(1) EXPERIMENTS AND OBSERVATIONS UPON COLOURS. SECT. I. lours, what it IS often thought that a diverfity of colours conftantly argues an Diverfity of o equal diverfity in the nature of the bodies wherein they re- fignifies. fide: but I cannot wholly give into this opinion. For not to mention changeable taffaties, the blue and golden feathers on the necks of pigeons, and feveral water-fowl, natural and artificial rainbows, &c. the colours whereof philofophers call not real, but apparent; we see that the contiguous feathers in various birds are fome of them red, others white, blue, yellow, c. and that in feveral parts of the fame feather there is frequently the greatest difparity of colours: fo in tulips, july-flowers, and other vegetables, even feveral parts of the very fame leaf are frequently found of different dyes; tho' no difference is obferved in their other properties. And fuch a variety we have much more remarkably in the marvel of Peru; for of the great profufion of fine flowers which that gaudy plant affords, I have fcarce obferved any two dyed perfectly alike. But tho' fuch particulars keep me from affirming, that a diverfity of colours always denotes fome great difference in bodies, yet that it often fignifies confiderable alterations in the difpofition of their parts, appears from the extraction of tinctures, wherein the change of colour is the chief, and fometimes the only thing by which the artist regulates his procedure in their preparation. Inftances of this are alfo obvious in feveral forts of fruit, wherein according as the vegetable fap is ripened, by paffing from one degree of maturation to another, the external part of VOL. II. B the PHYSICS. the fruit changes from one colour to another. A lefs obvious inftance of this kind is afforded us by the method of tempering fteel for gravers, drills, fprings, &c. which is this. First, the fteel to be tempered is hardened, by heating it in glowing coals, and not quenched as foon as taken from the fire, but held over a bafon of water till it defcend from a white heat to a red one; when 'tis immediately quenched in cold water. The fteel thus hardened will, if it be good, look whitish; and being brighten'd at the end, and held in the flame of a candle, that the bright end may lie about half an inch diftant from the flame, it will fwiftly pafs from one colour to another, as from a bright yellow to a deeper and reddish yellow, from that to a fainter firft, and then to a deeper blue; each of which fucceeding colours argues fuch a change made in the texture of the fteel, that if it be taken from the flame, and immediately quenched in tallow, whilft it is yellow, it will be of fuch a hardness as fits it for drills G. but if kept for a few minutes longer in the flame, till it turns blue, it becomes much fofter and proper to make fprings for watches; which are therefore commonly of that colour; laftly, if you keep the fteel in the flame after the deep blue has appeared, it will grow too foft even for penknives. Any perfon may eafily fatisfy himself of the different hardness of fteel of different colours, either by the file, or by breaking fome flender wires thus tempered, and obferving how they vary in point of brittlenefs. Colour, what? But before we defcend to a more particular confideration of our fubject, 'tis proper to obferve, that colours may be regarded either as a quality refiding in bodies to modify light after a particular manner, or elfe as light itself fo modified as to ftrike upon the organ of fight, and caufe the fenfation we call colour; and that this latter is the more proper acceptation of the word colour, will appear hereafter. And indeed it is the light itself, which after a certain manner, either mixed with fhades, or otherwife, ftrikes our eyes, and immediately produces that motion in the organ which gives us the colour of an object. Yet because there is in the power and difpofition to ftir up a fen"fation of this or that colour; for as "found in a bell, or mufical ftring, or "other founding body, is nothing but a trembling motion, and in the air nothing but that motion propagated from "the object, and in the fenforium 'tis a fenfe of that motion under the form of a found; fo colours in the object are "nothing but a difpofition to reflect this *The words of Sir Ifaac Newton excellently clear and illuftrate this matter. "The homogeneal light and rays (fays "that great philofopher) which appear "red, or rather make objects appear fo, "I call rubrific, or red-making; thofe "which make objects appear yellow, green, blue, and violet, I call yellow-" "making, green-making, blue-making, "violet-making, &c. And if at any time "I fpeak of light and rays as coloured, or or that fort of rays more copiously than "endued with colours, I would be under- "the reft in the rays they are nothing "flood to speak not philofophically and "but their difpofitions to propagate this "properly, but grofly, and according to "or that motion into the fenforium, and "fuch conceptions as vulgar people would "in the fenforium they are fenfations of "be apt to frame. For the rays, to fpeak" thofe motions under the forms of co"properly, are not coloured." In them "lours. Newton. Optic. p. 108, 109. "there is nothing elfe than a certain coloured coloured body a certain difpofition of the fuperficial particles to fend the PHYSICS. light reflected or refracted to our eyes, colour may affo, in fome fenfe, be faid to depend upon the vifible body; and therefore I fhall not reject the popular form of fpeaking of colours, provided we may have recourfe occafionally to the diftinction laid down. But ftill colour is fo far from being inherent in objects, that light itfelf produces the fenfation only as it caufes a determinate kind of local motion in fome part of the brain; for if the like motion happen from any other caufe, wherein the light is unconcerned, a man fhall think he fees the fame colour*. Thus it is ufual in dreams to fancy we behold coloured images; and the ftrange imaginations of diftracted perfons, with the flashings of light, and other appearances, upon fome diforders of the brain, farther confirm this particular. The like effect may alfo be produced from internal caufes which affect the optic nerve; for I remember in myfelf that, upon coughing with vehemence, fudden flashes of vivid flame have appeared, as it were, before my eyes t. And a lady, to whom I am related, affured me, that all the objects of her fight, once, of a fudden, appeared of various unufual colours, furprizingly bright and vivid; which fymptom was, the next day, followed by a fit of the hyfteric difeafe. The like fymptom a phyfician alfo informed me he had obferved to be a certain prognoftic of the plague, in a feafon when that diftemper raged; but that it generally went off after the exhibition of an emetic. As an appearance of colour may, therefore, be produced by internal motions, without the affiftance of an external object; fo the colour, which would otherwife be produced by an outward object, may, poffibly, be fometimes changed by a motion or new texture in the organ, as long as that motion or new difpofition continues. Thus I have often obferved, upon looking at the fun thro' a telescope, darkened fo as to make the fplendor of that luminary fupportable, the impreffion upon the retina, would nevertheless be fo vivid and permanent, that if afterwards I turned the eye, therein made ufe of, towards a flame, it would appear of a colour very different from its natural one. * "Tis certain, that whenever the fame eye is affected twice alike, vifion will be the fame in both cafes; but different when the eye is differently affected. And this is a juft, not an imaginary, foundation for diftinguishing red light from blue, yellow from green, &c. for that is properly a red light which excites in us the idea of red, a blue which excites in us the idea of blue, &c. Thefe feveral colours, therefore differently affe&t the eye, and confequently have different difpofitions. To this purpose Sir Ifaac Newton fpeaks thus: "When a man in the dark, fays he, preffes either corner of his eye "with his finger, and turns his eye away "from his finger, he will fee a circle of "colours like thofe in the feather of a B 2 And PHYSICS. And if I feveral times fucceffively fhut and opened the fame eye, this new colour feem'd changed or impaired by degrees; till at length the flame appeared to me of the fame colour it did to other fpectators. The like effect I have alfo found by looking upon the moon, when near the full, thro' a telescope, without employing a coloured glafs to defend the eye. And here 'tis remarkable, that tho' my right eye, with which I looked thro' the telescope, were thus affected by the light, yet if I fhut that eye, and looked upon the fame object with the other, it appeared of its ufual colour; but if I again opened and made ufe of the dazzled eye, the vivid adventitious colour would again return. "Tis further obfervable, that a vehement blow upon the organ of vifion, efpecially if it be naturally weak, may for a long time vitiate the action of vivid objects. I know a iady of unqueftionable veracity, who having, by a defperate fall, received feveral hurts, and particularly a confiderable one near her eye, had her fight fo difordered, that the next morning, when one of her fervants came to her bed-fide, his clothes appeared adorned with fuch a variety of dazzling colours, as prefently obliged her to command him to withdraw; and even the images in her hangings did, for many days after, appear to her, if the room were not greatly darkened, embellished with feveral offenfively vivid colours, which nobody elfe could fee in them. She faid alfo, that The fometimes thought the faw colours fo new and glorious, that they were of a peculiar kind, and fuch as fhe could not defcribe, by their likeness, to any fhe had feen either before or fince; and that white objects greatly difordered her fight; that for feveral days after her fall, if the looked upon the infide of a book, fhe fancied, the there faw colours like thofe of a rainbow. And even when she seemed pretty well recovered, and left her chamber, happening to come into a place where the walls were white, they appeared to her of fuch dazzling colours as much offended her fight. She added, that this diforder of her eyes lafted for five or fix weeks; tho' fince that time she hath been able to read and write confiderably, without finding the leaft inconvenience. A man of great learning coming to advife with me about a diftemper in his eyes, told me, that having once looked too attentively upon the fun thro' a telescope, without a dark glafs, the excefs of light fo ftrongly affected his eye, that ever fince, when he turns it towards a window, or any white object, he fancies he fees a globe of light about the fame bignefs the fun then appeared to him; tho' it were now ten years fince he firft obferved it. "Twere eafy from fome remarkable fymptoms, obferved by Epiphanius Ferdinandus, in perfons bitten by the tarantula, to fhew, that without any change in the object, an alteration in the inftrument of vifion may, for a great while, make fome colours appear delightful, and others difagreeable, and both to a high degree; tho' they had no fuch effects before. But thefe already mentioned may fuffice for our prefent purpose. We before obferv'd, that colour may, notwithstanding all this, be confider'd as a quality refiding in the body faid to be colour'd; and indeed moft of the following experiments refer to it principally under that notion. For For there is in colour'd bodies, and chiefly in their fuperficial parts, a certain PHYSICS. difpofition whereby they difturb the light that comes from them to our eye, fo as to make that diftin&t impreffion, upon whofe account we say the visible body is either white or black, red or yellow. But because we fhall hereafter more fully fhew, that the changes, and confequently the production and the appearance of colours often depend upon the continuation or alteration of the texture of the object, we shall here previously intimate two or three particulars relating to this matter. And firft, it is whether colours not without reafon, that I afcribe colour chiefly to the fuperficial parts of depend upon the bodies; for, not to examine how plentifully opake corpufcles may abound even in those bodies we call diaphanous, it is plain, that we fee little elfe than the fuperficies of dark objects; for if we found the rays of light reflected from the object, pierc'd deep into the body, we should not judge it opake, but either pellucid or femi-diaphanous +. The fchools feem to teach, that colour is a penetrating quality, reaching to the innermoft parts of the object; as if a piece of fealingwax be broken into ever fo many pieces, the internal fragments will be as red as the external furface. This is indeed a particular example; but it will not overthrow the foregoing doctrine; efpecially fince other examples of a contrary nature may be alledged. And two or three negative inftances are fufficient to overthrow the generality of a pofitive rule; at leaft when that is built but upon one, or a few examples. Not then to mention cherries, plums, &c. wherein the skin is of one colour, and the infide of another; I fhall offer an inftance or two of the colours of durable bodies, that are thought tolerably homogeneous, and whofe parts are neither organical, nor of a nature approaching thereto. And firft, I need only repeat, that blue, red, and yellow may be produced upon a piece of temper'd fteel: for thefe colours, tho' very vivid, yet if you break the metal they adorn, will appear to be fuperficial; not only the innermoft parts of the fteel, but thofe within a hair's breadth of its fuperficies, having none of these colours, but retaining that of the metal itself. Secondly, we melted a large quantity of pure lead with a ftrong fire; then immediately pouring it into a clean iron veffel, and carefully and nimbly *It follows from Sir Ifaac Newton's dif coveries, relating to light and colours, that all the productions and appearances of them in the world are derived not from any phyfical change caufed in light by refraction or reflection, but only from the various mixtures or feparations of rays, by virtue of their different refrangibility or reflexibility. And in this refpect, fays that great author," the fcience of co"lours becomes a fpeculation as truly "mathematical as any part of optics, fo "far as they depend on the nature of "light, and are not produced or altered "by the power of imagination, or by + Perhaps all opake bodies, when re- taking Surfaces of bodies. |