many of them, are but diverfify'd light, and not fuch real and inherent qualities as men commonly fuppofe.

Now fince we allow echoes, and other founds of bodies, to be true founds, all their odours to be true odours, and in fhort, other fenfible qualities to be true, because they are the proper objects of fome of our fenfes; I fee not why emphatical colours, being the proper and peculiar objects of the organ of fight, and capable to affect it as truly and as powerfully as other colours, fhould be reputed imaginary. And if we have proved, that colour, when taken in its more proper fenfe, is only modify'd light, there will be little reafon to deny, that these are true colours, which more manifeftly than others fhew themselves to be produced by the diverfifications of light. There is indeed a difference taken notice of betwixt these apparent colours, and thofe that are usually esteemed genuine, as to their duration; which has induced fome learned men to call the former rather evanid than fantastical. But, as Gaffendus obferves, if this way of arguing were true, the greennefs of a leaf ought to pafs for apparent, becaufe foon fading into a yellow, it cannot

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"fmaller corpufcles, (whereby a coloured

liquor may become tranfparent) or to "make many of them affociate into one "cluster, whereby two tranfparent li

quors may compofe a coloured one. "For we fee how apt thofe faline men"ftrua are to penetrate and diffolve fub"stances to which they are applied; and "fome of them to precipitate what others "diffolve. In like manner, if we confi"der the various phenomena of the at"mosphere, we may obferve, that when

"fome forts of painted glass, the infufion
"of Lignum Nephriticum, and fome other
"fubftances, reflect one colour, and tranf-
"mit another, like thin bodies also. And
"fome of thofe coloured powders which
painters use, may have their colours a
"little changed, by being very clabo-
"rately and finely ground. Where I fee
"not what can be juftly pretended for
"thofe changes, befides the breaking
"of their parts into lefs parts, by that
"contrition, after the fame manner that
"the colour of a thin plate is changed,
by varying its thickness: for which
"reafon alfo it is, that the coloured flow-
ers of plants and vegetables, by being
"bruifed, ufually become more tranfpa-
rent than before; or at leaft in fome
"degree or other change their colours.
"And thus, by mixing various liquors,"
very odd and remarkable productions
"and changes of colours may be effected,
"of which no caufe can be more obvious
"and rational, than that the faline cor-
"pufcles of one liquor do variously act
"upon, or unite with, the tinging cor-
"puscles of another, fo as to make them
"fwell or fhrink, (whereby not only
"their bulk, but their denfity alfo may
"be changed) or to divide them into

"

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vapours are firft raifed, they hinder not "the transparency of the air; being di"vided into parts too fmall to caufe any "reflexion "reflexion in their fuperficies. But "when, in order to compofe drops of rain, they begin to coalefce, and confti"tute globules of all intermediate fizes,

thofe globules, when they become of a "convenient fize to reflect fome colours, "and tranfmit others, may constitute "clouds of various colours, according to "their fizes. And I fee not what can be "rationally conceived in fo transparent

a fubftance as water, for the production "of these colours, befides the various fizes "of its fluid and globular parcels." Newton. Optic. p. 226-228.

be

be compared, in point of duration, with the greenness of an emerald. PHYSICS. But if the fun-beams pafs in a convenient manner thro' a glass prifm, fo as to be thrown upon fome well fhaded object, within a room; the rainbow thereby painted on the furface of a body that terminates the rays, may often laft longer than fome colours I have produced in certain bodies: which colours would juftly, and without fcruple, be accounted genuine, tho' they fuddenly degenerate, and lofe their nature.

A greater difparity betwixt emphatical colours, and others, may, perhaps, be urged; because genuine colours feem to be produced in opake bodies, by reflection; but apparent ones, in diaphanous bodies, principally, by refraction; for, in fome cafes, reflection, alfo, may concur. But ftill this feems not to prove, that thefe latter colours are falfe ones; nor must what we lately faid of the differences of true and apparent colours, be understood in too unlimited a fenfe: for if water be agitated into froth, it exhibits a white colour, which it foon after lofes upon the refolution of the bubbles into air and water; in which cafe, the white nefs of the froth is either a true colour, or not. If it be true, then true colours, fuppofing the water pure, may prove as fhort-lived as thofe of the rainbow; and the matter wherein the whitenefs refided, may, in a few moments, be perfectly deprived of it. Befides, even diaphanous bodies may be capable of exhibiting true colours, by reflection; for that whitenefs is fo produced, will fhortly appear. But if it be faid, that the whitenefs of froth is an emphatical colour; then it must no longer be faid, that fantastical colours require a certain pofition of the illuminating body and the eye; and must be varied or deftroyed by the change thereof: for froth appears white, whether the fun be rifing or fetting; in the meridian, or any where between that and the horizon; and in what part foever the fpectator's eye is placed. And fince, by making a liquor tenacious, without deftroying its tranfparency, or ftaining it with any colour, we can give the little films whereof the bubbles confift, fuch a texture as will make the froth laft for many hours, or days, together; it feems improper to affign duration for the diftinguishing character of genuine from fantastic colours: for fuch froth may long out-laft the true colours of fome natural productions. Thus in that gaudy plant, the marvel of Peru, the flowers often fade on the very fame day they are blown. And I have often feen a Virginia flower, which ufually withers within the compafs of a day. I am alfo credibly inform'd, that a curious neighbouring botanist has a plant whofe flowers perifh in about an hour's time.

But if the whitenefs of water, turn'd into froth, must therefore be reputed emphatical, because it appears not that the nature of the body, but only that the difpofition of its parts, with regard to the incident light, is changed; why may not that whitenefs be accounted emphatical too, which I fhall fhortly thew to be producible, barely by fuch another change in black horn? And yet this whitenefs, which is fo eafily acquir'd, feems to be as truly its colour as blacknefs was before; and is, at least, more permanent than the greennefs of leaves, the redness of roses, or the geVOL. II.

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nuine