Micrographia Part 6

Having therefore shewn that there is such a propriety in the _prisme_ and water _Globule_ whereby the pulse is made _oblique_ to the progressive, and that so much the more, by how much greater the refraction is, I shall in the next place consider, how this conduces to the production of colours, and what kind of impression it makes upon the bottom of the eye; and to this end it will be requisite to examine this _Hypothesis_ a little more particularly.

First therefore, if we consider the manner of the progress of the pulse, it will seem rational to conclude, that that part or end of the pulse which precedes the other, must necessarily be somwhat more _obtunded_, or _impeded_ by the resistance of the transparent _medium_, than the other part or end of it which is subsequent, whose way is, as it were, prepared by the other; especially if the adjacent _medium_ be not in the same manner enlightned or agitated. And therefore (in the fourth _Figure_ of the sixth _Iconism_) the Ray AAAHB will have its side HH more deadned by the resistance of the dark or quiet _medium_ PPP, Whence there will be a kind of deadness superinduc'd on the side HHH, which will continually increase from B, and strike deeper and deeper into the Ray by the line BR; Whence all the parts of the triangle, RBHO will be of a dead _Blue_ colour, and so much the deeper, by how much the nearer they lie to the line BHH, which is most deaded or impeded, and so much the more _dilute_, by how much the nearer it approaches the line BR. Next on the other side of the Ray AAN, the end A of the pulse AH will be promoted, or made stronger, having its pa.s.sage already prepar'd as 'twere by the other parts preceding, and so its impression wil be stronger; And because of its _obliquity_ to the Ray, there will be propagated a kind of faint motion into QQ the adjacent dark or quiet _medium_, which faint motion will spread further and further into QQ as the Ray is propagated further and further from A, namely, as far as the line MA, whence all the triangle MAN will be ting'd with a _Red_, and that _Red_ will be the deeper the nearer it approaches the line MA, and the _paler_ or _yellower_ the nearer it is the line NA. And if the Ray be continued, so that the lines AN and BR (which are the bounds of the _Red_ and _Blue diluted_) do meet and cross each other, there will be beyond that intersection generated all kinds of _Greens_.

Now, these being the proprieties of every single refracted Ray of light, it will be easie enough to consider what must be the result of very many such Rays collateral: As if we suppose infinite such Rays _interjacent_ between AKSB and An.o.b, which arc the terminating: For in this case the Ray AKSB will have its _Red_ triangle intire, as lying next to the dark or quiet _medium_, but the other side of it BS will have no _Blue_, because the _medium adjacent_ to it SBO, is mov'd or enlightned, and consequently that light does destroy the colour. So likewise will the Ray An.o.b lose its _Red_, because the _adjacent medium_ is mov'd or enlightned, but the other side of the Ray that is _adjacent_ to the dark, namely, AHO will preserve its _Blue_ entire, and these Rays must be so far produc'd as till AN and BR cut each other, before there will be any _Green_ produc'd. From these Proprieties well consider'd, may be deduc'd the reasons of all the _Phaenomena_ of the _prisme_, and of the _Globules_ or drops of Water which conduce to the production of the Rainbow.

Next for the impression they make on the _Retina_, we will further examine this _Hypothesis_: Suppose therefore ABCDEF, in the fifth _Figure_, to represent the Ball of the eye: on the _Cornea_ of which ABC two Rays GACH and KCAI (which are the terminating Rays of a luminous body) falling, are by the refraction thereof collected or _converg'd_ into two points at the bottom of the eye. Now, because these terminating Rays, and all the _intermediate_ ones which come from any part of the luminous body, are suppos'd by some sufficient refraction before they enter the eye, to have their pulses made _oblique_ to their progression, and consequently each Ray to have potentially _superinduc'd_ two proprieties, or colours, viz., a _Red_ on the one side, and a _Blue_ on the other, which notwithstanding are never actually manifest, but when this or that Ray has the one or the other side of it bordering on a dark or unmov'd _medium_, therefore as soon as these Rays are entred into the eye and so have one side of each of them bordering on a dark part of the humours of the eye, they will each of them actually exhibit some colour; therefore ADC the production of GACH will exhibit a _Blue_, because the side CD is _adjacent_ to the dark _medium_ CQDC, but nothing of a _Red_, because its side AD is _adjacent_ to the enlightned _medium_ ADFA: And all the Rays that from the points of the luminous body are collected on the parts of the _Retina_ between D and F shall have their _Blue_ so much the more _diluted_ by how much the farther these points of collection are distant from D towards F; and the Ray AFC the production of KCAI, will exhibit a _Red_, because the side AF is adjacent to the dark or quiet _medium_ of the eye APFA, but nothing of a _Blue_, because its side CF is _adjacent_ to the enlightned _medium_ CFDC, and all the Rays from the intermediate parts of the luminous body that are collected between F and D shall have their _Red_ so much the more diluted, by how much the farther they are distant from F towards D.

Now, because by the refraction in the _Cornea_, and some other parts of the eye, the sides of each Ray, which before were almost parallel, are made to _converge_ and meet in a point at the bottom of the eye, therefore that side of the _pulse_ which preceded before these refractions, shall first touch the _Retina_, and the other side last. And therefore according as this or that side, or end of the pulse shall be impeded, accordingly will the _impressions_ on the _Retina_ be varied; therefore by the Ray GACH refracted by the _Cornea_ to D there shall be on that point a stroke or impression confus'd, whose weakest end, namely, that by the line CD shall precede, and the stronger, namely, that by the line AD shall follow. And by the Ray KCAI refracted to F, there shall be on that part a confus'd stroke or impression, whose strongest part, namely, that by the line CF shal precede, and whose weakest or impeded, namely, that by the line AF shall follow, and all the intermediate points between F and D will receive impressions from the _converg'd_ Rays so much the more like the impressions on F and D by how much the nearer they approach that or this.

From the consideration of the proprieties of which impressions, we may collect these short definitions of Colours: That _Blue is an impression on the Retina of an oblique and confus'd pulse of light, whose weakest part precedes, and whose strongest follows._ And, that _Red is an impression on the Retina of an oblique and confus'd pulse of light, whose strongest part precedes, and whose weakest follows._

Which proprieties, as they have been already manifested, in the Prisme and falling drops of Rain, to be the causes of the colours there generated, may be easily found to be the efficients also of the colours appearing in thin _laminated_ transparent bodies; for the explication of which, all this has been premised.

And that this is so, a little closer examination of the _Phaenomena_ and the _Figure_ of the body, by this _Hypothesis_ will make evident.

For first (as we have already observed) the _laminated_ body must be of a determinate thickness, that is, it must not be thinner then such a determinate quant.i.ty; for I have always observ'd, that neer the edges of those which are exceeding thin, the colours disappear, and the part grows white; nor must it be thicker then another determinate quant.i.ty; for I have likewise observ'd, that beyond such a thickness, no colours appear'd, but the Plate looked white, between which two determinate thicknesses were all the colour'd Rings; of which in some substances I have found ten or twelve, in others not half so many, which I suppose depends much upon the transparency of the _laminated_ body. Thus though the consecutions are the same in the sc.u.mm or the skin on the top of metals; yet in those consecutions in the same colour is not so often repeated as in the consecutions in thin Gla.s.s, or in Sope-water, or any other more transparent and glutinous liquor; for in these I have observ'd, _Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, &c._ to succeed each other, ten or twelve times, but in the other more _opacous_ bodies the consecutions will not be half so many.

And therefore secondly, the _laminated_ body must be transparent, and this I argue from this, that I have not been able to produce any colour at all with an _opacous_ body, though never so thin. And this I have often try'd, by pressing small _Globule_ of _Mercury_ between two smooth Plates of Gla.s.s, whereby I have reduc'd that body to a much greater thinness then was requisite to exhibit the colours with a transparent body.

Thirdly, there must be a considerable reflecting body adjacent to the under or further side of the _lamina_ or _plate_: for this I always found, that the greater that reflection was, the more vivid were the appearing colours.

From which Observations, is most evident, that the reflection from the under or further side of the body is the princ.i.p.al cause of the production of these colours; which, that it is so, and how it conduces to that effect, I shall further explain in the following Figure, which is here described of a very great thickness, as if it had been view'd through the _Microscope_; and 'tis indeed much thicker than any _Microscope_ (I have yet us'd) has been able to shew me those colour'd plates of Gla.s.s, or _Muscovie-gla.s.s_, which I have not without much trouble view'd with it, for though I have endeavoured to magnifie them as much as the Gla.s.ses were capable of, yet are they so exceeding thin, that I have not hitherto been able positively to determine their thickness. This Figure therefore I here represent, is wholy _Hypothetical_.

Let ABCDHFE in the sixth Figure be a _frustum_ of _Muscovy-gla.s.s_, thinner toward the end AE, and thicker towards DF. Let us first suppose the Ray aghb coming from the Sun, of some remote luminous object to fall _obliquely_ on the thinner plate BAE, part therefore is reflected back by cghd, the first _Superficies_; whereby the perpendicular pulse ab is after reflexion propagated by cd, cd, equally remote from each other with ab, ab, so that ag + gc, or bh + hd are either of them equal to aa, as is also cc, but the body BAE being transparent, a part of the light of this Ray is refracted in the surface AB, and propagated by gikh to the surface EF, whence it is reflected and refracted again by the surface AB. So that after two refractions and one reflection, there is propagated a kind of fainter Ray emnf, whose pulse is not only weaker by reason of the two refractions in the surface AB, but by reason of the time spent in pa.s.sing and repa.s.sing between the two surfaces AB and EF, ef which is this fainter or weaker pulse comes behind the pulse cd; so that hereby (the surfaces AB, and EF being so neer together, that the eye cannot _discriminate_ them from one) this confus'd or _duplicated_ pulse, whose strongest part precedes, and whose weakest follows, does produce on the _Retina_, (or the _optick nerve_ that covers the bottom of the eye) the sensation of a _Yellow_.

And secondly, this _Yellow_ will appear so much the deeper, by how much the further back towards the middle between cd and cd the spurious pulse ef is remov'd, as in 2 where the surface BC being further remov'd from EF, the weaker pulse ef will be nearer to the middle, and will make an impression on the eye of a _Red_.

But thirdly, if the two reflecting surfaces be yet further remov'd asunder (as in 3 CD and EF are) then will the weaker pulse be so farr behind, that it will be more then half the distance between cd and cd. And in this case it will rather seem to precede the following stronger pulse, then to follow the preceding one, and consequently a _Blue_ will be generated. And when the weaker pulse is just in the middle between two strong ones, then is a deep and lovely _Purple_ generated; but when the weaker pulse ef is very neer to cd, then is there generated a _Green_, which will be _bluer_, or _yellower_, according as the _approximate_ weak pulse does precede or follow the stronger.

Now fourthly, if the thicker Plate chance to be cleft into two thinner Plates, as CDFE is divided into two Plates by the surface GH then from the composition arising from the three reflections in the surfaces CD, GH, and EF, there will be generated several compounded or mixt colours, which will be very differing, according as the proportion between the thicknesses of those two divided Plates CDHG, and GHFE are varied.

And _fifthly_, if these surfaces CD and FE are further remov'd asunder, the weaker pulse will yet lagg behind much further, and not onely be _coincident_ with the second, cd, but lagg behind that also, and that so much the more, by how much the thicker the Plate be; so that by degrees it will be _coincident_ with the third cd backward also, and by degrees, as the Plate grows thicker with a fourth, and so onward to a fifth, sixth, seventh, or eighth; so that if there be a thin transparent body, that from the greatest thinness requisite to produce colours, does, in the manner of a Wedge, by degrees grow to the greatest thickness that a Plate can be of, to exhibit a colour by the reflection of Light from such a body, there shall be generated several consecutions of colours, whose order from the thin end towards the thick, shall be _Yellow, Red, Purple, Blue, Green; Yellow, Red, Purple, Blue, Green; Yellow, Red, Purple, Blue, Green; Yellow_, &c. and these so often repeated, as the weaker pulse does lose paces with its _Primary_, or first pulse, and is _coincident_ with a second, third, fourth, fifth, sixth, &c. pulse behind the first. And this, as it is _coincident_, or follows from the first _Hypothesis_ I took of colours, so upon experiment have I found it in mult.i.tudes of instances that seem to prove it. One thing which seems of the greatest concern in this _Hypothesis_, is to determine the greatest or least thickness requisite for these effects, which, though I have not been wanting in attempting, yet so exceeding thin are these coloured Plates, and so imperfect our _Microscope_, that I have not been hitherto successfull, though if my endeavours shall answer my expectations, I shall hope to gratifie the curious Reader with some things more remov'd beyond our reach hitherto.

Thus have I, with as much brevity as I was able, endeavoured to explicate (_Hypothetically_ at least) the causes of the _Phaenomena_ I formerly recited, on the consideration of which I have been the more particular.

First, because I think these I have newly given are capable of explicating all the _Phaenomena_ of colours, not onely of those appearing in the _Prisme_, Water-drop, or Rainbow, and in _laminated_ or plated bodies, but of all that are in the world, whether they be fluid or solid bodies, whether in thick or thin, whether transparent, or seemingly opacous, as I shall in the next Observation further endeavour to shew. And secondly, because this being one of the two ornaments of all bodies discoverable by the sight, whether looked on with, or without a _Microscope_, it seem'd to deserve (somewhere in this Tract, which contains a description of the Figure and Colour of some minute bodies) to be somewhat the more intimately enquir'd into.

Observ. X. _Of _Metalline_, and other real Colours._

Having in the former Discourse, from the Fundamental cause of Colour, made it probable, that there are but two Colours, and shewn, that the _Phantasm_ of Colour is caus'd by the sensation of the _oblique_ or uneven pulse of Light which is capable of no more varieties than two that arise from the two sides of the _oblique_ pulse, though each of those be capable of infinite gradations or degrees (each of them beginning from _White_, and ending the one in the deepest _Scarlet_ or _Yellow_, the other in the deepest _Blue_) I shall in this _Section_ set down some Observations which I have made of other colours, such as _Metalline_ powders tinging or colour'd bodies and several kinds of tinctures or ting'd liquors, all which, together with those I treated of in the former Observation will, I suppose, comprise the several subjects in which colour is observ'd to be inherent, and the several manners by which it _inheres_, or is apparent in them. And here I shall endeavour to shew by what composition all kind of compound colours are made, and how there is no colour in the world but may be made from the various degrees of these two colours, together with the intermixtures of _Black_ and _White_.

And this being so, as I shall anon shew, it seems an evident argument to me, that all colours whatsoever, whether in fluid or solid, whether in very transparent or seemingly _opacous_, have the same efficient cause, to wit, some kind of _refraction_ whereby the Rays that proceed from such bodies, have their pulse _obliquated_ or confus'd in the manner I explicated in the former _Section_; that is, a _Red_ is caus'd by a duplicated or confus'd pulse, whose strongest pulse precedes, and a weaker follows: and a _Blue_ is caus'd by a confus'd pulse, where the weaker pulse precedes, and the stronger follows. And according as these are, more or less, or variously mixt and compounded, so are the _sensations_, and consequently the _phantasms_ of colours _diversified_.

To proceed therefore; I suppose, that all transparent colour'd bodies, whether fluid or solid, do consist at least of two parts, or two kinds of substances, the one of a substance of a somewhat differing _refraction_ from the other. That one of these substances which may be call'd the _tinging_ substance, does consist of distinct parts, or particles of a determinate bigness which are _disseminated_, or dispers'd all over the other: That these particles, if the body be equally and uniformly colour'd, are evenly rang'd and dispers'd over the other contiguous body; That where the body is deepest ting'd, there these particles are rang'd thickest, and where 'tis but faintly ting'd, they are rang'd much thinner, but uniformly.

That by the mixture of another body that unites with either of these, which has a differing refraction from either of the other, quite differing effects will be produc'd, that is, the _consecutions_ of the confus'd pulses will be much of another kind, and consequently produce other _sensations_ and _phantasms_ of colours, and from a _Red_ may turn to a _Blue_, or from a _Blue_ to a _Red_, &c.

Now, that this may be the better understood, I shall endeavour to explain my meaning a little more sensible by a _Scheme_: Suppose we therefore in the seventh _Figure_ of the sixth _Scheme_, that ABCD represents a Vessel holding a ting'd liquor, let IIIII, &c. be the clear liquor, and let the tinging body that is mixt with it be EE, &c. FF, &c. GG, &c. HH, &c. whose particles (whether round, or some other determinate Figure is little to our purpose) are first of a determinate and equal bulk. Next, they are rang'd into the form of _Quincunx_, or _Equilaterotriangular_ order, which that probably they are so, and why they are so, I shall elsewhere endeavour to shew. Thirdly, they are of such a nature, as does either more easily or more difficultly transmit the Rays of light then the liquor; if more easily, a _Blue_ is generated, and if more difficultly, a _Red_ or _Scarlet_.

And first, let us suppose the tinging particles to be of a substance that does more _impede_ the Rays of light, we shall find that the pulse or wave of light mov'd from AD to BC, will proceed on, through the containing _medium_ by the pulses or waves KK, LL, MM, NN, OO; but because several of these Rays that go to the const.i.tution of these pulses will be slugged or stopped by the tinging particles E, F, G, H; therefore there shall be _secundary_ and weak pulse that shall follow the Ray, namely PP which will be the weaker: first, because it has suffer'd many refractions in the impeding body; next, for that the Rays will be a little dispers'd or confus'd by reason of the refraction in each of the particles, whether _round_ or _angular_; and this will be more evident, if we a little more closely examine any one particular tinging _Globule_.

Suppose we therefore AB in the eighth _Figure_ of the sixth _Scheme_, to represent a tinging _Globule_ or particle which has a greater refraction than the liquor in which it is contain'd: Let CD be a part of the pulse of light which is _propagated_ through the containing _medium_; this pulse will be a little stopt or impeded by the _Globule_, and so by that time the pulse is past to EF that part of it which has been impeded by pa.s.sing through the _Globule_, will get but to LM, and so that pulse which has been _propagated_ through the _Globule_, to wit, LM, NO, PQ, will always come behind the pulses EF, GH, IK, &c.

Next, by reason of the greater impediment in AB, and its _Globular_ Figure, the Rays that pa.s.s through it will be dispers'd, and very much scatter'd.

Whence CA and DB which before went _direct_ and _parallel_, will after the refraction in AB, _diverge_ and spread by AP, and BQ; so that as the Rays do meet with more and more of these tinging particles in their way, by so much the more will the pulse of light further lagg behind the clearer pulse, or that which has fewer refractions, and thence the deeper will the colour be, and the fainter the light that is trajected through it; for not onely many Rays are reflected from the surfaces of AB, but those Rays that get through it are very much disordered.

By this _Hypothesis_ there is no one experiment of colour that I have yet met with, but may be, I conceive, very rationably solv'd, and perhaps, had I time to examine several particulars requisite to the demonstration of it, I might prove it more than probable, for all the experiments about the changes and mixings of colours related in the Treatise of Colours, published by the _Incomparable_ Mr. _Boyle_, and mult.i.tudes of others which I have observ'd, do so easily and naturally flow from those principles, that I am very apt to think it probable, that they own their production to no other _secundary_ cause: As to instance in two or three experiments. In the twentieth Experiment, this _n.o.ble Authour_ has shewn that the deep _bluish purple-colour_ of _Violets_, may be turn'd into a _Green_, by _Alcalizate Salts_, and to a _Red_ by acid; that is, a _Purple_ consists of two colours, a deep _Red_, and a deep _Blue_; when the _Blue_ is diluted, or altered, or destroy'd by _acid Salts_, the _Red_ becomes predominant, but when the _Red_ is diluted by _Alcalizate_, and the _Blue_ heightned, there is generated a _Green_; for of a _Red_ diluted, is made a _Yellow_, and _Yellow_ and _Blue_ make a _Green_.

Now, because the _spurious_ pulses which cause a _Red_ and a _Blue_, do the one follow the clear pulse, and the other precede it, it usually follows, that those _Saline_ refracting bodies which do _dilute_ the colour of the one, do deepen that of the other. And this will be made manifest by almost all kinds of _Purples_, and many sorts of _Greens_, both these colours consisting of mixt colours; for if we suppose A and A in the ninth Figure, to represent two pulses of clear light, which follow each other at a convenient distance, AA, each of which has a _spurious_ pulse preceding it, as BB, which makes a _Blue_, and another following it, as CC, which makes a _Red_, the one caus'd by tinging particles that have a greater refraction, the other by others that have a less refracting quality then the liquor or _Menstruum_ in which these are dissolv'd, whatsoever liquor does so alter the refraction of the one, without altering that of the other part of the ting'd liquor, must needs very much alter the colour of the liquor; for if the refraction of the _dissolvent_ be increas'd, and the refraction of the tinging particles not altered, then will the preceding _spurious_ pulse be shortned or stopt, and not out-run the clear pulse so much; so that BB will become EE, and the _Blue_ be _diluted_, whereas the other _spurious_ pulse which follows will be made to lagg much more, and be further behind AA than before, and CC will become _ff_, and so the _Yellow_ or _Red_ will be heightned.

A _Saline_ liquor therefore, mixt with another ting'd liquor, may alter the colour of it several ways, either by altering the refraction of the liquor in which the colour swims: or secondly by varying the refraction of the coloured particles, by uniting more intimately either with some particular _corpuscles_ of the tinging body, or with all of them, according as it has a _congruity_ to some more especially, or to all alike: or thirdly, by uniting and interweaving it self with some other body that is already joyn'd with the tinging particles, with which substance it may have a _congruity_, though it have very little with the particles themselves: or fourthly, it may alter the colour of a ting'd liquor by dis-joyning certain particles which were before united with the tinging particles, which though they were somewhat _congruous_ to these particles, have yet a greater _congruity_ with the newly _infus'd Saline menstruum_. It may likewise alter the colour by further dissolving the tinging substance into smaller and smaller _particles_, and so _diluting_ the colour; or by uniting several _particles_ together as in precipitations, and so deepning it, and some such other ways, which many experiments and comparisons of differing trials together, might easily inform one of.

From these Principles applied, may be made out all the varieties of colours observable, either in liquors, or any other ting'd bodies, with great ease, and I hope intelligible enough, there being nothing in the _notion_ of colour, or in the suppos'd production, but is very conceivable, and may be possible.

The greatest difficulty that I find against this _Hypothesis_, is, that there seem to be more distinct colours then two, that is, then Yellow and Blue. This Objection is grounded on this reason, that there are several Reds, which _diluted_, make not a Saffron or pale Yellow, and therefore Red, or Scarlet seems to be a third colour distinct from a deep degree of Yellow.

To which I answer, that Saffron affords us a deep Scarlet tincture, which may be _diluted_ into as pale a Yellow as any, either by making a weak solution of the Saffron, by infusing a small parcel of it into a great quant.i.ty of liquor, as in spirit of Wine, or else by looking through a very thin quant.i.ty of the tincture, and which may be heightn'd into the loveliest Scarlet, by looking through a very thick body of this tincture, or through a thinner parcel of it, which is highly _impregnated_ with the tinging body, by having had a greater quant.i.ty of the Saffron dissolv'd in a smaller parcel of the liquor.

Now, though there may be some particles of other tinging bodies that give a lovely Scarlet also, which though _diluted_ never so much with liquor, or looked on through never so thin a parcel of ting'd liquor, will not yet afford a pale Yellow, but onely a kind of faint Red; yet this is no argument but that those ting'd particles may have in them the faintest degree of Yellow, though we may be unable to make them exhibit it; For that power of being _diluted_ depending upon the divisibility of the ting'd body, if I am unable to make the tinging particles so thin as to exhibit that colour, it does not therefore follow, that the thing is impossible to be done; now, the tinging particles of some bodies are of such a nature, that unless there be found some way of comminuting them into less bulks then the liquor does dissolve them into, all the Rays that pa.s.s through them must necessarily receive a tincture so deep, as their appropriate refractions and bulks compar'd with the proprieties of the dissolving liquor must necessarily dispose them to empress, which may perhaps be a pretty deep Yellow, or pale Red.

And that this is not _gratis dictum_, I shall add one instance of this kind, wherein the thing is most manifest.

If you take Blue _Smalt_, you shall find, that to afford the deepest Blue, which _caeteris paribus_ has the greatest particles or sands; and if you further divide, or grind those particles on a Grindstone, or _porphyry_ stone, you may by _comminuting_ the sands of it, _dilute_ the Blue into as pale a one as you please, which you cannot do by laying the colour thin; for wheresoever any single particle is, it exhibits as deep a Blue as the whole ma.s.s. Now, there are other Blues, which though never so much ground, will not be _diluted_ by grinding, because consisting of very small particles, very deeply ting'd, they cannot by grinding be actually separated into smaller particles then the operation of the fire, or some other dissolving _menstruum_, reduc'd them to already.

Thus all kind of _Metalline_ colours, whether _precipitated_, _sublim'd_, _calcin'd_, or otherwise prepar'd, are hardly chang'd by grinding, as _ultra marine_ is not more _diluted_; nor is _Vermilion_ or _Red-lead_ made of a more faint colour by grinding; for the smallest particles of these which I have view'd with my greatest Magnifying-Gla.s.s, if they be well enlightned, appear very deeply ting'd with their peculiar colours; nor, though I have magnified and enlightned the particles exceedingly, could I in many of them, perceive them to be transparent, or to be whole particles, but the smallest specks that I could find among well ground _Vermilion_ and _Red-lead_, seem'd to be a Red ma.s.s, compounded of a mult.i.tude of less and less motes, which sticking together, compos'd a bulk, not one thousand thousandth part of the smallest visible sand or mote.

And this I find generally in most _Metalline_ colours, that though they consist of parts so exceedingly small, yet are they very deeply ting'd, they being so ponderous, and having such a mult.i.tude of terrestrial particles throng'd into a little room; so that 'tis difficult to find any particle transparent or resembling a pretious stone, though not impossible; for I have observ'd divers such shining and resplendent colours intermixt with the particles of _Cinnaber_, both natural and artificial, before it hath been ground and broken or flaw'd into _Vermilion_: As I have also in _Orpiment_, _Red-lead_, and _Bise_, which makes me suppose, that those _metalline_ colours are by grinding, not onely broken and separated actually into smaller pieces, but that they are also flaw'd and brused, whence they, for the most part, become _opacous_, like flaw'd Crystal or Gla.s.s, &c. But for _Smalts_ and _verditures_, I have been able with a _Microscope_ to perceive their particles very many of them transparent.

Now, that the others also may be transparent, though they do not appear so to the _Microscope_, may be made probable by this Experiment: that if you take _ammel_ that is almost _opacous_, and grind it very well on a _Porphyry_, or _Serpentine_, the small particles will by reason of their flaws, appear perfectly _opacous_; and that 'tis the flaws that produce this _opacousness_, may be argued from this, that particles of the same _Ammel_ much thicker if unflaw'd will appear somewhat transparent even to the eye; and from this also, that the most transparent and clear Crystal, if heated in the fire, and then suddenly quenched, so that it be all over flaw'd, will appear _opacous_ and white.

And that the particles of _Metalline_ colours are transparent, may be argued yet further from this, that the Crystals, or _Vitriols_ of all Metals, are transparent, which since they consist of _metalline_ as well as _saline_ particles, those _metalline_ ones must be transparent, which is yet further confirm'd from this, that they have for the most part, _appropriate_ colours; so the _vitriol_ of Gold is Yellow; of Copper, Blue, and sometimes Green; of Iron, green; of Tinn and Lead, a pale White; of Silver, a pale Blue, _&._

And next, the _Solution_ of all Metals into _menstruums_ are much the same with the _Vitriols_, or Crystals. It seems therefore very probable, that those colours which are made by the _precipitation_ of those particles out of the _menstruums_ by transparent _precipitating_ liquors should be transparent also. Thus Gold _precipitates_ with _oyl of Tartar_, or _spirit of Urine_ into a brown Yellow, Copper with spirit of _Urine_ into a Mucous blue, which retains its transparency. A solution of sublimate (as the same Ill.u.s.trious Authour I lately mention'd shews in his 40. Experiment) _precipitates_ with oyl of _Tartar_ _per deliquium_, into an Orange colour'd _precipitate_; nor is it less probable, that the _calcination_ of those _Vitriols_ by the fire, should have their particles transparent: Thus _Saccarum Saturni_, or the _Vitriol of Lead_ by _calcination_ becomes a deep Orange-colour'd _minium_, which is a kind of _precipitation_ by some Salt which proceeds from the fire; common _Vitriol_ _calcin'd_, yields a deep Brown Red, etc.

A third Argument, that the particles of Metals are transparent, is, that being _calcin'd_, and melted with Gla.s.s, they tinge the Gla.s.s with transparent colours. Thus the _Calx_ of Silver tinges the Gla.s.s on which it is anneal'd with a lovely Yellow, or Gold colour, &c.

And that the parts of Metals are transparent, may be farther argued from the transparency of Leaf-gold, which held against the light, both to the naked eye, and the _Microscope,_ exhibits a deep Green. And though I have never seen the other Metals _laminated_ so thin, that I was able to perceive them transparent, yet, for Copper and Bra.s.s, if we had the same conveniency for _laminating_ them, as we have for Gold, we might, perhaps, through such plates or leaves, find very differing degrees of Blue, or Green; for it seems very probable, that those Rays that rebound from them ting'd, with a deep Yellow, or pale Red, as from Copper, or with a pale Yellow, as from Bra.s.s, have past through them; for I cannot conceive how by reflection alone those Rays can receive a tincture, taking any _Hypothesis_ extant.

So that we see there may a sufficient reason be drawn from these instances, why those colours which we are unable to _dilute_ to the palest Yellow, or Blue, or Green, are not therefore to be concluded not to be a deeper degree of them; for supposing we had a great company of small _Globular_ essence Bottles, or round Gla.s.s bubbles, about the bigness of a Walnut, fill'd each of them with a very deep mixture of Saffron, and that every one of them did appear of a deep Scarlet colour, and all of them together did _exhibit_ at a distance, a deep dy'd Scarlet body. It does not follow, because after we have come nearer to this _congeries_, or ma.s.s, and divided it into its parts, and examining each of its parts severally or apart, we find them to have much the same colour with the whole mats; it does not, I say, therefore follow, that if we could break those _Globules_ smaller, or any other ways come to see a smaller or thinner parcel of the ting'd liquor that fill'd those bubbles, that that ting'd liquor must always appear Red, or of a Scarlet hue, since if Experiment be made, the quite contrary will ensue; for it is capable of being _diluted_ into the palest Yellow.

Now, that I might avoid all the Objections of this kind, by exhibiting an Experiment that might by ocular proof convince those whom other reasons would not prevail with, I provided me a _Prismatical Gla.s.s_, made hollow, just in the form of a Wedge, such as is represented in the tenth _Figure_ of the sixth _Scheme_. The two _parallelogram_ sides ABCD, ABEF, which met at a point, were made of the clearest Looking-gla.s.s plates well ground and polish'd that I could get; these were joyn'd with hard cement to the _triangular_ sides, BCE, ADF, which were of Wood; the _Parallelogram_ base BCEF, likewise was of Wood joyn'd on to the rest with hard cement, and the whole _Prismatical_ Box was exactly stopt every where, but onely a little hole near the base was left, whereby the Vessel could be fill'd with any liquor, or emptied again at pleasure.

One of these Boxes (for I had two of them) I fill'd with a pretty deep tincture of _Aloes_, drawn onely with fair Water, and then stopt the hole with a piece of Wax, then, by holding this Wedge against the Light, and looking through it, it was obvious enough to see the tincture of the liquor near the edge of the Wedge where it was but very thin, to be a pale but well colour'd Yellow, and further and further from the edge, as the liquor grew thicker and thicker, this tincture appear'd deeper and deeper, so that near the blunt end, which was seven Inches from the edge and three Inches and an half thick; it was of a deep and well colour'd Red. Now, the clearer and purer this tincture be, the more lovely will the deep Scarlet be, and the fouler the tincture be, the more dirty will the Red appear; so that some dirty tinctures have afforded their deepest Red much of the colour of burnt Oker or _Spanish_ brown; others as lovely a colour as _Vermilion_, and some much brighter; but several others, according as the tinctures were worse or more foul, exhibited various kinds of Reds, of very differing degrees.

The other of these Wedges, I fill'd with a most lovely tincture of Copper, drawn from the filings of it, with spirit of _Urine_, and this Wedge held as the former against the Light, afforded all manner of Blues, from the faintest to the deepest, so that I was in good hope by these two, to have produc'd all the varieties of colours imaginable; for I thought by this means to have been able by placing the two _Parallelogram_ sides together, and the edges contrary ways, to have so mov'd them to and fro one by another, as by looking through them in several places, and through several thicknesses, I should have compounded, and consequently have seen all those colours, which by other like compositions of colours would have ensued.