them, than that which our blind man allowed, when, comparing black PHYSICS. and white bodies, he faid, that the latter were the lefs rough of the two. Nor is every kind of roughnefs, tho' fenfible, inconfiftent with whitenefs, for the phyfical fuperficies of a body is fometimes made, by the fame operation, both rough and white; as when the level furface of clear water being agitated and made rough, with a multitude of unequal bubbles, thereby acquires a whitenefs; and as a fmooth piece of glass by being fcratched with a diamond, difclofes the fame colour. This minutenefs and figure of the extant particles being fuppofed, the depth of the little cavities intercepted between the extant particles, without being fo much greater in black bodies than in white ones, as to be very perceptible to the grofs organs of touch, may be vaftly greater with regard to their disposition of reflecting the fubtile rays of light. For in black bodies thofe fmall intercepted cavities, and other depreffions, may be fo figured, fo narrow and fo deep, that the incident rays of light, which the more extant parts of the phyfical fuperficies are difpofed to reflect inwards, may be detained there, and prove unable to emerge; whilft in a white body the flender particles may not only, by their figure, be fitted to reflect the light copioufly outwards, but the intercepted cavities being fhallow, and perhaps fomewhat wide, the bottoms of them may be fo conftituted, as to be fit to reflect outwards much of the light that falls upon them. There is no neceffity of concluding, from the blind man's relation, that tho', according to his touch, black was the rougheft, as it is the darkeft of colours, that therefore white, which is the lighteft, fhould also be the fmootheft; for he makes yellow to be two degrees rougher than blue, and as much fmoother than green; tho' yellow not only appears to the eye a lighter colour than blue, but we fhall hereafter fhew, that yellow reflects much more light than blue, and manifeftly more than green; which feems to ftrengthen our conjecture, that there was fomething in the kinds of afperity, as well as in the degrees of it, that enabled Vermaafen to diftinguifh colours; and, at leaft fhews, that we cannot, in all cafes, from the bare difference in the degrees of afperity in colours, fafely conclude that the rougher reflects the leaft light. Thus much, however, we gain from the teftimony of this blind man, that fince many colours may be felt with the circumstances above related, the furfaces of fuch coloured bodies muft certainly have different degrees, and in all probability different forms, or kinds of afperity belonging to them; which is all the ufe I here defigned to make of the hiftory of this blind man for hence it fufficiently appears, that colour greatly depends upon the difpofition of the fuperficial parts of bodies, and, probably, wherein fuch a difpofition principally confifts*. The permanent colours of natural bodies arife from hence, that some such bodies reflect fome forts of rays, and others other forts more copiously than the reft. Minium, fays Sir Isaac New But quors. PHYSICS. But to return; the caufes of the feveral forms of afperity, that may diverfify the furfaces of coloured bodies, will, perhaps, afford us fome How changes general conjectures as to feveral of the ways whereby 'tis poffible to are produced in colours, by li- produce the fudden changes of colours, we fhall hereafter find in our experiments; for moft of thofe phenomena being produced by means of liquors, which generally abound with very minute, active, and varioufly figured faline corpufcles, they may well very fuddenly alter the texture of the body they are employed to work on; and fo change their form of afperity, and thereby make them reflect to the eye the light that falls on them, after another manner than they did before, and by that means vary the colour fo far as it depends upon the texture or difpofition 66 "other bodies. Every body reflects the violet blue light, is moft refplendent, and 46 green, are more ftrongly reflected than "the other colours, red and violet, &c. "But to make these experiments the more "manifeft, fuch bodies ought to be cho"fen as have the fulleft and most vivid co"lours, and two of those bodies are to "be compared together. Thus, for in“stance, if cinnabar and ultramarine blue, of "doth, and that the ultramarine reflects "the blue light much more copiously than "the cinnabar doth. The fame experi"ment may be made fuccefsfully with red "lead and indigo, or with any other two "coloured bodies, if due allowance be "made for the different ftrength or weak"nefs of their colour and light. And "fince these reflected lights, which differ "in colour, differ alfo in degree of re"frangibility, it's certain that fome bodies "reflect the more refrangible, and others "the lefs refrangible rays more copiously; "and that this is the only reason of these "colours, appears from hence, that the co"lour of homogeneal light cannot be changed by the reflexion of natural bodies. "For if bodies, by reflexion, cannot in "the least change the colour of any fort of 66 66 rays, they cannot appear coloured by any "other means, than by reflecting those "which are either of their own colour, or "which, by mixture, muft produce it. "But in making experiments of this kind, care must be had that the light be fuffici"ently homogeneal." Newton. Optic. p. 156,- 158. "In tranfparently coloured liquors, 'tis "obfervable, that their colour uses to vary with their thickness. Thus a red li quor, in a conical glass, held between the "light and the eye, looks of a pale and dilute "yellow at the bottom where 'tis thin, and 66 a little higher where 'tis thicker, grows or fome other full blue be held toge"ther in the red homogeneal light, they " will both appear red, but the cinnabar "will appear of a ftrongly luminous and "refplendent red, and the ultramarine blue, "of a faint, obfcure, and dark red; and "if they be held together in the blue "homogeneal light, they will both appear blue, but the ultramarine will appear of a ftrongly luminous and refplendent blue, orange; and where 'tis ftill thicker, be"and the cinnabar of a faint and dark comes red; and where 'tis thickest, the "blue; which puts it paft difpute, that "red is deepest and darkest. For it is to be "the cinnabar reflects the red light much "conceived, that fuch a liquor ftops the more copiously than the ultramarine" indigo-making and violet-making rays 66 66 of the visible parts of the object; for I do not abfolutely exclude all other PHYSICS. ways of modifying the rays of light, between their parting from the lu cid body, and their reception into the common organ of vifion. Now there seem to be feveral ways whereby liquors may fuddenly alter the colour of one another, but I fhall only touch upon a few. And, firft, the minute corpufcles that compofe a liquor may eafily infinuate themselves into thofe pores of bodies whereto their fize and figure adapt them; and thefe pores they may either exactly or inadequately fill; in which latter cafe they will, for the most part, alter the number and figure, but always moft eafily, the blue-making rays more "difficultly, and the red-making most "difficultly; and that if the thickness "of the liquor be only fo much as fuffi"ces to flop a competent number of the "violet-making and indigo-making rays, "without diminishing much the number "of the reft, the reft must compound a pale "yellow. But if the liquor be so much "thicker, as to ftop also a great number of "the blue making rays, and fome of the "green-making, the reft muft compound "an orange; and where it is fo thick as to "ftop also a great number of the green"making, and a confiderable number of the yellow-making, the reft muft begin to compound a red, and this red must "grow deeper and darker, as the yellow"making and orange-making rays are more and more stopped by the increa fing thickness of the liquor; fo that few rays befides the red-making can get "through. 06 66 66 "Of this kind is an experiment lately "related to me by Dr. Halley, who, diving "deep into the fea in a diving veffel, "found, in a clear funfhine day, that "when he was funk many fathoms deep "into the water, the upper part of his "hand, on which the fun fhone directly "through the water, and through a small glass-window in the veffel, like that of a damask rose, and the water below and "the under part of his hand illuminated "by light reflected from the water below, "looked green. From thence it may be 66 66 the appear red, and the greater the depth "is, the fuller and intenfer muft that red "be. And at fuch depths as the violet"making rays can scarce penetrate to, the " blue-making, green-making and yellow"making rays being reflected from below "more copioufly than the red-making 66 66 ones, mult compound a green. Now if "there be two liquors of full colours, "fuppofe a red and a blue, and both of "them fo thick as fuffices to make their "colours fufficiently full, though either "liquor be fufficiently tranfparent a-part, yet will you not be able to fee through "both together; for if only the red-making rays pass through one liquor, and "only the blue-making through the other, no rays can pafs through both. This Dr. "Hook tried cafually with glafs wedges, "filled with red and blue liquors, and was furprized at the event. 66 66 66 "Now whilft bodies become coloured, "by reflecting or tranfmitting this or "that fort of rays more copiously than "the reft, it is to be conceived that they stop and stifle in themselves the 66 66 rays which they do not reflect or tranf"mit. For if gold be foliated, and held "between the eye and the light, the "light looks of a greenish blue; and "therefore maffy gold lets into its body the blue-making rays, to be reflected "to and fro within it, till they be ftopIped and ftifled, whilst it reflects the yellow-making outwards, and thereby "looks yellow. And much after the fame manner that leaf- gold is yellow by reflected, and blue by tranfmitted light, and maffy gold is yellow in all pofitions of the eye; there are some 66 66 66 gathered, that the fea-water reflects "back the violet and blue-making rays "moft eafily, and lets the red-making "pafs moft freely and copioufly to great depths; for thereby the fun's direct" liquors, as the tincture of Lignum Ne" phriticum, and fome forts of glafs, "which tranfmit one fort of light most " copiously, PHYSICS. quors. But to return; the causes of the feveral forms of afperity, that may diverfify the furfaces of coloured bodies, will, perhaps, afford us fome How changes general conjectures as to feveral of the ways whereby 'tis poffible to are produced in colours, by li- produce the fudden changes of colours, we fhall hereafter find in our experiments; for most of those phenomena being produced by means of liquors, which generally abound with very minute, active, and variously figured faline corpufcles, they may well very fuddenly alter the texture of the body they are employed to work on; and fo change their form of afperity, and thereby make them reflect to the eye the light that falls on them, after another manner than they did before, and by that means vary the colour fo far as it depends upon the texture or difpofition of 66 "doth, and that the ultramarine reflects "the blue light much more copiously than "the cinnabar doth. The fame experi"ment may be made fuccefsfully with red "lead and indigo, or with any other two "coloured bodies, if due allowance be "made for the different strength or weak"nefs of their colour and light. And "fince these reflected lights, which differ "in colour, differ alfo in degree of re"frangibility, it's certain that fome bodies "reflect the more refrangible, and others "the lefs refrangible rays more copiously; "and that this is the only reafon of these "colours, appears from hence, that the co"lour of homogeneal light cannot be chan "other bodies. Every body reflects the 46 ged by the reflexion of natural bodies. "For if bodies, by reflexion, cannot in "the least change the colour of any fort of "rays, they cannot appear coloured by any "other means, than by reflecting those " which are either of their own colour, or green, are more ftrongly reflected than "the other colours, red and violet, &c. "But to make these experiments the more "manifeft, fuch bodies ought to be cho"fen as have the fulleft and most vivid co"lours, and two of those bodies are to" which, by mixture, muft produce it. "be compared together. Thus, for in"ftance, if cinnabar and ultramarine blue, "But in making experiments of this kind, care must be had that the light be fuffici"ently homogeneal." Newton. Optic. p. 156,158. "In tranfparently coloured liquors, 'tis "obfervable, that their colour uses to va66 ry with their thickness. Thus a red li quor, in a conical glass, held between the "light and the eye, looks of a pale and dilute "yellow at the bottom where 'tis thin, and or fome other full blue be held toge- 66 a little higher where 'tis thicker, grows orange; and where 'tis ftill thicker, becomes red; and where 'tis thickest, the red is deepest and darkeft. For it is to be conceived, that fuch a liquor ftops the indigo-making and violet-making rays of the vifible parts of the object; for I do not abfolutely exclude all other PHYSICS: ways of modifying the rays of light, between their parting from the lu cid body, and their reception into the common organ of vifion. Now there feem to be feveral ways whereby liquors may fuddenly alter the colour of one another, but I fhall only touch upon a few. And, firft, the minute corpufcles that compofe a liquor may eafily infinuate themselves into thofe pores of bodies whereto their fize and figure adapt them; and these pores they may either exactly or inadequately fill; in which latter cafe they will, for the most part, alter the number and figure, but always 66 the appear red, and the greater the depth "is, the fuller and intenfer must that red "be. And at fuch depths as the violet"making rays can scarce penetrate to, the "blue-making, green-making and yellow"making rays being reflected from below more copiously than the red-making 66 ones, mult compound a green. Now if "there be two liquors of full colours, "fuppofe a red and a blue, and both of "them fo thick as fuffices to make their "colours fufficiently full, though either "liquor be fufficiently tranfparent a-part, "moft eafily, the blue-making rays more 66 46 66 66 66 yet will you not be able to fee through "both together; for if only the red-ma"king rays pafs through one liquor, and "only the blue-making through the other, "Now whilft bodies become coloured, "by reflecting or tranfmitting this or "that fort of rays more copiously than "the reft, it is to be conceived that they stop and stifle in themselves the rays which they do not reflect or tranf"mit. For if gold be foliated, and held "between the eye and the light, the "light looks of a greenish blue; and "therefore maffy gold lets into its body "Of this kind is an experiment lately "related to me by Dr. Halley, who, diving. deep into the fea in a diving veffel, 66 found, in a clear funfhine day, that "when he was funk many fathoms deep "into the water, the upper part of his "hand, on which the fun fhone directly through the water, and through a small "glass-window in the veffel, like that of 66 a damask rofe, and the water below and "the under part of his hand illuminated "by light reflected from the water below, "looked green. From thence it may be 66 gathered, that the fea-water reflects "back the violet and blue-making rays "moft eafily, and lets the red-making pafs moft freely and copiously to great depths; for thereby the fun's direct light, at all great depths, by reason of "the predominating red-making rays muft 66 the blue-making rays, to be reflected "to and fro within it, till they be stop ped and ftifled, whilft it reflects the "yellow-making outwards, and thereby "looks yellow. And much after the "fame manner that leaf- gold is yellow by reflected, and blue by tranfmitted light, and maffy gold is yellow in all pofitions of the eye; there are fome liquors, as the tincture of Lignum Nephriticum, and fome forts of glass, "which tranfmit one fort of light most "copiously, |