Micrographia Part 21

Observ. LIV. _Of a Louse._

This is a Creature so officious, that 'twill be known to every one at one time or other, so busie, and so impudent, that it will be intruding it self in every ones company, and so proud and aspiring withall, that it fears not to trample on the best, and affects nothing so much as a Crown; feeds and lives very high, and that makes it so saucy, as to pull any one by the ears that comes in its way, and will never be quiet till it has drawn blood: it is troubled at nothing so much as at a man that scratches his head, as knowing that man is plotting and contriving some mischief against it, and that makes it oftentime sculk into some meaner and lower place, and run behind a mans back, though it go very much against the hair; which ill conditions of it having made it better known then trusted, would exempt me from making any further description of it, did not my faithful _Mercury_, my _Microscope_, bring me other information of it. For this has discovered to me, by means of a very bright light cast on it, that it is a Creature of a very odd shape; it has a head shap'd like that exprest in 35. _Scheme_ marked with A, which seems almost Conical, but is a little flatted on the upper and under sides, at the biggest part of which, on either side behind the head (as it were, being the place where other Creatures ears stand) are placed its two black shining goggle eyes BB, looking backwards, and fenced round with several small _cilia_, or hairs that incompa.s.s it, so that it seems this Creature has no very good foresight: It does not seem to have any eye-lids, and therefore perhaps its eyes were so placed, that it might the better cleanse them with its fore-legs; and perhaps this may be the reason, why they so much avoid and run from the light behind them, for being made to live in the shady and dark recesses of the hair, and thence probably their eye having a great aperture, the open and clear light, especially that of the Sun, must needs very much offend them; to secure these eyes from receiving any injury from the hairs through which it pa.s.ses, it has two horns that grow before it, in the place where one would have thought the eyes should be; each of these CC hath four joynts, which are fringed, as 'twere, with small brisles, from which to the tip of its snout D, the head seems very round and tapering, ending in a very sharp nose D, which seems to have a small hole, and to be the pa.s.sage through which he sucks the blood. Now whereas if it be plac'd on its back, with its belly upwards, as it is in the 35. _Scheme_, it seems in several Positions to have a resemblance of chaps, or jaws, as is represented in the Figure by EE, yet in other postures those dark strokes disappear; and having kept several of them in a box for two or three dayes, so that for all that time they had nothing to feed on, I found, upon letting one creep on my hand, that it immediately fell to sucking, and did neither seem to thrust its nose very deep into the skin, nor to open any kind of mouth, but I could plainly perceive a small current of blood, which came directly from its snout, and past into its belly; and about A there seem'd a contrivance, somewhat resembling a Pump, pair of Bellows, or Heart, for by a very swift _systole_ and _diastole_ the blood seem'd drawn from the nose, and forced into the body. It did not seem at all, though I viewed it a good while as it was sucking, to thrust more of its nose into the skin then the very snout D, nor did it cause the least discernable pain, and yet the blood seem'd to run through its head very quick and freely, so that it seems there is no part of the skin but the blood is dispers'd into, nay, even into the _cuticula_; for had it thrust its whole nose in from D to CC, it would not have amounted to the supposed thickness of that _tegument_, the length of the nose being not more then a three hundredth part of an inch.

It has six legs, covered with a very transparent sh.e.l.l, and joynted exactly like a Crab's, or Lobster's; each leg is divided into six parts by these joynts, and those have here and there several small hairs; and at the end of each leg it has two claws, very properly adapted for its peculiar use, being thereby inabled to walk very securely both on the skin and hair; and indeed this contrivance of the feet is very curious, and could not be made more commodiously and compendiously, for performing both these requisite motions, of walking and climbing up the hair of a mans head, then it is: for, by having the lesser claw (a) set so much short of the bigger (b) when it walks on the skin the shorter touches not, and then the feet are the same with those of a Mite, and several other small Insects, but by means of the small joynts of the longer claw it can bend it round, and so with both claws take hold of a hair, in the manner represented in the Figure, the long transparent Cylinder FFF, being a Man's hair held by it.

The _Thorax_ seem'd cas'd with another kind of substance then the belly, namely, with a thin transparent h.o.r.n.y substance, which upon the fasting of the Creature did not grow flaccid; through this I could plainly see the blood, suck'd from my hand, to be variously distributed, and mov'd to and fro; and about G there seem'd a pretty big white substance, which seem'd to be moved within its _thorax_; besides, there appear'd very many small milk-white vessels, which crost over the breast between the legs, out of which, on either side, were many small branchings, these seem'd to be the veins and arteries, for that which is a.n.a.logus to blood in all Insects is milk-white.

The belly is covered with a transparent substance likewise, but more resembling a skin then a sh.e.l.l, for 'tis grain'd all over the belly just like the skin in the palms of a man's hand, and when the belly is empty, grows very flaccid and wrinkled; at the upper end of this is placed the stomach HH, and perhaps also the white spot II may be the liver or _pancreas_, which, by the _peristalick_ motion of the guts, is a little mov'd to and fro, not with a _systole_ and _diastole_, but rather with a thronging or justling motion. Viewing one of these Creatures, after it had fasted two dayes, all the hinder part was lank and flaccid, and the white spot II hardly mov'd, most of the white branchings disappear'd, and most also of the redness or sucked blood in the guts, the _peristaltick_ motion of which was scarce discernable; but upon the suffering it to suck, it presently fill'd the skin of the belly, and of the six scolop'd embosments on either side, as full as it could be stuft, the stomach and guts were as full as they could hold; the _peristaltick_ motion of the gut grew quick, and the justling motion of II accordingly; mult.i.tudes of milk-white vessels seem'd quickly filled, and turgid, which were perhaps the veins and arteries and the Creature was so greedy, that though it could not contain more, yet it continued sucking as fast as ever, and as fast emptying it self behind: the digestion of this Creature must needs be very quick, for though I perceiv'd the blood thicker and blacker when suck'd, yet, when in the guts, it was of a very lovely ruby colour, and that part of it, which was digested into the veins, seemed white; whence it appears, that a further digestion of blood may make it milk, at least of a resembling colour: What is else observable in the figure of this Creature, may be seen by the 35. _Scheme_.

Observ. LV. _Of _Mites_._

The least of _Reptiles_ I have hitherto met with, is a Mite, a Creature whereof there are some so very small, that the sharpest sight, una.s.sisted with Gla.s.ses, is not able to discern them, though, being white of themselves, they move on a black and smooth surface; and the Eggs, out of which these Creatures seem to be hatch'd, are yet smaller, those being usually not above a four or five hundredth part of a well grown Mite, and those well grown Mites not much above one hundredth of an inch in thickness; so that according to this reckoning there may be no less then a million of well grown Mites contain'd in a cubick inch, and five hundred times as many Eggs.

Notwithstanding which minuteness a good _Microscope_ discovers those small movable specks to be very prettily shap'd Insects, each of them furnished with eight well shap'd and proportion'd legs, which are each of them joynted or bendable in eight several places, or joynts, each of which is covered, for the most part, with a very transparent sh.e.l.l, and the lower end of the sh.e.l.l of each joynt is fringed with several small hairs; the contrivance of the joynts seems the very same with that of Crabs and Lobsters legs, and like those also, they are each of them terminated with a very sharp claw or point; four of these legs are so placed, that they seem to draw forwards, the other four are placed in a quite contrary position, thereby to keep the body backwards when there is occasion.

[15]The body, as in other larger Insects, consists of three regions or parts; the hinder or belly A, seems covered with one intire sh.e.l.l, the middle, or chest, seems divided into two sh.e.l.ls BC. which running one within the other, the Mite is able to shrink in and thrust out as it finds occasion, as it can also the snout D. The whole body is pretty transparent, so that being look'd on against the light, divers motions within its body may be perceived; as also all the parts are much more plainly delineable, then in other postures, to the light. The sh.e.l.l, especially that which covers the back, is curiously polisht, so that 'tis easie to see, as in a _convex_ Looking-gla.s.s, or _foliated_ Gla.s.s-ball, the picture of all the objects round about; up and down, in several parts of its body, it has several small long white hairs growing out of its sh.e.l.l, which are often longer then the whole body, and are represented too short in the first and second Figures; they seem all pretty straight and plyable, save only two upon the fore-part of its body, which seem to be the horns, as may be seen in the Figures; the first whereof is a prospect of a smaller sort of Mites (which are usually more plump) as it was _pa.s.sant_ to and fro; the second is the prospect of one fixt on its tail (by means of a little mouth-glew rub'd on the object plate) exhibiting the manner of the growing of the legs, together with their several joynts.

This Creature is very much diversify'd in shape, colour, and divers other properties, according to the nature of the substance out of which it seems to be ingendred and nourished, being in one substance more long, in another more round, in some more hairy, in others more smooth, in this nimble, in that slow, here pale and whiter, there browner, blacker, more transparent, &c. I have observed it to be resident almost on all kinds of substances that are mouldy, or putrifying, and have seen it very nimbly meshing through the thickets of mould, and sometimes to lye _dormant_ underneath them; and 'tis not unlikely, but that it may feed on that vegetating substance, _spontaneous Vegetables_ seeming a food proper enough for _spontaneous Animals_,

But whether indeed this Creature, or any other, be such or not, I cannot positively, from any Experiment, or Observation, I have yet made, determine. But, as I formerly hinted, it seems probable, that some kind of wandring Mite may sow, as 'twere, the first seeds, or lay the first eggs, in those places, which Nature has instructed them to know convenient for the hatching and nourishing their young; and though perhaps the prime Parent might be of a shape very differing from what the offspring, after a little while, by reason of the substance they feed on, or the Region (as 'twere) they inhabite; yet perhaps even one of these alter'd progeny, wandering again from its native soil, and lighting on by chance the same place from whence its prime Parent came, and there settling, and planting, may produce a generation of Mites of the same shapes and properties with the first wandring Mite: And from some such accidents as these, I am very apt to think, the most sorts of Animals, generally accounted _spontaneous_, have their _origination_, and all those various sorts of Mites, that are to be met with up and down in divers putrifying substances, may perhaps be all of the same kind, and have sprung from one and the same sort of Mites at the first.

Observ. LVI. _Of a small Creature hatch'd on a Vine._

There is, almost all the Spring and Summer time, a certain small, round, white Cobweb, as 'twere, about the bigness of a Pea, which sticks very close and fast to the stocks of Vines nayl'd against a warm wall: being attentively viewed, they seem cover'd, upon the upper side of them, with a small husk, not unlike the scale, or sh.e.l.l of a Wood-louse, or Hog-louse, a small Insect usually found about rotten wood, which upon touching presently rouls it self into the form of a peppercorn: Separating several of these from the stock, I found them, with my _Microscope_, to consist of a sh.e.l.l, which now seemed more likely to be the husk of one of these Insects: And the fur seem'd a kind of cobweb, consisting of abundance of small filaments, or sleaves of cobwebs. In the midst of this, if they were not hatch'd, and run away before, the time of which hatching was usually about the latter end of _June_, or beginning of _July_, I have often found abundance of small brown Eggs, such as A and B in the second Figure of the 36. _Scheme_, much about the bigness of Mites Eggs; and at other times, mult.i.tudes of small Insects, shaped exactly like that in the third Figure marked with X. Its head large, almost half the bigness of its body, which is usual in the _foetus_ of most Creatures. It had two small black eyes _aa_, and two small long joynted and brisled horns _bb_. The hinder part of its body seem'd to consist of nine scales, and the last ended in a forked tayl, much like that of a _Cutio_, or Wood louse, out of which grew two long hairs; they ran to and fro very swiftly, and were much of the bigness of a common Mite, but some of them less: The longest of them seem'd not the hundredth part of an inch, and the Eggs usually not above half as much.

They seemed to have six legs, which were not visible in this I have here delineated, by reason they were drawn under its body.

If these Minute creatures were _Wood-lice_ (as indeed from their own shape and from the frame, the skin, or sh.e.l.l, that grows on them, one may with great probability ghess) it affords us an Instance, whereof perhaps there are not many like in Nature, and that is, of the prodigious increase of these Creatures, after they are hatch'd and run about; for a common Wood-louse, of about half an inch long, is no less then a hundred and twenty five thousand times bigger then one of these, which though indeed it seems very strange, yet I have observed the young ones of some Spiders have almost kept the same proportion to their Dam.

This, methinks, if it be so, does in the next place hint a Quaery, which may perhaps deserve a little further examination: And that is, Whether there be not many of those minute Creatures, such as Mites, and the like, which, though they are commonly thought of otherwise, are only the _pully_, or young ones, of much bigger Insects, and not the generating, or parent _Insect_, that has layd those Eggs; for having many times observ'd those Eggs, which usually are found in great abundance where Mites are found, it seems something strange, that so small an Animal should have an Egg so big in proportion to its body. Though on the other side, I must confess, that having kept divers of those Mites inclosed in a box for a good while, I did not find them very much augmented beyond their usual bigness.

What the husk and cobweb of this little white substance should be, I cannot imagine, unless it be, that the old one, when impregnated with Eggs, should there stay, and fix it self on the Vine, and dye, and all the body by degrees should rot, save only the husk, and the Eggs in the body: And the heat, or fire, as it were, of the approaching Sun-beams should vivifie those Relicts of the corrupted Parent, and out of the ashes, as 'twere, (as it is fabled of the _Phoenix_) should raise a new _offspring_ for the perpetuation of the _Species_. Nor will the cobweb, as it were, in which these Eggs are inclos'd, make much against this Conjecture; for we may, by those cobwebs that are carried up and down the Air after a Fog (which with my _Microscope_ I have discovered to be made up of an infinite company of small filaments or threads) learn, that such a texture of body may be otherwise made then by the spinning of a Worm.

Observ. LVII. _Of the _Eels_ in Vinegar._

Of these small Eels, which are to be found in divers sorts of Vinegar, I have little to add besides their Picture, which you may find drawn in the third Figure of the 25. _Scheme_: That is, they were shaped much like an Eel, save only that their nose A, (which was a little more opacous then the rest of their body) was a little sharper, and longer, in proportion to their body, and the wrigling motion of their body seem'd to be onely upwards and downwards, whereas that of Eels is onely side wayes: They seem'd to have a more opacous part about B, which might, perhaps, be their Gills; it seeming always the same proportionate distant from their nose, from which, to the tip of their tail, C, their body seem'd to taper.

Taking several of these out of their Pond of Vinegar, by the net of a small piece of filtring Paper, and laying them on a black smooth Gla.s.s plate, I found that they could wriggle and winde their body, as much almost as a Snake, which made me doubt, whether they were a kind of Eal or Leech.

I shall add no other observations made on this minute Animal, being prevented herein by many excellent ones already publish'd by the ingenious, Doctor _Power_, among his _Microscopical_ Observations, save onely that a quant.i.ty of Vinegar repleat with them being included in a small Viol, and stop'd very close from the ambient air, all the included Worms in a very short time died, as if they had been stifled.

And that their motion seems (contrary to what we may observe in the motion of all other Infects) exceeding slow. But the reason of it seems plain, for being to move to and fro after that manner which they do, by waving onely, or wrigling their body; the tenacity, or glutinousness, and the density or resistance of the fluid _medium_ becomes so exceeding sensible to their extremely minute bodies, that it is to me indeed a greater wonder that they move them so fast as they do, then that they move them no faster. For what a vastly greater proportion have they of their superficies to their bulk, then Eels or other larger Fishes, and next, the tenacity and density of the liquor being much the same to be moved, both by the one and the other, the resistance or impediment thence arising to the motions made through it, must be almost infinitely greater to the small one then to the great. This we find experimentally verify'd in the Air, which though a _medium_ a thousand times more rarify'd then the water, the resistance of it to motions made through it, is yet so sensible to very minute bodies, that a Down-feather (the least of whose parts seem yet bigger then these Eels, and many of them almost incomparably bigger, such as the quill and stalk) is suspended by it, and carried to and fro as if it had no weight.

Observ. LVIII. _Of a new Property in the _Air_, and several other transparent _Mediums_ nam'd _Inflection_, whereby very many considerable _Phaenomena_ are attempted to be solv'd, and divers other uses are hinted._

Since the Invention (and perfecting in some measure) of _Telescopes_, it has been observ'd by several, that the Sun and Moon neer the Horizon, are disfigur'd (losing that exactly-smooth terminating circular limb, which they are observ'd to have when situated neerer the Zenith) and are bounded with an edge every way (especially upon the right and left sides) ragged and indented like a Saw: which inequality of their limbs, I have further observ'd, not to remain always the same, but to be continually chang'd by a kind of fluctuating motion, not unlike that of the waves of the Sea, so as that part of the limb, which was but even now nick'd or indented in, is now protuberant, and will presently be sinking again; neither is this all but the whole body of the Luminaries, do in the _Telescope_, seem to be depress'd and slatted, the upper, and more especially the under side appearing neerer to the middle then really they are, and the right and left appearing more remote: whence the whole _Area_ seems to be terminated by a kind of Oval. It is further observ'd, that the body, for the most part, appears red, or of some colour approaching neer unto it, as some kind of yellow; and this I have always mark'd, that the more the limb is slatted or ovalled, the more red does the body appear, though not always the contrary.

It is further observable, that both fix'd Stars and Planets, the neerer they appear to the Horizon, the more red and dull they look, and the more they are observ'd to twinkle; in so much, that I have seen the Dog-starr to vibrate so strong and bright a radiation of light, as almost to dazle my eyes, and presently, almost to disappear. It is also observable, that those bright scintillations neer the Horizon, are not by much so quick and sudden in their consecutions of one another, as the nimbler twinklings of Stars neerer the Zenith. This is also notable, that the Starrs neer the Horizon, are twinkled with several colours; so as sometimes to appear red, sometimes more yellow, and sometimes blue, and this when the Starr is a pretty way elevated above the Horizon. I have further, very often seen some of the small Starrs of the fifth or sixth magnitude, at certain times to disappear for a small moment of time, and again appear more conspicuous, and with a greater l.u.s.ter. I have several times, with my naked eye, seen many smaller Starrs, such as may be call'd of the seventh or eighth magnitude to appear for a short s.p.a.ce, and then vanish, which, by directing a small _Telescope_ towards that part they appear'd and disappear'd in; I could presently find to be indeed small Starrs so situate, as I had seen them with my naked eye, and to appear twinkling like the ordinary visible Stars; nay, in examining some very notable parts of the Heaven, with a three foot Tube, me thought I now and then, in several parts of the constellation, could perceive little twinklings of Starrs, making a very short kind of apparition, and presently vanishing, but noting diligently the places where they thus seem'd to play at boe-peep, I made use of a very good twelve foot Tube, and with that it was not uneasie to see those, and several other degrees of smaller Starrs, and some smaller yet, that seem'd again to appear and disappear, and these also by giving the same Object-gla.s.s a much bigger aperture, I could plainly and constantly see appear in their former places; so that I have observ'd some twelve several magnitudes of Starrs less then those of the six magnitudes commonly recounted in the Globes.

It has been observ'd and confirm'd by the accuratest Observations of the best of our modern Astronomers, that all the Luminous bodies appear above the Horizon, when they really are below it. So that the Sun and Moon have both been seen above the Horizon, whil'st the Moon has been in an Eclipse.

I shall not here instance in the great refractions, that the tops of high mountains, seen at a distance, have been found to have; all which seem to argue the Horizontal refraction, much greater then it is. .h.i.therto generally believ'd.

I have further taken notice, that not onely the Sun, Moon and Starrs, and high tops of mountains have suffer'd these kinds of refraction, but Trees, and several bright Objects on the ground: I have often taken notice of the twinkling of the reflections of the Sun from a Gla.s.s-window at a good distance, and of a Candle in the night, but that is not so conspicuous, and in observing the setting Sun, I have often taken notice of the tremulation of the Trees and Bushes, as well as of the edges of the Sun. Divers of these _Phaenomena_ have been taken notice of by several, who have given several reasons of them, but I have not yet met with any altogether satisfactory, though some of their conjectures have been partly true, but partly also false. Setting my self therfore upon the inquiry of these _Phaenomena_, I first endeavour'd to be very diligent in taking notice of the several particulars and circ.u.mstances observable in them; and next, in making divers particular Experiments, that might cleer some doubts, and serve to determine, confirm, and ill.u.s.trate the true and adaequate cause of each; and upon the whole, I find much reason to think, that the true cause of all these _Phaenomena_ is from the _inflection_, or _multiplicate refraction_ of those Rays of light within the body of the _Atmosphere_, and that it does not proceed from a _refraction_ caus'd by any terminating _superficies_ of the Air above, nor from any such exactly defin'd _superficies_ within the body of the _Atmosphere_.

This Conclusion is grounded upon these two Propositions:

First, that a _medium_, whose parts are unequally _dense_, and mov'd by various motions and transpositions as to one another, will produce all these visible effects upon the Rays of light, without any other _coefficient_ cause.

Secondly, that there is in the Air or _Atmosphere_ such a variety in the const.i.tuent parts of it, both as to their _density_ and _rarity_, and as to their divers mutations and positions one to another.

By _Density_ and _Rarity_, I understand a property of a transparent body, that does either more or less refract a Ray of light (coming obliquely upon its superficies out of a third _medium_) toward its perpendicular: As I call Gla.s.s a more dense body then Water, and Water a more rare body then Gla.s.s, because of the refractions (more or less deflecting towards the perpendicular) that are made in them, of a Ray of light out of the Air that has the same inclination upon either of their superficies.

So as to the business of Refraction, spirit of Wine is a more _dense_ body then Water, it having been found by an accurate Instrument that measures the angles of Refractions to Minutes that for the same refracted angle of 30.00'. in both those _Mediums_, the angle of incidence in Water was but 41.35'. but the angle of the incidence in the trial with spirit of Wine was 42.45'. But as to gravity, Water is a more _dense_ body then spirit of Wine, for the proportion of the same Water, to the same very well rectify'd spirit of Wine was, as 21. to 19.

So as to Refraction, Water is more Dense then Ice; for I have found by a most certain Experiment, which I exhibited before divers ill.u.s.trious Persons of the _Royal Society_, that the Refraction of Water was greater then that of Ice, though some considerable Authors have affirm'd the contrary, and though the Ice be a very hard, and the Water a very fluid body.

That the former of the two preceding Propositions is true, may be manifested by several Experiments; As first, if you take any two liquors differing from one another in density, but yet such as will readily mix: as Salt Water, or Brine, & Fresh; almost any kind of Salt dissolv'd in Water, and filtrated, so that it be cleer, spirit of Wine and Water; nay, spirit of Wine, and spirit of Wine, one more highly rectify'd then the other, and very many other liquors; if (I say) you take any two of these liquors, and mixing them in a Gla.s.s Viol, against one side of which you have fix'd or glued a small round piece of Paper, and shaking them well together (so that the parts of them may be somewhat disturb'd and move up and down) you endeavour to see that round piece of Paper through the body of the liquors, you shall plainly perceive the Figure to wave, and to be indented much after the same manner as the limb of the Sun through a _Telescope_ seems to be, save onely that the mutations here, are much quicker. And if, in steed of this bigger Circle, you take a very small spot, and fasten and view it as the former, you will find it to appear much like the twinkling of the Starrs, though much quicker: which two _Phaenomena_, (for I shall take notice of no more at present, though I could instance in mult.i.tudes of others) must necessarily be caus'd by an _inflection_ of the Rays within the terminating superficies of the compounded _medium_, since the surfaces of the transparent body through which the Rays pa.s.s to the eye, are not at all altered or chang'd.

This _inflection_ (if I may so call it) I imagine to be nothing else, but a _multiplicate refraction_, caused by the unequal _density_ of the const.i.tuent parts of the _medium_, whereby the motion, action or progress of the Ray of light is hindred from proceeding in a streight line, and _inflected_ or _deflected_ by a _curve_. Now, that it is a _curve_ line is manifest by this Experiment: I took a Box, such as ADGE, in the first _Figure_ of the 37. _Scheme_, whose sides ABCD, and EFGH, were made of two smooth flat plates of Gla.s.s, then filling it half full with a very strong solution of Salt, I filled the other half with very fair fresh water, then exposing the opacous side, DHGC, to the Sun, I observ'd both the _refraction_ and _inflection_ of the Sun beams, ID & KH, and marking as exactly as I could, the points, P, N, O, M, by which the Ray, KH, pa.s.sed through the compounded _medium_, I found them to be in a _curve_ line; for the parts of the _medium_ being continually more dense the neerer they were to the bottom, the Ray _pf_ was continually more and more deflected downwards from the streight line.

This Inflection may be mechanically explained, either by Monsieur _Des Cartes_ principles by conceiving the Globuls of the third Element to find less and less resistance against that side of them which is downwards, or by a way, which I have further explicated in the Inquisition about Colours, to be from an obliquation of the pulse of light, whence the under part is continually promoted, and consequently refracted towards the perpendicular, which cuts the Orbs at right angles. What the particular Figure of the _Curve line_, describ'd by this way of light, is, I shall not now stand to examine, especially since there may be so many sorts of it as there may be varieties of the Positions of the _intermediat_ degrees of _density_ and _rarity_ between the bottom and the top of the inflecting Medium.

I could produce many more Examples and Experiments, to ill.u.s.trate and prove this first Proposition, _viz._ that there is such a const.i.tution of some bodies as will cause inflection. As not to mention those I have observ'd in _Horn_, _Tortoise-Sh.e.l.l_, _transparent Gums_, and _resinous Substances_: The _veins_ of Gla.s.s, nay, of melted _Crystal_, found, and much complained of by Gla.s.s-grinders, and others, might sufficiently demonstrate the truth of it to any diligent Observator.

But that, I presume, I have by this Example given proof sufficient (_viz.

ocular demonstration_) to evince, that there is such a modulation, or bending of the rayes of light, as I have call'd _inflection_, differing both from _reflection_, and _refraction_ (since they are both made in the superficies, this only in the middle); and likewise, that this is able or sufficient to produce the effects I have ascribed to it.

It remains therefore to shew, that there is such a property in the Air, and that it is sufficient to produce all the above mentioned _Phaenomena_, and therefore may be the princ.i.p.al, if not the only cause of them.

First, That there is such a property, may be proved from this, that the parts of the Air are some of them more condens'd, others more rarified, either by the differing heat, or differing pressure it sustains, or by the somewhat heterogeneous vapours interspers'd through it. For as the Air is more or less rarified, so does it more or less refract a ray of light (that comes out of a denser medium) from the perpendicular. This you may find true, if you make tryal of this Experiment.

Take a small Gla.s.s-bubble, made in the form of that in the second Figure of the 37. _Scheme_, and by heating the Gla.s.s very hot, and thereby very much rarifying the included Air, or, which is better, by rarifying a small quant.i.ty of water, included in it, into vapours, which will expel the most part, if not all the Air, and then sealing up the small neck of it, and letting it cool, you may find, if you place it in a convenient Instrument, that there will be a manifest difference, as to the refraction.

As if in this second Figure you suppose A to represent a small sight or hole, through which the eye looks upon an object, as C, through the Gla.s.s-bubble B, and the second sight L; all which remain exactly fixt in their several places, the object C being so cized and placed, that it may just seem to touch the upper and under edge of the hole L: and so all of it be seen through the small Gla.s.s-ball of rarified Air; then by breaking off the small seal'd neck of the Bubble (without at all stirring the sights, object, or gla.s.s) and admitting the external Air, you will find your self unable to see the utmost ends of the object; but the terminating rayes AE and AD (which were before refracted to G and F by the rarified Air) will proceed almost directly to I and H; which alteration of the rayes (seeing there is no other alteration made in the Organ by which the Experiment is tryed, save only the admission, or exclusion of the condens'd Air) must necessarily be caused by the variation of the _medium_ contain'd in the Gla.s.s B; the greatest difficulty in the making of which Experiment, is from the uneven surfaces of the bubble, which will represent an uneven image of the object.

Now, that there is such a difference of the upper and under parts of the Air is clear enough evinc'd from the late improvement of the _Torricellian_ Experiment, which has been tryed at the tops and feet of Mountains; and may be further ill.u.s.trated, and inquired into, by a means, which some whiles since I thought of, and us'd, for the finding by what degrees the Air pa.s.ses from such a degree of Density to such a degree of Rarity. And another, for the finding what pressure was requisite to make it pa.s.s from such a degree of Rarefaction to a determinate Density: Which Experiments, because they may be useful to ill.u.s.trate the present Inquiry, I shall briefly describe.