Micrographia Part 5

Thirdly, That all Metals (excepting Gold and Silver, which do not so much with the bare fire, unless a.s.sisted by other saline Bodies) do more or less _vitrifie_ by the strength of fire, that is, are corroded by a saline Substance, which I elsewhere shew to be the true cause of fire; and are thereby, as by several other _Menstruums_ converted into _Scoria_; And this is called, _calcining_ of them, by Chimists. Thus Iron and Copper by heating and quenching do turn all of them by degrees into _Scoria_, which are evidently _vitrified_ Substances, and unite with Gla.s.s, and are easily _fusible_; and when cold, very hard, and very brittle.

Fourthly, That most kind of _Vitrifications_ or _Calcinations_ are made by Salts, uniting and incorporating with the metalline Particles. Nor do I know any one _calcination_ wherein a _Saline_ body may not, with very great probability, be said to be an agent or coadjutor.

Fifthly, That Iron is converted into Steel by means of the incorporation of certain salts, with which it is kept a certain time in the fire.

Sixthly, That any Iron may, in a very little time, be _case hardned_, as the Trades-men call it, by casing the iron to be hardned with clay, and putting between the clay and iron a good quant.i.ty of a mixture of _Urine_, _Soot_, _Sea-salt_, and _Horses hoofs_ (all which contein great quant.i.ties of Saline bodies) and then putting the case into a good strong fire, and keeping it in a considerable degree of heat for a good while, and afterwards heating, and quenching or cooling it suddenly in cold water.

Seventhly, That all kind of vitrify'd substances, by being suddenly cool'd, become very hard and brittle. And thence arises the pretty _Phaenomena_ of the Gla.s.s Drops, which I have already further explained in its own place.

Eighthly, That those metals which are not so apt to vitrifie, do not acquire any hardness by quenching in water, as Silver, Gold, &c.

These considerations premis'd, will, I suppose, make way for the more easie reception of this following Explication of the _Phaenomena_ of hardned and temper'd Steel. That Steel is a substance made out of Iron, by means of a certain proportionate _Vitrification_ of several parts, which are so curiously and proportionately mixt with the more tough and unalter'd parts of the Iron, that when by the great heat of the fire this vitrify'd substance is melted, and consequently rarify'd, and thereby the pores of the Iron are more open, if then by means of dipping it in cold water it be suddenly cold, and the parts hardned, that is, stay'd in that same degree of _Expansion_ they were in when hot, the parts become very hard and brittle, and that upon the same account almost as small parcels of gla.s.s quenched in water grow brittle, which we have already explicated. If after this the piece of Steel be held in some convenient heat, till by degrees certain colours appear upon the surface of the brightned metal, the very hard and brittle tone of the metal, by degrees relaxes and becomes much more tough and soft; namely, the action of the heat does by degrees loosen the parts of the Steel that were before streached or set _atilt_ as it were, and stayed open by each other, whereby they become relaxed and set at liberty, whence some of the more brittle interjacent parts are thrust out and melted into a thin skin on the surface of the Steel, which from no colour increases to a deep Purple, and so onward by these _gradations_ or consecutions, _White, Yellow, Orange, Minium, Scarlet, Purple, Blew, Watchet,_ &c. and the parts within are more conveniently, and proportionately mixt; and so they gradually subside into a texture which is much better proportion'd and closer joyn'd, whence that rigidnesse of parts ceases, and the parts begin to acquire their former _ductilness_.

Now, that 'tis nothing but the vitrify'd metal that sticks upon the surface of the colour'd body, is evident from this, that if by any means it be sc.r.a.ped and rubb'd off, the metal underneath it is white and clear; and if it be kept longer in the fire, so as to increase to a considerable thickness, it may, by blows, be beaten off in flakes. This is further confirm'd by this observable, that that Iron or Steel will keep longer from rusting which is covered with this vitrify'd case: Thus also Lead will, by degrees, be all turn'd into a litharge; for that colour which covers the top being sc.u.m'd or shov'd aside, appears to be nothing else but a litharge or vitrify'd Lead.

This is observable also in some sort, on Bra.s.s, Copper, Silver, Gold, Tin, but is most conspicuous in Lead: all those Colours that cover the surface of the Metal being nothing else, but a very thin vitrifi'd part of the heated Metal.

The other Instance we have, is in Animal bodies, as in Pearls, Mother of Pearl-shels, Oyster-shels, and almost all other kinds of stony shels whatsoever. This have I also sometimes with pleasure observ'd even in Muscles and Tendons. Further, if you take any glutinous substance and run it exceedingly thin upon the surface of a smooth gla.s.s or a polisht metaline body, you shall find the like effects produced: and in general, wheresoever you meet with a transparent body thin enough, that is terminated by reflecting bodies of differing refractions from it, there will be a production of these pleasing and lovely colours.

Nor is it necessary, that the two _terminating_ Bodies should be both of the same kind, as may appear by the _vitrified Laminae_ on _Steel_, _Lead_, and other Metals, one surface of which _Laminae_ is contiguous to the surface of the Metal, the other to that of the Air.

Nor is it necessary, that these colour'd _Laminae_ should be of an even thickness, that is, should have their edges and middles of equal thickness, as in a Looking-gla.s.s-plate, which circ.u.mstance is only requisite to make the Plate appear all of the same colour; but they may resemble a _Lens_, that is, have their middles thicker then their edges; or else a _double concave_, that is, be thinner in the middle then at the edges; in both which cases there will be various coloured rings or lines, with differing consecutions or orders of Colours; the order of the first from the middle outwards being Red, Yellow, Green, Blew, &c. And the latter quite contrary.

But further, it is altogether necessary, that the Plate, in the places where the Colours appear, should be of a determinate thickness: First, It must not be more then such a thickness, for when the Plate is increased to such a thickness, the Colours cease; and besides, I have seen in a thin piece of _Muscovy-gla.s.s_, where the two ends of two Plates, which appearing both single, exhibited two distinct and differing Colours; but in that place where they were united, and const.i.tuted one double Plate (as I may call it) they appeared transparent and colourless. Nor, Secondly, may the Plates be _thinner_ then such a determinate _cize_; for we alwayes find, that the very outmost Rim of these flaws is terminated in a white and colourless Ring.

Further, in this Production of Colours there is no need of a determinate Light of such a bigness and no more, nor of a determinate position of that Light, that it should be on this side, and not on that side; nor of a terminating shadow, as in the Prisme, and Rainbow, or Water-ball: for we find, that the Light in the open Air, either in or out of the Sun-beams, and within a Room, either from one or many Windows, produces much the same effect: only where the Light is brightest, there the Colours are most _vivid_. So does the light of a Candle, collected by a Gla.s.s-ball. And further, it is all one whatever side of the coloured Rings be towards the light; for the whole Ring keeps its proper Colours from the middle outwards in the same order as I before related, without varying at all, upon changing the position of the light.

But above all it is most observable, that here are all kind of Colours generated in a _pellucid_ body, where there is properly no such refraction as _Des Cartes_ supposes his _Globules_ to acquire a _vertuity_ by: For in the plain and even Plates it is manifest, that the second refraction (according to _Des Cartes_ his Principles in the _fifth section of the eighth Chapter of his Meteors_) does regulate and restore the supposed _turbinated Globules_ unto their former uniform motion. This Experiment therefore will prove such a one as our _thrice excellent Verulam_ calls _Experimentum Crucis_, serving as a Guide or Land-mark, by which to direct our course in the search after the true cause of Colours. Affording us this particular negative Information, that for the production of Colours there is not necessary either a great refraction, as in the Prisme; nor Secondly, a determination of Light and shadow, such as is both in the Prisme and Gla.s.s-ball. Now that we may see likewise what affirmative and positive Instruction it yields, it will be necessary, to examine it a little more particularly and strictly; which that we may the better do, it will be requisite to premise somewhat in general concerning the nature of Light and Refraction.

And first for Light it seems very manifest, that there is no luminous Body but has the parts of it in motion more or less.

First, That all kind of _fiery burning Bodies_ have their parts in motion, I think, will be very easily granted me. That the _spark_ struck from a Flint and Steel is in a rapid agitation, I have elsewhere made probable.

And that the Parts of _rotten Wood_, _rotten Fish_ and the like, are also in motion, I think, will as easily be conceded by those, who consider, that those parts never begin to shine till the Bodies be in a state of putrefaction; and that is now generally granted by all, to be caused by the motion of the parts of putrifying bodies. That the _Bononian stone_ shines no longer then it is either warmed by the Sun-beams, or by the flame of a Fire or of a Candle, is the general report of those that write of it, and of others that have seen it. And that heat argues a motion of the internal parts is (as I said before) generally granted.

But there is one Instance more, which was first shewn to the _Royal Society_ by Mr. _Clayton_ a worthy Member thereof, which does make this a.s.sertion more evident then all the rest: And that is, That a _Diamond_ being _rub'd_, _struck_ or _heated_ in the dark, shines for a pretty while after, so long as that motion, which is imparted by any of those Agents, remains (in the same manner as a Gla.s.s, rubb'd, struck, or (by a means which I shall elsewhere mention) heated, yields a sound which lasts as long as the vibrating motion of that _sonorous_ body) several Experiments made on which Stone, are since published in a Discourse of Colours, by the truly honourable Mr. _Boyle_. What may be said of those _Ignes fatui_ that appear in the night, I cannot so well affirm, having never had the opportunity to examine them my self, nor to be inform'd by any others that had observ'd them: And the relations of them in Authors are so imperfect, that nothing can be built on them. But I hope I shall be able in another place to make it at least very probable, that there is even in those also a Motion which causes this effect. That the shining of _Sea-water_ proceeds from the same cause, may be argued from this, That it shines not till either it be beaten against a Rock, or be some other wayes broken or agitated by Storms, or Oars, or other _percussing_ bodies. And that the Animal _Energyes_ or Spirituous _agil_ parts are very active in _Cats eyes_ when they shine, seems evident enough, because their eyes never shine but when they look very intensly either to find their prey, or being hunted in a dark room, when they seek after their adversary, or to find a way to escape. And the like may be said of the shining _Bellies of Gloworms_; since 'tis evident they can at pleasure either increase or extinguish that Radiation.

It would be somewhat too long a work for this place _Zetetically_ to examine, and positively to prove, what particular kind of motion it is that must be the efficient of Light; for though it be a motion, yet 'tis not every motion that produces it, since we find there are many bodies very violently mov'd, which yet afford not such an effect; and there are other bodies, which to our other senses, seem not mov'd so much, which yet shine.

Thus Water and quick-silver, and most other liquors heated, shine not; and several hard bodies, as Iron, Silver, Bra.s.s, Copper, Wood, &c. though very often struck with a hammer, shine not presently, though they will all of them grow exceeding hot; whereas rotten Wood, rotten Fish, Sea water, Gloworms, &c. have nothing of tangible heat in them, and yet (where there is no stronger light to affect the Sensory) they shine some of them so Vividly, that one may make a shift to read by them.

It would be too long, I say, here to insert the discursive progress by which I inquir'd after the proprieties of the motion of Light, and therefore I shall only add the result.

And, First, I found it ought to be exceeding _quick_, such as those motions of _fermentation_ and _putrefaction_, whereby, certainly, the parts are exceeding nimbly and violently mov'd; and that, because we find those motions are able more minutely to shatter and divide the body, then the most violent heats _menstruums_ we yet know. And that fire is nothing else but such a _dissolution_ of the Burning body, made by the most _universal menstruum_ of all _sulphureous bodies_, namely, the Air, we shall in an other place of this Tractate endeavour to make probable. And that, in all extreamly hot shining bodies, there is a very quick motion that causes Light, as well as a more robust that causes Heat, may be argued from the celerity wherewith the bodyes are dissolv'd.

Next, it must be a _Vibrative motion_. And for this the newly mention'd _Diamond_ affords us a good argument; since if the motion of the parts did not return, the Diamond must after many rubbings decay and be wasted: but we have no reason to suspect the latter, especially if we consider the exceeding difficulty that is found in cutting or wearing away a Diamond.

And a Circular motion of the parts is much more improbable, since, if that were granted, and they be suppos'd irregular and Angular parts, I see not how the parts of the Diamond should hold so firmly together, or remain in the same sensible dimensions, which yet they do. Next, if they be _Globular_, and mov'd only with a _turbinated_ motion, I know not any cause that can impress that motion upon the _pellucid medium_, which yet is done.

Thirdly, any other _irregular_ motion of the parts one amongst another, must necessarily make the body of a fluid consistence, from which it is far enough. It must therefore be a _Vibrating_ motion.

And Thirdly, That it is a very _short-vibrating motion_, I think the instances drawn from the shining of Diamonds will also make probable. For a Diamond being the hardest body we yet know in the World, and consequently the least apt to yield or bend, must consequently also have its _vibrations_ exceeding short.

And these, I think, are the three princ.i.p.al proprieties of a motion, requisite to produce the effect call'd Light in the Object.

The next thing we are to consider, is the way or manner of the _trajection_ of this motion through the interpos'd pellucid body to the eye: And here it will be easily granted,

First, That it must be a body _susceptible_ and _impartible_ of this motion that will deserve the name of a Transparent. And next, that the parts of such a body must be _h.o.m.ogeneous_, or of the same kind. Thirdly, that the const.i.tution and motion of the parts must be such, that the appulse of the luminous body may be communicated or propagated through it to the greatest imaginable distance in the least imaginable time, though I see no reason to affirm, that it must be in an instant: For I know not any one Experiment or observation that does prove it. And, whereas it may be objected, That we see the Sun risen at the very instant when it is above the sensible Horizon, and that we see a Star hidden by the body of the Moon at the same instant, when the Star, the Moon, and our Eye are all in the same line; and the like Observations, or rather suppositions, may be urg'd. I have this to answer, That I can as easily deny as they affirm; for I would fain know by what means any one can be a.s.sured any more of the Affirmative, then I of the Negative. If indeed the propagation were very slow, 'tis possible something might be discovered by Eclypses of the Moon; but though we should grant the progress of the light from the Earth to the Moon, and from the Moon back to the Earth again to be full two Minutes in performing, I know not any possible means to discover it; nay, there may be some instances perhaps of Horizontal Eclypses that may seem very much to favour this supposition of the slower progression of Light then most imagine. And the like may be said of the Eclypses of the Sun, &c. But of this only by the by. Fourthly, That the motion is propagated every way through an _h.o.m.ogeneous medium_ by _direct_ or _straight_ lines extended every way like Rays from the center of a Sphere. Fifthly, in an _h.o.m.ogeneous medium_ this motion is propagated every way with _equal velocity_, whence necessarily every _pulse_ or _vitration_ of the luminous body will generate a Sphere, which will continually increase, and grow bigger, just after the same manner (though indefinitely swifter) as the waves or rings on the surface of the water do swell into bigger and bigger circles about a point of it, where, by the sinking of a Stone the motion was begun, whence it necessarily follows, that all the parts of these Spheres undulated through an _h.o.m.ogeneous medium_ cut the Rays at right angles.

But because all transparent _mediums_ are not _h.o.m.ogeneous_ to one another, therefore we will next examine how this pulse or motion will be propagated through differingly transparent _mediums_. And here, according to the most acute and excellent Philosopher _Des Cartes_, I suppose the sign of the angle of inclination in the first _medium_ to be to the sign of refraction in the second, As the density of the first, to the density of the second.

By density, I mean not the density in respect of gravity (with which the refractions or transparency of _mediums_ hold no proportion) but in respect onely to the _trajection_ of the Rays of light, in which respect they only differ in this; that the one propagates the pulse more easily and weakly, the other more slowly, but more strongly. But as for the pulses themselves, they will by the refraction acquire another propriety, which we shall now endeavour to explicate.

We will suppose therefore in the first Figure ACFD to be a physical Ray, or ABC and DEF to be two Mathematical Rays, _trajected_ from a very remote point of a luminous body through an _h.o.m.ogeneous_ transparent _medium_ LLL, and DA, EB, FC, to be small portions of the orbicular impulses which must therefore cut the Rays at right angles; these Rays meeting with the plain surface NO of a _medium_ that yields an easier _transitus_ to the propagation of light, and falling _obliquely_ on it, they will in the _medium_ MMM be refracted towards the perpendicular of the surface. And because this _medium_ is more easily _trajected_ then the former by a third, therefore the point C of the orbicular pulse FC will be mov'd to H four s.p.a.ces in the same time that F the other end of it is mov'd to G three s.p.a.ces, therefore the whole refracted pulse GH shall be _oblique_ to the refracted Rays CHK and GI; and the angle GHC shall be an acute, and so much the more acute by how much the greater the refraction be, then which nothing is more evident, for the sign of the inclination is to the sign of refraction as GF to TC the distance between the point C and the perpendicular from G on CK, which being as four to three, HC being longer then GF is longer also then TC, therefore the angle GHC is less than GTC.

So that henceforth the parts of the pulses GH and IK are mov'd ascew, or cut the Rays at _oblique_ angles.

It is not my business in this place to set down the reasons why this or that body should impede the Rays more, others less: as why Water should transmit the Rays more easily, though more weakly than air. Onely thus much in general I shall hint, that I suppose the _medium_ MMM to have less of the transparent undulating subtile matter, and that matter to be less implicated by it, whereas LLL I suppose to contain a greater quant.i.ty of the fluid undulating substance, and this to be more implicated with the particles of that _medium_.

But to proceed, the same kind of _obliquity_ of the Pulses and Rays will happen also when the refraction is made out of a more easie into a more difficult _mediu_; as by the calculations of GQ & CSR which are refracted from the perpendicular. In both which calculations 'tis _obvious_ to observe, that always that part of the Ray towards which the refraction is made has the end of the _orbicular pulse_ precedent to that of the other side. And always, the oftner the refraction is made the same way, Or the greater the single refraction is, the more is this unequal progress. So that having found this odd propriety to be an inseparable concomitant of a refracted Ray, not streightned by a contrary refraction, we will next examine the refractions of the Sun-beams, as they are suffer'd onely to pa.s.s through a small pa.s.sage, _obliquely_ out of a more difficult, into a more easie _medium_.

Let us suppose therefore ABC in the second Figure to represent a large _Chimical Gla.s.s-body_ about two foot long, filled with very fair Water as high as AB, and inclin'd in a convenient posture with B towards the Sun: Let us further suppose the top of it to be cover'd with an _opacous_ body, all but the hole ab, through which the Sun-beams are suffer'd to pa.s.s into the Water, and are thereby refracted to cdef, against which part, if a Paper be expanded on the outside, there will appear all the colours of the Rain-bow, that is, there will be generated the two princ.i.p.al colours, _Scarlet_ and _Blue_, and all the _intermediate_ ones which arise from the composition and dilutings of these two, that is, cd shall exhibit a _Scarlet_, which toward d is diluted into a _Yellow_; this is the refraction of the Ray, ik, which comes from the underside of the Sun; and the Ray ef shall appear of a deep _Blue_, which is gradually towards e diluted into a pale _Watchet-blue_. Between d and e the two _diluted_ colours. _Blue_ and _Yellow_ are mixt and compounded into a _Green_; and this I imagine to be the reason why _Green_ is so acceptable a colour to the eye, and that either of the two extremes are, if intense, rather a little offensive, namely, the being plac'd in the middle between the two extremes, and compounded out of both those, _diluted_ also, or somewhat qualifi'd, for the _composition_, arising from the mixture of the two extremes _undiluted_, makes a _Purple_, which though it be a lovely colour, and pretty acceptable to the eye, yet is it nothing comparable to the ravishing pleasure with which a curious and well tempered _Green_ affects the eye. If removing the Paper, the eye be plac'd against cd, it will perceive the lower side of the Sun (or a Candle at night which is much better, because it offends not the eye, and is more easily manageable) to be of a deep _Red_, and if against ef it will perceive the upper part of the luminous body to be of a deep _Blue_; and these colours will appear deeper and deeper, according as the Rays from the luminous body fall more _obliquely_ on the surface of the Water, and thereby suffer a greater refraction, and the more distinct, the further cdef is removed from the trajecting hole.

So that upon the whole, we shall find that the reason of the _Phaenomena_ seems to depend upon the _obliquity_ of the _orbicular pulse_, to the Lines of Radiation, and in particular, that the Ray cd which const.i.tutes the _Scarlet_ has its inner parts, namely those which are next to the middle of the luminous body, precedent to the outermost which are contiguous to the dark and _unradiating_ skie. And that the Ray ef which gives a _Blue_, has its outward part, namely, that which is contiguous to the dark side precedent to the pulse from the innermost, which borders on the bright _area_ of the luminous body.

We may observe further, that the cause of the _diluting_ of the colours towards the middle, proceeds partly from the wideness of the hole through which the Rays pa.s.s, whereby the Rays from several parts of the luminous body, fall upon many of the same parts between c and f as is more manifest by the Figure: And partly also from the nature of the refraction it self, for the vividness or strength of the two terminating colours, arising chiefly as we have seen, from the very great difference that is betwixt the outsides of those _oblique undulations_ & the dark Rays circ.u.mambient, and that disparity betwixt the _approximate_ Rays, decaying gradually: the further inward toward the middle of the luminous body they are remov'd, the more must the colour approach to a white or an undisturbed light.

Upon the calculation of the refraction and reflection from a Ball of Water or Gla.s.s, we have much the same _Phaenomena_, namely, an _obliquity_ of the undulation in the same manner as we have found it here. Which, because it is very much to our present purpose, and affords such an _Instancia crucis_, as no one that I know has. .h.i.therto taken notice of, I shall further examine. For it does very plainly and positively distinguish, and shew, which of the two _Hypotheses_, either the _Cartesian_ or this is to be followed, by affording a generation of all the colors in the Rainbow, where according to the _Cartesian Principles_ there should be none at all generated. And secondly, by affording an instance that does more closely confine the cause of these _Phaenomena_ of colours to this present _Hypothesis_.

And first, for the _Cartesian_, we have this to object against it, That whereas he says (_Meteorum Cap. 8. Sect. 5._) _Sed judicabam unicam (refractione scilicet) ad minimum requiri, & quidem talem ut ejus effectus alia contraria (refractione) non destruatur: Nam experientia docet si superficies _NM_ & _NP_ (nempe refringentes) Parallelae forent, radios tantundem per alteram iterum erectos quantum per unam frangerentur, nullos colores depicturos_; This Principle of his holds true indeed in a prisme where the refracting surfaces are plain, but is contradicted by the Ball or Cylinder, whether of Water Or Gla.s.s, where the refracting surfaces are Orbicular or Cylindrical. For if we examine the pa.s.sage of any _Globule_ or Ray of the primary _Iris_, we shall find it to pa.s.s out of the Ball or Cylinder again, with the same inclination and refraction that it enter'd in withall, and that that last refraction by means of the _intermediate_ reflection shall be the same as if without any reflection at all the Ray had been twice refracted by two Parallel surfaces.

And that this is true, not onely in one, but in every Ray that goes to the const.i.tution of the Primary Iris; nay, in every Ray, that suffers only two refractions, and one reflection, by the surface of the round body, we shall presently see most evident, if we repeat the _Cartesian Scheme_, mentioned in the tenth _Section_ of the eighth _Chapter_ of his _Meteors_, where EFKNP in the third Figure[9] is one of the Rays of the Primary Iris, twice refracted at F and N, and once reflected at K by the surface of the Water-ball. For, first it is evident, that KF and KN are equal, because KN being the reflected part of KF they have both the same inclination on the surface K that is the angles FKT, and NKV made by the two Rays and the Tangent of K are equal, which is evident by the Laws of reflection; whence it will follow also, that KN has the same inclination on the surface N, or the Tangent of it XN that the Ray KF has to the surface F, or the Tangent of it FY, whence it must necessarily follow, that the refractions at F and N are equal, that is, KFE and KNP are equal. Now, that the surface N is by the reflection at K made parallel to the surface at F, is evident from the principles of reflection; for reflection being nothing but an inverting of the Rays, if we re-invert the Ray KNP, and make the same inclinations below the line TKV that it has above, it will be most evident, that KH the inverse of KN will be the continuation of the line FK, and that LHI the inverse of OX is parallel to FY. And HM the inverse of NP is Parallel to EF for the angle KHI is equal to KNO which is equal to KFY, and the angle KHM is equal to KNP which is equal to KFE which was to be prov'd.

So that according to the above mentioned _Cartesian_ principles there should be generated no colour at all in a Ball of Water or Gla.s.s by two refractions and one reflection, which does hold most true indeed, if the surfaces be plain, as may be experimented with any kind of prisme where the two refracting surfaces are equally inclin'd to the reflecting; but in this the _Phaenomena_ are quite otherwise.

The cause therefore of the generation of colour must not be what _Des Cartes_ a.s.signs, namely, a certain _rotation_ of the _Globuli aetherei_, which are the particles which he supposes to const.i.tute the _Pellucid medium_, But somewhat else, perhaps what we have lately supposed, and shall by and by further prosecute and explain.

But, First I shall crave leave to propound some other difficulties of his, notwithstanding exceedingly ingenious _Hypothesis_, which I plainly confess to me seem such; and those are,

First, if that light be (as is affirmed, _Diopt._ cap. 1. --. 8.) not so properly a motion, as an action or propension to motion, I cannot conceive how the eye can come to be sensible of the _verticity_ of a _Globule_, which is generated in a drop of Rain, perhaps a mile off from it. For that _Globule_ is not carry'd to the eye according to his formerly recited Principle; and if not so, I cannot conceive how it can communicate its _rotation_, or circular motion to the line of the _Globules_ between the drop and the eye. It cannot be by means of every ones turning the next before him; for if so, then onely all the _Globules_ that are in the odd places must be turned the same way with the first, namely, the 3. 5. 7. 9.

11, &c. but all the _Globules_ interposited between them in the even places; namely, the 2. 4. 6. 8. 10. &c. must be the quite contrary, whence, according to the _Cartesian Hypothesis_, there must be no distinct colour generated, but a confusion. Next, since the _Cartesian Globuli_ are suppos'd (_Principiorum Philosoph._ Part. 3. --. 86.) to be each of them continually in motion about their centers, I cannot conceive how the eye is able to distinguish this new generated motion from their former inherent one, if I may so call that other wherewith they are mov'd or _turbinated_, from some other cause than refraction. And thirdly, I cannot conceive how these motions should not happen sometimes to oppose each other, and then, in stead of a _rotation_, there would be nothing but a direct motion generated, and consequently no colour. And fourthly, I cannot conceive, how by the _Cartesian Hypothesis_ it is possible to give any plausible reason of the nature of the Colours generated in the thin _laminae_ of these our _Microscopical Observations_; for in many of these, the refracting and reflecting surfaces are parallel to each other, and consequently no _rotation_ can be generated, nor is there any necessity of a shadow or termination of the bright Rays, such as is suppos'd (_Chap._ 8. --. 5. _Et praeterea observavi umbram quoque, aut limitationem luminis requiri:_ and _Chap._ 8. --. 9.) to be necessary to the generation of any distinct colours; Besides that, here is oftentimes one colour generated without any of the other appendant ones, which cannot be by the _Cartesian Hypothesis_.

There must be therefore some other propriety of refraction that causes colour. And upon the examination of the thing, I cannot conceive any one more general, inseparable, and sufficient, than that which I have before a.s.sign'd. That we may therefore see how exactly our _Hypothesis_ agrees also with the _Phaenomena_ of the refracting round body, whether _Globe_ or _Cylinder_, we shall next subjoyn our _Calculation_ or _Examen_ of it.

And to this end, we will calculate any two Rays: as for instance;[10] let EF be a Ray cutting the _Radius_ CD (divided into 20. parts) in G 16. parts distant from C, and ef another Ray, which cuts the same _Radius_ in g 17.

parts distant, these will be refracted to K and k, and from thence reflected to N and n, and from thence refracted toward P and p; therefore the Arch Ff will be 5.d 5'. The Arch FK 106.d 30'. the Arch fk 101.d 2'.

The line FG 6000. and fg 5267. therefore hf. 733. therefore Fc 980, almost.

The line FK 16024. and fk 15436. therefore Nd 196. and no 147 almost, the line Nn 1019 the Arch Nn 5.d 51'. therefore the Angle Nno is 34.d 43'.

therefore the Angle Non is 139.d 56'. which is almost 50.d more than a right Angle.

It is evident therefore by this _Hypothesis_, that at the same time that ef touches f. EF is arrived at c. And by that time efkn is got to n, EFKN is got to d and when it touches N, the pulse of the other Ray is got to o. and no farther, which is very short of the place it should have arriv'd to, to make the Ray np to cut the _orbicular pulse_ No at right Angles: therefore the Angle Nop is an acute Angle, but the quite contrary of this will happen, if 17. and 18. be calculated in stead of 16. and 17. both which does most exactly agree with the _Phaenomena_: For if the Sun, or a Candle (which is better) be placed about Ee, and the eye about Pp, the Rays EFef at 16. and 17. will paint the side of the luminous object toward np _Blue_, and towards NP _Red_. But the quite contrary will happen when EF is 17. and ef 18. for then towards NP shall be a _Blue_, and towards np a _Red_, exactly according to the calculation. And there appears the _Blue_ of the Rainbow, where the two _Blue_ sides of the two Images unite, and there the _Red_ where the two _Red_ sides unite, that is, where the two Images are just disappearing; which is, when the Rays EF and NP produc'd till they meet, make an Angle of about 41. and an half; the like union is there of the two Images in the Production of the _Secundary Iris_, and the same causes, as upon calculation may appear; onely with this difference, that it is somewhat more faint, by reason of the duplicate reflection, which does always weaken the impulse the oftner it is repeated.

Now, though the second refraction made at Nn be convenient, that is, do make the Rays glance the more, yet is it not altogether requisite; for it is plain from the calculation, that the pulse dn is sufficiently _oblique_ to the Rays KN and kn, as wel as the pulse fc is _oblique_ to the Rays FK & fk. And therefore if a piece of very fine Paper be held close against Nn and the eye look on it either through the Ball as from D, or from the other side, as from B. there shall appear a Rainbow, or colour'd line painted on it with the part toward X appearing _Red_, towards O, _Blue_; the same also shall happen, if the Paper be placed about Kk, for towards T shall appear a _Red_, and towards V a _Blue_, which does exactly agree with this my _Hypothesis_, as upon the calculation of the progress of the pulse will most easily appear.

Nor do these two observations of the colours appearing to the eye about p differing from what they appear on the Paper at N contradict each other; but rather confirm and exactly agree with one another, as will be evident to him that examines the reasons set down by the ingenious. _Des Cartes_ in the 12. _Sect._ of the 8. _Chapter of his Meteors_, where he gives the true reason why the colours appear of a quite contrary order to the eye, to what they appear'd on the Paper if the eye be plac'd in steed of the Paper: And as in the Prisme, so also in the Water-drop, or Globe the _Phaenomena_, and reason are much the same.