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PHYSICS. nuine colours of moft natural productions. It may, indeed, be further objected, that, acccording as the fun or other luminous body changes place, thefe emphatical colours alter, or vanish. If a piece of cloth, in a draper's fhop, where the light is feldom primary, be variously folded, it will appear of different colours, as the parts happen to be more illuminated or fhaded; and if you ftretch it flat, it will commonly exhibit fome one uniform colour; yet thefe are not reputed emphatical: fo that the difference feems to be chiefly this, that in the cafe of the rain-bow, and the like, the pofition of the illuminating body varies the colour; and, in cloth, the pofition of the object does it. Yet I am not forward to allow, that in all cafes the apparition of emphatical colours requires a determinate pofition of the eye; for if men will have the whitenefs of froth emphatical, we have already feen what may be inferred from thence. Befides, the fun-beams tranfmitted thro' a triangular prism of glass, after the manner lately mentioned, will, upon the body that terminates them, paint a rain-bow, vifible to the eye, whether placed on the right hand of it, or on the left; above it, or beneath it; before it, or behind it and tho' there may appear fome little variation in the colours of the rain-bow, beheld from different parts of the room; yet the like may be obferved by an attentive eye, in real colours, view'd under the like circumftances. Nor will it follow, that, because there remain no footfteps of the colour upon the object, when the prifm is removed, that therefore the colour was not real; fince the light was truly modify'd by the refraction* and reflection it fuffered in paffing thro' the prifm: and the object, in our cafe, ferved as a fpeculum to reflect that colour to the eye. For a rough and coloured object may ferve for a fpeculum to reflect the artificial rain-bow we fpeak of; fince, in darkned rooms, a wall, conveniently fituated within, will fo reflect the colours of bodies external to the room, that they may very clearly be difcern'd and diftinguished and yet it is taken for granted, that the colours feen in a darkned room, tho' they leave no traces upon the wall or body that receives them, are the true colours of the external objects; together with which the colours of the images are moved, or reft. And the error lies not in the eye, whofe office is only to perceive the appearance of things, and truly does fo; but in the judgment that, by miftake, concludes the colour belongs to the wall, which really belongs to the object; becaufe the wall is that from whence the rays of light which carry the vifible fpecies, come in ftrait lines directly to the eye. And thus, for the fame reafon, we fuppofe at a certain diftance from concave fpherical glaffes,

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*We must again repeat, that Sir "of colour, made by refra&tion, do not Ifaac Newton demonftrates, all the colours "arife from any new modification of the in the universe, which are made of light, rays impreffed by thofe refractions; and depend not on the power of imagi- "and, by the various terminations of nation, are either the colours of homoge-"light and fhadow; as has been the neal lights, or compounded of them in a "conftant and general opinion of phicertain proportion. "For the changes" lofophers." Newton. Optic. p. 99. & p. 138.

that

that we fee the image ftand out to meet us, and hang in the air betwixt PHYSICS. the glafs and us; becaufe the reflected rays, that compofe the image, orofs one another in that place where the image appears to be; and thence in direct lines take their courfe to the eye. I might here add what will more fully appear hereafter, that colours, called emphatical, because not inherent in the bodies where they appear, may be compounded with one another, as thofe that are confeffedly genuine.

I

SECT. II.

Come now to inquire into the nature of whitenefs and blackness. The nature of whiteness. Whiteness, confidered as a quality in the object, feems, in the general, chiefly to depend upon the roughnefs of the furface of the body, call'd white; which gives it innumerable fmall fuperficies, that acting like fo many little specula in various pofitions, they reflect the rays of light that fall on them, not towards one another, but externally towards the fpectator's eye. The fun, and other very lucid bodies, not only offend or dazle our eyes; but if any colour is afcribed to them, it should be whitenefs: for the fun at noon-day, and in clear weather, when his face is ferene,

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*White, in ftri&tnefs, we have already obferved to be no fimple colour; but a compound of all colours in a due proportion. "When the feveral forts of rays are mixed, fays Sir Ifaac Newton, and, "in croffing, pafs thro' the fame space, they act not upon one another, fo as "to change their colour-making qualities; but, by mixing their actions in "the fenforium, beget a fenfation, differ"6 ing from what either would do a-part : that is, a fenfation of a mean colour between their proper colours. And particularly, when, by the concourfe "and mixtures of all forts of rays, a "white colour is produced; the white

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is a mixture of the colours which the rays would have a-part: the rays in "that mixture do not alter their feveral colour-making qualities; but, by all "their various kinds of actions, mixed "in the fenforium, beget a fenfation of a ་་ middling colour, between all their co"lours, which is whitenefs. For white"nefs is a mean between all cciours; "having itfelf indifferently to them all; "fo as with equal facility to be tinged "with any of them. A red powder A red powder "mixed with a little blue, or a blue "with a little red, doth not prefently

"lofe its colour; but a white powder,
"mixed with any colour, is prefently
"tinged with that colour, and is equally
capable of being tinged with any co-
"lour whatever." See Newton. Optic. p.
117, 118: 138, 139.

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We farther learn from Sir Ifaac Newton,
that as a mixture of all the prifmatic co-
lours makes perfect white light; fo if
foapy water be agitated into a froth, va-
rious colours will appear therein, when
viewed near; and, at a distance, when
thofe colours can
no longer be diftin-
guifhed, the froth will feem perfectly
white. But, by mixing coloured pow-
ders, he tells us, we are not to expect a
ftrong and full white, but fome dusky
obfcure one; because they fupprefs and
ftop in them a very confiderable of
the light by which they are illuminated.
Thus he produced a dark white, or a
dun colour, by mixing one part of red
lead with five parts of verdigreafe; for
thefe two colours were feverally so com-
pounded of others, that, in both toge-
ther, there was a mixture of all colours.

part

Again, one part of red lead, and four parts of blue bife, compofed a dun colour, varying a little to purple; and, by adding to it a certain mixture of orpiE 2

ment

PHYSICS. ferene, and his rays pafs thro' a much lefs part of the atmosphere to our eyes, appears of a colour more approaching to white than when he is nearer the horizon; in which cafe the interpofition of certain fumes and vapours make him oftentimes appear either red or yellow. And when the fun fhines upon fmooth water, that part of it which appears moft illumined, feems far whiter than the reft. And I have fometimes found, that when the fun was veil'd with a thin white cloud, tho' ftill too bright to be look'd upon directly; that by cafting my eyes upon ftill water, his body, being not far from the meridian, appeared to me exceedingly white. And tho' we vulgarly fay, in English, a thing is red-hot, to express a superlative degree of heat; yet, at the forges and furnaces of artificers, by a white heat they understand a further degree of ignition, than by a red one.

2. Common experience informs us, that as too much light overpowers the eye; fo when the ground is covered with fnow, thofe who have a weak fight, complain that this profpect is offenfive to them. And even thofe who have good eyes, are from hence generally fenfible of an extraordinary light in the air; and, if obliged to look very long upon the fnow, find their fight injured by it. Thus Xenophor relates, that Cyrus marching his army, for many days, over mountains covered with fnow, the dazling fplendour of its whitenefs prejudiced the fight of a great number of his foldiers, and blinded fome of them; and other accounts of the fame nature may be met with in writers of good note. The like has also been affirmed to me by credible perfons of my own acquaintance, and especially by one, who, during

ment and verdigreafe, in due proportion, it loft its purple tin&ture, and became perfectly dun. But the experiment fucceeded beft without red lead, thus: "To orpiment, fays Sir Isaac, I "added, by little and little, a certain "full bright purple, which painters "ufe, till the orpiment ceased to be "yellow, and became of a pale red: "then I diluted that red, by adding a "little verdigreafe and a little more blue bife than verdigreafe, till it became of fuch a grey or pale white as va"ried to no one of the colours more than "to another. For thus it became of a "colour equal in whiteness to that of "afhes, or of wood newly cut, or of a "man's skin. The orpiment reflected 66 more light than did any other of the "powders; and therefore conduced more "to the whiteness of the compounded "colour than they. To affign the pro"portions accurately, may be difficult,

by reafon of the different goodness of "powders of the fame kind. As the co

"lour of any powder is more or lefs full "and luminous, it ought to be used in a "lefs or greater proportion. Now con"fidering that thefe grey and dun co"lours may be alfo produced, by mixing "whites and blacks; and, by confe

quence, differ from perfect whites, not "in fpecies of colours, but only in de

gree of luminoufnefs; it is manifeft "that there is nothing more requifite to "make them perfectly white, than to "increase their light fufficiently; and, "on the contrary, if, by increasing their "light, they can be brought to perfect "whiteness; it will thence alfo follow, "that they are of the fame fpecies of "colour with the beft whites; and dif"fer from them only in the quantity "of light." And accordingly, by placing fome powder, compofed of orpiment, purple, bife, and verdigreafe, in the fun's rays; and, viewing it at a diftance, it appeared intenfely white. Newton. Optic.

P, 129-134.

his ftay in Muscovy, found his eyes much impaired, by being frequently PHYSICS. obliged to travel in the fnow and this weakness of fight did not leave him when he left that country, but ftill continues, tho' he be a young man. I myself alfo, as well as others, have obferved, that upon travelling by night, when the ground was all covered with fnow, tho' it would otherwife have been dark, yet we could very well fee to chuse our way. But much more to my prefent purpose is that account given us by Olaus Magnus of a way of travelling in the Northern regions during the. winter, where the days of that feafon are very fhort. "In the day-time, fays he, they travel twelve Italian miles, but twice or thrice as far in "the night, and that with ease; for the light of the moon, reflected by the "fnow, renders both hills and vales confpicuous; fo that then they can "fee not only precipices afar off, but the wild beafts they would avoid.'

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This teftimony I the lefs fcruple to alledge, because it agrees very well with what has been affirmed to me by a phyfician of Mofcow; who informed me, that he could fee things at a far greater diftance, and with more clearness, when he travelled by night on the fnow there, tho' without the affiftance of the moon-fhine, than we, in thefe parts, would eafily believe. 'Tis true, indeed, the intenfenefs of the cold might contribute fomething to the confiderableness of the effect, by clearing the air of dark steams, which, in thefe more temperate climates, are usually thick in fnowy weather: for this phyfician, and the ingenious navigator Captain James agree, that in dark frofty nights, in frozen climates, they could difcover more ftars, and have a clearer profpect of the heavens, than we in England. I know, indeed, many learned men fuppofe fnow thus ftrongly affects our eyes, not by a borrowed, but a native light. I venture, however, to give it as a proof, that white bodies reflect more light than others, becaufe having once placed a parcel of fnow in a room, carefully darkened, that no celeftial light might fall upon it, neither I, nor an ingenious perfon skilled in optics, could find it had any light befides what it received; and 'tis ufual, among fuch as travel in dark nights, to make their guides wear fomething of white to be difcern'd by; for there is fcarce any night fo dark, but that, in the free air, fome light remains, tho' broken and debilitated, perhaps, by a thoufand reflections from the opake corpufcles, that fwim in the atmosphere, and fend it to one another before it arrives at the eye.

3. And the better to fhew that white bodies reflect much more light than others, I held in the darkened room, formerly mentioned, not far from the hole at which the light entered, a fheet of white paper; from whence cafting the fun-beams upon a white wall, it manifeftly appeared both to me, and to a perfon I took to be witness of the experiment, to reflect a far greater light than any of the other colours; for the wall itself was not only thus notably enlightened, but alfo a confiderable part of the room. And, further, to fhew that white bodies reflect the rays outwards, let me add, that ordinary burning-glaffes will not, in a great while, burn or difcolour white paper; fo that when I was a boy, and delighted to

make

PHYSICS. make trials with fuch glaffes, I could not but wonder at this odd phenomenon, which fet me very early upon gueffing at the nature of whitenefs.; and the more, becaufe I took notice that the image of the fun, upon white paper, was not fo well defined as upon black; and because that, upon inking over the paper, the moisture would be quickly dried up, and the paper, that I could not burn before, would now p elently take fire. I have alfo try'd, that by expofing my hand, with a thin black glove on it, to the warm fun, it would thereby very fuddenly be more confiderably heated, than if I took off the glove, and held my naked hand to its rays, or put on another glove of thin white leather.

4. And to fhew that white bodies are apt, like fpecula, to reflect the light that falls on them, we have found, in a darken'd room, that the fun-beams being caft from a coloured body upon a white wall, the determinate colour of the body was from the wall reflected to the eye; whilft we could, in many cafes, manifeftly alter the colour arriving at the eye, by fubftituting, at a convenient diftance, a colour'd gloffy body inftead of the white wall; thus by throwing the rays from a yellow body upon a blue, there would be exhibited a kind of green. I might alfo here take notice, that when looking upon the calm and fmooth furface of a river, lying betwixt my eye and the fun, it appeared to be a natural fpeculum, wherein that part, which reflected to my eye the entire and defined image of the fun and rays adjacent thereto, appeared of a great whitish brightnefs, but the reft comparatively dark; when, if afterwards the fuperficies chanced to be a little ruffled by a gentle breath of wind, and thereby reduced into a multitude of fmall, fmooth fpecula, the furface of the river would, at a diftance, appear almoft white; tho' it loft that appearance upon a return of the furface to a calm and uniform level. And I have fometimes, as an experiment, brought in a lenticular glafs, the image of a river illumined by the fun, into a darkened room, at about the diftance of a quarter of a mile; by which means the numerous declining furfaces of the water were fo contracted, that, upon the body which received the image, the whole river appeared a very white object at two or three paces diftance. But if we drew near it, this whitenefs feemed to proceed from innumerable lucid reflections, made by the fuperficies of the water gently waved, which looked, when clofely viewed, like a multitude of very little fhining fcales of fifh; many whereof every moment difappeared, while as many were by the fun, wind, and river, generated anew. But tho' this obfervation feemed fufficiently to difcover how the apparent whitenefs, in that cafe, was produced; yet in fome other cafes, water may have the fame, tho' not fo vivid a colour, upon other accounts; for it often happens that the fmooth furface of the water appears bright or whitish, by reafon of the reflection, not immediately of the image of the fun, but of the brightness of the sky; and in fuch cafes a convenient wind may, where it paffes along, make the furface look black, by caufing many fuch furrows and cavities, as make the inflected fuperficies of the water reflect the brightnefs of the sky, rather internal

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