Micrographia — Observ. IX. (part 1)

Observ.

IX.

_Of the Colours observable in Muscovy-Glass, and other thin Bodies_.

Moscovy-glass, or _Lapis specularis_, is a Body that seems to have as many Curiosities in its Fabrick as any common Mineral I have met with: for first, It is transparent to a great thickness: Next, it is compounded of an infinite number of thin flakes joyned or generated one upon another so close & smooth, as with many hundreds of them to make one smooth and thin Plate of a transparent flexible substance, which with care and diligence may be slit into pieces so exceedingly thin as to be hardly perceivable by the eye, and yet even those, which I have thought the thinnest, I have with a good _Microscope_ found to be made up of many other Plates, yet thinner;

and it is probable, that, were our _Microscopes_ much better, we might much further discover its divisibility.

Nor are these flakes only regular as to the smoothness of their Surfaces, but thirdly, In many Plates they may be perceived to be terminated naturally with edges of the figure of a _Rhomboeid_.

This Figure is much more conspicuous in our English talk, much whereof is found in the Lead Mines, and is commonly called _Spar_, and _Kauck_, which is of the same kind of substance with the _Selenitis_, but is seldom found in so large flakes as that is, nor is it altogether so tuff, but is much more clear and transparent, and much more curiously shaped, and yet may be cleft and flak’d like the other _Selenitis_.

But fourthly, this stone has a property, which in respect of the _Microscope_, is more notable, and that is, that it exhibits several appearances of Colours, both to the naked Eye, but much more conspicuously to the _Microscope_; for the exhibiting of which, I took a piece of _Muscovy-glass_, and splitting or cleaving it into thin Plates, I found that up and down in several parts of them I could plainly perceive several white specks or flaws, and others diversly coloured with all the Colours of the _Rainbow_; and with the _Microscope_ I could perceive, that these Colours were ranged in rings that incompassed the white speck or flaw, and were round or irregular, according to the shape of the spot which they terminated; and the position of Colours, in respect of one another, was the very same as in the _Rainbow_.

The consecution of those Colours from the middle of the spot outward being Blew, Purple, Scarlet, Yellow, Green;

Blew, Purple, Scarlet, and so onwards, sometimes half a score times repeated, that is, there appeared six, seven, eight, nine or ten several coloured rings or lines, each incircling the other, in the same manner as I have often seen a very _vivid Rainbow_ to have four or five several Rings of Colours, that is, accounting all the Gradations between Red and Blew for one: But the order of the Colours in these Rings was quite contrary to the primary or innermost _Rainbow_, and the same with those of the secondary or outermost Rainbow; these coloured Lines or _Irises_, as I may so call them, were some of them much brighter then others, and some of them also very much broader, they being some of them ten, twenty, nay, I believe, neer a hundred times broader then others; and those usually were broadest which were neerest the center or middle of the flaw.

And oftentimes I found, that these Colours reacht to the very middle of the flaw, and then there appeared in the middle a very large spot, for the most part, all of one colour, which was very vivid, and all the other Colours incompassing it, gradually ascending, and growing narrower towards the edges, keeping the same order, as in the _secundary Rainbow_, that is, if the middle were Blew, the next incompassing it would be a Purple, the third a Red, the fourth a Yellow, &c. as above; if the middle were a Red, the next without it would be a Yellow, the third a Green, the fourth a Blew, and so onward.

And this order it alwayes kept whatsoever were the middle Colour.

There was further observable in several other parts of this Body, many Lines or Threads, each of them of some one peculiar Colour, and those so exceedingly bright and vivid, that it afforded a very pleasant object through the _Microscope_.

Some of these _threads_ I have observed also to be pieced or made up of several short lengths of differently coloured _ends_ (as I may so call them) as a line appearing about two inches long through the _Microscope_, has been compounded of about half an inch of a Peach colour, ⅛ of a lovely Grass-green, ¾ of an inch more of a bright Scarlet, and the rest of the line of a Watchet blew.

Others of them were much otherwise coloured; the variety being almost infinite.

Another thing which is very observable, is, that if you find any place where the colours are very broad and conspicuous to the naked eye, you may, by pressing that place with your finger, make the colours change places, and go from one part to another.

There is one _Phænomenon_ more, which may, if care be used, exhibit to the beholder, as it has divers times to me, an exceeding pleasant, and not less instructive Spectacle; And that is, if curiosity and diligence be used, you may so split this admirable Substance, that you may have pretty large Plates (in companion of those smaller ones which you may observe in the Rings) that are perhaps an ⅛ or a ⅙ part of an inch over, each of them appearing through the _Microscope_ most curiously, intirely, and uniformly adorned with some one vivid colour: this, if examined with the _Microscope_, may be plainly perceived to be in all parts of it equally thick.

Two, three, or more of these lying one upon another, exhibit oftentimes curious compounded colours, which produce such a _Compositum_, as one would scarce imagine should be the result of such _ingredients_: As perhaps a _faint yellow_ and a _blew_ may produce a very _deep purple_.

But when anon we come to the more strict examination of these _Phænomena_, and to inquire into the causes and reasons of these productions, we shall, I hope, make it more conceivable how they are produced, and shew them to be no other then the natural and necessary effects arising from the peculiar union of concurrent causes.

These _Phænomena_, being so various, and so truly admirable, it will certainly be very well worth our inquiry, to examine the causes and reasons of them, and to consider, whether from these causes demonstratively evidenced, may not be deduced the true causes of the production of all kind of Colours.

And I the rather now do it, instead of an Appendix or Digression to this History, then upon the occasion of examining the Colours in Peacocks, or other Feathers, because this Subject, as it does afford more variety of particular Colours, so does it afford much better wayes of examining each circumstance.

And this will be made manifest to him that considers, first, that this laminated body is more simple and regular then the parts of Peacocks feathers, this consisting only of an indefinite number of plain and smooth Plates, heaped up, or _incumbent_ on each other.

Next, that the parts of this body are much more manageable, to be divided or joyned, then the parts of a Peacocks feather, or any other substance that I know.

And thirdly, because that in this we are able from a colourless body to produce several coloured bodies, affording all the variety of Colours imaginable: And several others, which the subsequent Inquiry will make manifest.

To begin therefore, it is manifest from several circumstances, that the material cause of the _apparition_ of these several Colours, is some _Lamina_ or Plate of a transparent or pellucid body of a thickness very determinate and proportioned according to the greater or less refractive power of the _pellucid_ body.

And that this is so, abundance of Instances and particular Circumstances will make manifest.

As _first_, if you take any small piece of the _Muscovy-glass_, and with a Needle, or some other convenient Instrument, cleave it oftentimes into thinner and thinner _Laminæ_, you shall find, that till you come to a determinate thinness of them, they shall all appear transparent and colourless, but if you continue to split and divide them further, you shall find at last, that each Plate, after it comes to such a determinate thickness, shall appear most lovely ting’d or imbued with a determinate colour.

If _further_, by any means you so flaw a pretty thick piece, that one part does begin to cleave a little from the other, and between those two there be by any means gotten some pellucid _medium_, those _laminated_ pellucid bodies that fill that space, shall exhibit several Rainbows or coloured Lines, the colours of which will be disposed and ranged according to the various thicknesses of the several parts of that Plate.

That this is so, is yet _further_ confirmed by this Experiment.

Take two small pieces of ground and polisht Looking-glass-plate, each about the bigness of a shilling, take these two dry, and with your fore-fingers and thumbs press them very hard and close together, and you shall find, that when they approach each other very near, there will appear several _Irises_ or coloured Lines, in the same manner almost as in the _Muscovy-glass_; and you may very easily change any of the Colours of any part of the interposed body, by pressing the Plates closer and harder together, or leaving them more lax; that is, a part which appeared coloured with a red, may be presently ting’d with a yellow, blew, green, purple, or the like, by altering the appropinquation of the terminating Plates.

Now that air is not necessary to be the interposed body, but that any other transparent fluid will do much the same, may be tryed by wetting those approximated Surfaces with Water, or any other transparent Liquor, and proceeding with it in the same manner as you did with the Air; and you will find much the like effect, only with this difference, that those comprest bodies, which differ most, in their refractive quality, from the compressing bodies, exhibit the most strong and vivid tinctures.

Nor is it necessary, that this _laminated_ and _ting’d_ body should be of a fluid substance, any other substance, provided it be thin enough and transparent, doing the same thing: this the _Laminæ_ of our _Muscovy-glass_ hint; but it may be confirm’d by multitudes of other Instances.

And first, we shall find, that even Glass it self may, by the help of a Lamp, be blown thin enough to produce these _Phænomena_ of Colours: which _Phænomena_ accidentally happening, as I have been attempting to frame small Glasses with a Lamp, did not a little surprize me at first, having never heard or seen any thing of it before; though afterwards comparing it with the _Phænomena_, I had often observed in those Bubbles which Children use to make with Soap-water, I did the less wonder; especially when upon Experiment I found, I was able to produce the same _Phænomena_ in thin Bubbles made with any other transparent Substance.

Thus have I produced them with Bubbles of _Pitch_, _Rosin_, _Colophony_, _Turpentine_, _Solutions_ of several _Gums_, as _Gum-Arabick_ in water; any _glutinous_ Liquor, as _Wort_, _Wine_, _Spirit of Wine_, _Oyl of Turpentine_, _Glare of Snails_, &c. It would be needless to enumerate the several Instances, these being enough to shew the generality or universality of this propriety.

Only I must not omit, that we have instances also of this kind even in metalline Bodies and animal; for those several Colours which are observed to follow each other upon the polisht surface of hardned Steel, when it is by a sufficient degree of heat gradually tempered or softened, are produced, from nothing else but a certain thin _Lamina_ of a _vitrum_ or _vitrified_ part of the Metal, which by that degree of heat, and the concurring action of the ambient Air, is driven out and fixed on the surface of the Steel.

And this hints to me a very probable (at least, if not the true) cause of the hardning and tempering of Steel, which has not, I think, been yet given, nor, that I know of been so much as thought of by any.

And that is this, that the hardness of it arises from a greater proportion of a vitrified Substance interspersed through the pores of the Steel.

And that the tempering or softning of it arises from the proportionate or smaller parcels of it left within those pores.

This will seem the more probable, if we consider these Particulars.

First, That the pure parts of Metals are of themselves very _flexible_ and _tuff_; that is, will indure bending and hammering, and yet retain their continuity.

Next, That the Parts of all vitrified Substances, as all kinds of Glass, the _Scoria_ of Metals, &c. are very hard, and also very brittle, being neither _flexible_ nor _malleable_, but may by hammering or beating be broken into small parts or powders.

Thirdly, That all Metals (excepting Gold and Silver, which do not so much with the bare fire, unless assisted 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 Glass, 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 Tradesmen call it, by casing the iron to be hardned with clay, and putting between the clay and iron a good quantity of a mixture of _Urine_, _Soot_, _Sea-salt_, and _Horses hoofs_ (all which contein great quantities 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 _Phænomena_ of the Glass 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 _Phænomena_ 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 glass 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 rigidness 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 scraped 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 scum’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 Brass, 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 glass 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 Laminæ_ on _Steel_, _Lead_, and other Metals, one surface of which _Laminæ_ is contiguous to the surface of the Metal, the other to that of the Air.

Nor is it necessary, that these colour’d _Laminæ_ should be of an even thickness, that is, should have their edges and middles of equal thickness, as in a Looking-glass-plate, which circumstance 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-glass_, 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 constituted 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 Glass-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 _verticity_ 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 Glass-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 Assertion 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 Glass, 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 _Energies_ 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, Brass, 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 or _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 principal 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 _Homogeneous_, or of the same kind.

Thirdly, that the constitution 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 assured 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 _Homogeneous medium_ by _direct_ or _straight_ lines extended every way like Rays from the center of a Sphere.

Fifthly, in an _Homogeneous medium_ this motion is propagated every way with _equal velocity_, whence necessarily every _pulse_ or _vibration_ 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 _Homogeneous medium_ cut the Rays at right angles.

But because all transparent _mediums_ are not _Homogeneous_ 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 _Homogeneous_ 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 spaces in the same time that F the other end of it is mov’d to G three spaces, 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 quantity 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 pass through a small passage, _obliquely_ out of a more difficult, into a more easie _medium_.