PHYSICS. nuine colours of moft natural productions. It may, indeed, be further objected, that, according as the fun or other luminous body changes place, these 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 stretch it flat, it will commonly exhibit some one uniform colour; yet thefe are not reputed emphatical: so that the difference feems to be chiefly this, that in the case of the rain-bow, and the like, the pofition of the illuminating body varies the colour; and, in cloth, the position 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 whiteness of froth emphatical, we have already feen what may be inferred from thence. Befides, the fun-beams tranfmitted thro' a triangular prifm of glass, after the manner lately mentioned, will, upon the body that terminates them, paint a rain-bow, visible 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 circumstances. 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 fuffer'd 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 serve for a fpeculum to re-flect 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 discern'd and distinguished: 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 mistake, concludes the colour belongs to the wall, which really belongs to the object; because the wall is that from whence the rays of light which carry the visible species, come in ftrait lines directly to the eye. And thus, for the fame reason, we fuppofe at a certain diftance from concave spherical glaffes, that we see the image ftand out to meet us, and hang in the air betwixt PHYSICS. the glafs and us; because the reflected rays, that compofe the image, cross one another in that place where the image appears to be; and thence in direct lines take their course 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 those that are confeffedly genuine. I SECT. II. Come now to inquire into the nature of whiteness and blackness. The nature of Whiteness, confidered as a quality in the object, seems, in the general, whiteness. 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 fpecula in various pofitions, they reflect the rays of light that fall on them, not towards one another, but externally towards the spectator'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 fhould be whitenefs: * for the fun at noon-day, and in clear weather, when his face is ferene, White, in ftrictnefs, 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, "pass 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 " sensation, differing from what either "would do a part: that is, a fenfation of 66 a mean colour between their proper co"lours. And particularly, when, by the "concourse and mixtures of all forts of 66 ་་ "der, mixed with any colour, is presently We farther learn from Sir Isaac Newton, 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 orpi E 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 case 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 most illumined, seems 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 exprefs a fuperlative 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, those who have a weak fight, complain that this profpect is offenfive to them. And even those 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 Xenophon relates, that Cyrus marching his army, for many days, over mountains covered with fnow, the dazling fplendor of its whiteness 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 propor- 66 "lour of any powder is more or less 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 confequence, differ from perfect whites, not " in fpecies of colours, but only in degree 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 increafing 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 differ "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 stay 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 otherwise have been dark, yet we could very well fee to chufe our way. But much more to my prefent purpofe 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 eafe; 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." This teftimony I the less fcruple to alledge, because it agrees very well with what has been affirmed to me by a physician of Moscow; who informed me, that he could fee things at a far greater diftance, and with more clearness, when he travelled by right on the fnow there, tho' without the affiftance of the moon-fhine, than we, in thefe parts, would eafily believe. 'Tis true, indeed, the intenfeness 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 ufually thick in fnowy weather: for this phyfician, and the ingenious navigator Captain James agree, that in dark frosty nights, in frozen climates, they could discover more stars, and have a clearer profpect of the heavens, than we in England. I know, indeed, many learned men fuppofe fnow thus strongly affects our eyes, not by a borrowed, but a native light. F 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 such as travel in dark nights, to make their guides wear fomething of white to be discern'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 thousand 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 also 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, because 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 prefently 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; whilst we could, in many cafes, manifeftly alter the colour arriving at the eye, by fubftituting, at a convenient distance, a colour'd gloffy body instead 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 smooth 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 distance, 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 glass, 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 whiteness feemed to proceed from innumerable lucid reflections, made by the fuperficies of the water gently waved, which looked, when closely viewed, like a multitude of very little fhining scales of fish; many whereof every moment disappeared, 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 smooth furface of the water appears bright or whitish, by reason 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 causing many fuch furrows and cavities, as make the inflected fuperficies of the water reflect the brightness of the sky, rather internal |