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 bason 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 brigh ten'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 swiftly 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; &c. but if kept for a few minutes longer in the flame, till it turns blue, it becomes much fofter and proper to make springs for watches; which are therefore commonly of that colour; laftly, if you keep the steel 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 steel of different colours, either by the file, or by breaking fome flender wires thus tempered, and obferving how they vary in point of brittleness. Colour, what? But before we defcend to a more particular confideration of our subject, 'tis proper to obferve, that colours may be regarded either as a quality refiding in bodies to modify light after a particular manner, or else as light itself fo modified as to strike upon the organ of fight, and cause 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 itfelf, which after a certain manner, either mixed with fhades, or otherwise, 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 "difpofition to ftir up a fenfation of this or "that colour; for as found in a bell, or mu 66 66 fical ftring, or other founding body, is no"thing but a trembling motion, and in the "air nothing but that motion propagated "from the object, and in the fenforium 'tis a fense of that motion under the form of a found; fo colours in the object are nothing but a difpofition to reflect this or "that fort of rays more copiously than the "reft in the rays they are nothing but "their difpofitions to propagate this or that "motion into the fenforium, and in the "fenforium they are fenfations of those mo❝tions under the forms of colours. Optic. p. 108, 109. Newton. coloured body a certain difpofition of the fuperficial particles to fend the PHYSICS. light reflected or refracted to our eyes, colour may alfo, in fome fenfe, be U faid to depend upon the visible body; and therefore I fhall not reject the popular form of speaking 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 itself 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 some disorders of the brain, farther confirm this particular. The like effect may also be produced from internal caufes which affect the optic nerve; for I remember in myself that, upon coughing with vehemence, fudden flashes of vivid flame have appeared, as it were, before my eyes. And a lady, to whom I am related, affured me, that all the objects of her fight, once, of a fudden, appeared of various unusual colours, furprizingly bright and vivid; which fymptom was, the next day, followed by a fit of the hysteric difeafe. The like fymptom a phyfician alfo informed me he had obferved to be a certain prognoftic of the plague, in a seafon 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 distinguishing red light from blue, yellow from green, . 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 affect 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 pea"cock's tail. If the eye and the finger re"main quiet, these colours vanish in a se"cond minute of time; but if the finger "be moved with a quavering motion, they appear again. Do not thefe colours arife from fuch motions excited in the bottom "of the eye by the preffure and motion of "the finger, as at other times are excited "there by light fór caufing vifion? And "do not the motions, once excited, conti"nue about a fecond of time before they "cease? And when a man by a stroke upon "his eye fees a flash of light, are not the "like motions excited in the retina by the "Stroke?" Newton. Opt. f. 321, 322. 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 glass 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 lady of unquestionable veracity, who having, by a defperate fali, 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, embellifhed with feveral offenfively vivid colours, which nobody elfe could fee in them. She faid alfo, that fhe fometimes thought fhe 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 he looked upon the infide of a book, fhe fancied fhe there faw colours like thofe of a rainbow. And even when the feemed 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 he hath been able to read and write confiderably, without finding the least 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 glass, the excess 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 first observed it. 'Twere eafy from fome remarkable symptoms, 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 these already mentioned may fuffice for our present 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 most of the following experiments refer to it principally under that notion. For there is in colour'd bodies, and chiefly in their fuperficial parts, a cer- PHYSICS. tain difpofition whereby they disturb the light that comes from them to our eye, fo as to make that diftinct impreffion, upon whofe account we fay the vifible 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 fhall here previously intimate two or three particulars relating to this matter. And first, it is Whether conot without reason, that I afcribe colour chiefly to the fuperficial parts of lours depend bodies; for, not to examine how plentifully opake corpufcles may abound even in those bodies we call diaphancus, it is plain, that we fee little else than the fuperficies of dark objects; for if we found the rays of light re flected from the object, pierc'd deep into the body, we should not judge it opake, but either pellucid or femi-diaphanous +. The schools 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 least 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 first, I need only repeat, that blue, red, and yellow may be produced upon a piece of temper'd steel: for these colours, tho' very vivid, yet if you break the metal they adorn, will appear to be fuperficial; not only the innermost 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 discoveries, 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 colours becomes a fpeculation "as truly mathematical as any part of op"tics, fo far as they depend on the nature "of light, and are not produced or altered "by the power of imagination, or by ftri- + Perhaps all opake bodies, when reduced upon the fur faces of bodies. PHYSICS. taking off the fcum, we perceiv'd the smooth gloffy surface of the melted matter to be adorn'd with a very delightful colour, which almost immediately gave place to another vivid one, that was as immediately fucceeded by a third, and this by a fourth. Thus thefe wonderfully vivid colours fucceffively appear'd and vanish'd, till the metal cooling, put a ftop to this pleasant fight; tho' the colours which chanced to adorn the surface, when the lead began to cool, remain'd upon it, but were fo fuperficial, that how little foever we fcraped off from the furface of the lead, all the colour came away, and discovered only that which is natural to the metal. But unless lead be brought to a very high degree of fufion or fluidity, the phenomena will scarce appear. However, the fame colours did neither always, nor regularly, fucceed one another, as is usual in steel; but in the following diversify'd order, which paffed fo fuddenly, that I was scarce able to commit it to paper; blue, yellow, purple, blue, green, purple, blue, yellow, red, purple blue, yellow and blue, yellow, blue, purple, green mix'd, yellow, red, blue, green, yellow, red, purple, green. To the fame purpose I might add, that viewing a sphere of rock crystal, which was fawn afunder by a lapidary, and expofing the flat furfaces to the fun beams, the little particles that, notwithstanding their feeming fmoothness in the fhade, render'd their surfaces rough, so refracted and reflected the light, as to exceed the vivid colours of the rainbow; but in an interrupted order, fometimes on one part of the furface, fometimes on another, as it happen'd to be fituated with regard to the fun. And having caufed a fine-grain'd touchftone to be fawn afunder by the fame artificer, I obferved upon the new furfaces, tho' to the touch they feem'd smooth and polifh'd, the vivid colours, as above-mention'd, according as the furfaces were put into various pofitions, in respect of the fun and the eye; fo that, notwithstanding the great tranfparency of the cryftal, and the great opacity of the touch-stone, their fuperficial corpufcles were found fit to exhibit the vivid colours we admire in the rain-bow. The atomifts of old, and fome learned men of late, have attempted to explain the variety of colours in opake bodies from the various figures of their fuperficial parts. The attempt is indeed ingenious, and the doctrine feems partly true; but I confefs other things appear to me neceffary to be taken in, as contributing to thofe different forms of afperity whereon the colours of opake bodies depend. But in order to prove it, we must affume, that the furfaces of all fuch bodies, how fmooth foever they may appear to our fight and touch, are only fo in a popular, or a phyfical fenfe. The truth hereof is evident from the ufe of microscopes, which fhew us in many bodies, that feem fmooth to our naked eyes, little protuberancies rifing above that which may be conceiv'd to be the plain of the furface; as alfo numerous depreffions beneath that level. And of this fort of cavities we have, by the help of an excellent magnifier, on the furface of a thin piece of cork, that appeared fmooth to the eye, observ'd fixty in a row, within the compafs of that glafs, tho' not above of an inch; and thefe too, which made that little piece of cork look almoft like an |