hand in the production of colours, appears from light paffing thro' drops PHYSICS. of 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 those 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 visible, without affording any lively colours: which feems to argue, that this duft, or minute fragments of feveral 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 also obferve, that several bodies, which pafs for opake, appear in great measure tranfparent, when reduced into thin parts, and held against a strong light. This I have not only taken notice of in pieces of ivory reduced to thick ifh leaves, in many confiderable thick fhells of fish, and in fhavings of wood; but have alfo found, that a piece of very thick deal purposely 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, though of a pretty thickness, being within a due distance 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; for tho' plates of this mineral of a moderate thicknefs often appear opake, yet if one of these be dextrously split into the thinnest 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 obferved, are yet known

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 homogenial 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 reflections or refractions whatever; that the fun's light is composed of all the primary colours to

and a single plate of it will be fo

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 owing
to the difpofition of them to reflect
fome one fort of rays, as red, blue, &r.
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 vifible. And multitudes of bodies there are, whofe fragments feem opake to the naked eye, which yet, when included in good microscopes, appear tranfparent. 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 fteel 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 best 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 strongly 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 seems 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 glass +.

For a clear account of opacity and transparency, let us hear the excellent Sir If. Newton. The opacity of bodies proceeds (according to that great Philofopher) from the multitude of reflections caufed in their internal parts, by a difcontinuity 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 substances "become transparent 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 "steeped in water, linen cloth oil'd or "varnish'd, and many other fubftances "foaked in fuch liquors as will inti"mately pervade their little pores, become, "by that means, more transparent than "otherwife: fo, on the contrary, the

By

evacuating their pores, or feparating their parts, be render'd fufficiently opake; as "falts or wet paper, or the Oculus "mundi ftone, by being dried; horn, "by being fcraped; glafs, by being re"duced to powder, or otherwife, flaw'd;

66

turpentine, by being stirred about with "water, till they mix imperfectly; and water, by being form'd into many small "bubbles, either alone, in the form of "froth, or by fhaking it together with "oil 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 these bodies, it conduces fomething, that the reflexions of very "thin, transparent fubftances are confimade derably stronger than thofe "by the fame fubftances of a greater "thicknefs." Newton. Optic. p. 223, 224.

66

Sir Ifaac Newton farther obferves, that the parts of bodies, and their interstices, must not be lefs than of fome definite

Whether ob

By what has hitherto been delivered we may be affifted to judge of that PHYSICS. famous controversy anciently held, betwixt the atomifts on one fide, and moft other philofophers on the other fide; the former denying bodies to be coloured in the dark, and the latter maintaining colour to be an inhe-loured in the rent quality, as well as figure, hardness, weight, &c. For, if colour dark? 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 fenfe, 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.

imaginary?

But 'tis of greater moment in the inquiry into the nature of colours, Whether emto decide that controversy, whether those of the rain-bow; those often phatical colours be real or seen 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 easily 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 perfectly tranf"parent. Thus water, falt, glass, stones, "and fuch like fubftances, are transparent; "for, upon divers confiderations, they "feem to be as full of pores or interstices "between their parts as other bodies are; "but yet their parts and interstices seem to "be too fmall to cause reflexions in their "common furfaces." Newton. Optic. p.

225.

"The transparent part of bodies, fays "Sir Ifaac Newton, according to their fe"veral fizes, reflect rays of one colour, “and transmit those of another, on the "fame grounds that thin plates or bubbles "do reflect or transmit those rays; " and this Sir Ifaac takes to be the ground of all their colours: 66 for if, fays he, a thinned "or plated body, which being of an even "thickness, appears all over of one uni"form colour, fhould be flit into threads, "or broken into fragments, I fee no rea"fon why every thread or fragment should "not keep its colour; and, by confe

66

quence, why a heap of thofe threads or fragments, thould not conftitute a mass "or powder of the fame colour which the "plate exhibited before it was broken.

with

"And the parts of all natural bodies be-
"ing like fo many fragments of a plate,
"muft, on the fame grounds, exhibit the
"fame colour. Now that they do fo, will
"appear by the affinity of their proper-
"ties. The finely coloured feathers of
"fome birds, and particularly thofe of

66

peacocks tails, do, in the very fame part "of the feather, appear of feveral colours "in feveral pofitions of the eye, after the

very fame manner that thin plates do. "And therefore their colours arife from "the thinness of the transparent parts of "the feathers; that is, from the flender"nefs of the very fine hairs, or Capilla"menta, which grow out of the fides of "the groffer lateral branches or fibres of "those feathers. And to the fame pur"pofe it is, that the webs of fome fpi

ders, by being spun very fine, have ap

peared coloured; and that the coloured "fibres of fome filks, by varying the po"fition of the eye, do vary their colour. "Alfo the colours of filks, cloths, and "other fubftances which water or oil

PHYSICS. with the concurrence fometimes of reflections, and perhaps other accidents depending on them both; if these emphatical colours be resolved genuine, it will feem to follow, that colours, or at least 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 sounds of bodies, to be true founds, all their odours to be true odours, and in fhort, other fenfible qua lities to be true, because they are the proper objects of fome of our fenses; 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 greenness of a leaf ought to pafs for apparent, becaufe foon fading into a yellow, it cannot

fome forts of painted glafs, the infufion "of Lignum Nephriticum, and fome other "fubftances, reflect one colour, and tranf"mit another, like thin bodies alfo. And "fome of those coloured powders which "painters use, may have their colours a "little changed, by being very elaborate"ly and finely ground. Where I fee not "what can be justly pretended for those "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 flowers of plants and "vegetables, by being bruifed, ufually be"come more transparent than before; or "at least in fome degree or other change "their colours. And thus, by mixing va"rious liquors, very odd and remarkable "productions and changes of colours may "be effected, of which no caufe can be 66 more obvious and rational, than that the faline corpufcles of one liquor do varioufly act upon, or unite with, the tin"ging corpufcles of another, fo as to make

them fwell or shrink, (whereby not only "their bulk, but their density also may Abe changed) or to divide them into fmal

"ler corpufcles, (whereby a coloured li

66

quor may become transparent) or to "make many of them affociate into one "cluster, whereby two transparent liquors

may compofe a coloured one. For we "fee how apt thofe faline menftrua are to penetrate and dissolve substances to which "they are applied; and some of them to 66 precipitate what others diffolve. In "like manner, if we confider the various "phenomena of the atmosphere, we may obferve, that when vapours are first rai"fed, they hinder not the transparency of "the air; being divided into parts too "fmall to caufe any reflexion in their fu"perficies. But when, in order to com

66

pofe drops of rain, they begin to coa"lefce, and conftitute globules of all inter"mediate fizes, thofe globules, when they "become of a convenient fize to reflect "fome colours, and tranfmit others, may "conftitute clouds of various colours, ac"cording to their fizes. And I fee not "what can be rationally conceived in fo "transparent a substance as water, for the

production of thefe colours, befides the "various fizes of its fluid and globular par"cels." Newton. Opt. p. 226,—228.

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 glafs 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 these latter colours are falfe ones; Dor muft 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 whiteness of the froth is either a true colour, or not. If it be true, then true colours, supposing 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 whiteness 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 spectator's eye is placed. And fince, by making a liquor tenacious, without deftroying its transparency, or staining 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 seen 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, muft 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 fhew 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 blackness was before; and is, at least, more permanent than the greennefs of leaves, the rednefs of rofes, or the geVOL. II.

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nuine