Opticks (1721, 3rd Edition) — part 11 of 18

But farther, that this difcontinuity of parts is the principal Caufe of the opacity of Bodies^ will appear by coniidering, that opake Subltan- ces become traniparent by filling their Pores with any Subftanee of equal or almoft equal den- fity with their parts. Thus Paper dipped in Water or Oil, the Oculus Mundi Stone Iteep'd in Water, Linen Cloth oiled or varnifli'd, and and many other Subltances foaked in fuch Li- quors as will intimately pervade their little Pores, become by that means more tranrparent than otherwife ; fo, on the contrary, the mofl tranfparent Subftances may by evacuating their* Pores, or feparating their parts, be render'd fufficiently opake, or Salts or Wet Paper, or th& Oculus Mundi Stone by being dried, Horn by being fcraped, Glafs by being reduced to Pow- der, or otherwife flawed. Turpentine by being Itirred about with Water till they mix imper- fectly, and Water by being form'd into many fmall Bubbles, either alone in the form of Froth, or by fhaking it together with Oil of Turpen- tine, or Oil Olive, or with fome other conve- nient Liquor, with which it will not perfedly incorporate. And to the increafe of the opaci- ty of thefe Bodies it conduces fomething, that by the isd Obfervation the Reflexions of very thin tranfparent Subftances are confiderably flronger than thofe made by the fame Subltances of a greater thicknefs.

Prop.

[ 225 3

Prop. IV.

The farts of Bodies and their Interftices muft not be lefs than of fome definite bignefs , to render them opake arid colour' d.

FOR the opakeft Bodies, if their parts be fubtily divided, (as Metals by being diirol- ved in acid Menftruums, '^c.) become perfeft- ly tranfparent. And you may alfo remember, that in the eighth Obfervation there was no fenfible reflexion at the Superficies of the Ob- jeft glafTes where they were very near one an- other, though they did not abfolutely touch. And in the 17th Obfervation the Reflexion of the Water-bubble where it became thinnell was almoll infenfible, fo as to caufe very black Spots to appear on the top of the Bubble by the want of reflected Light.

On thefe grounds I perceive it is that Water, Salt, Glafs, Stones, and fuch like Subflances, are tranfparent. For^ upon divers Confiderations, they feem to be as full of Pores or Interilices between their parts as other Bodies arej but yet their Parts and Interllices to be too fmall to caufe Reflexions in their common Surfaces,

Q Prop.

[ 226 ]

P R OB. V.

The tranjparent parts of Bodies according to their fever a I Jizes reflect Rays of one Colour, and tranfmit thofe of another , on the fame grounds that thin Tlates or Bubbles do refle^f or tranfmit thofe Rays. And this I take to be the ground of all their Colours.

FOR if a thinn'd or plated Body, which being of an even thicknefs^ appears all o- ver of one uniform Colour, fliould be Hit into Threds, or broken into Fragments, of the fame thicknefs with the Plate ; I fee no reafon why every Thred or Fragment fliould not keep its Colour, and by confequence why a heap of thofe Threds or Fragments Ihould not confli- tute a Mais or Powder of the fame Colour, which the Plate exhibited before it was broken. And the parts of all natural Bodies being like fo many Fragments of a Plate, mult on the fame grounds exhibit the fame Colours.

Now that they do fo, will appear by the affi- nity of their Properties. The hnely colour'd Feathers of fome Birds, and particularly thofe of Peacocks Tails, do in the very fame part of the Feather appear of feveral Colours in feveral Pofitions of the Eye, after the very fame man- ner that thin Plates Vv^ere found to do in the 7th and 19th Oblervations, and therefore their Colours arife from the thinnefs of the tranfpa- rent parts of the Feathers ; that is, from the flendernefs of the very tine Hairs, or Capilla- menta, which grow out of the fides of the

grofTer

[ 227 I

grofTer lateral Branches or Fibres of thofe Fea- thers. And to the fame purpofe it is, that the Webs of fome Spiders by being fpun very fine have appeared colour'd, as fome have obferv'd, and that the colour'd Fibres of fome Sill^s by varying the Pofition of the Eye do vary their Colour. Alfo the Colours of Siliis, Cloths, and other Sublfances, which Water or Oil cari intimately penetrate, become more fi\int and obfcure by being immerged in thofe Liquors, and recover their Vigour again by being dried, much after the manner declared of thin Bodies in the loth and iifl Obfervarions. Leaf Gold, fome forts of painted Glafs, the Infufion of Lignum Nephriticum^ and fome other Subihn- ces refleft one Colour, and tranfmit another, like thin Bodies in the 9th and loth Obferva- tions. And fome of thofe colour'd Powders which Painters ufe, may have their Colours a little changed, by being very elaborately and finely ground. • Where I fee not what can be juflly pretended for thofe changes, befides the breaking of their parts into lefs parts by thaif contrition after the fame manner that the Co- lour of a thin Plate is changed by varying its thicknefs. For which reafon alfo it is that the colour'd Flowers of Plants and Vegetables by being bruifed ufually become more tranfparent than before, or at leaft in fome degree or o^ ther change their Colours. Nor is it much kfs to my purpofe, that by mixing divers Liquors very odd and remarkable Produdions and Changes of Colours may be effeded, of which no caufe can be more obvious and rational than

Q ^ that

[ 228 ]

the faline Corpufcles of one Liquor do vari- oufly ad upon or unite with the tinging Cor- pufcles of another, fo as to malve them fwell, or flirink (whereby not only their bulk but their denfity alfo may be changed ) or to divide them into fmaller Corpufcles, (whereby a colour'd Liquor may become tranfparent) or to make many of them allbciate into one clufter, where- . by two tranfparent Liquors may compofe a co- lour'd one. For we fee how apt thofe faline Menflruums are to penetrate and diflblve Sub- ftances to which they are applied, and fome of them to percipitate what others dilTolve. In like manner, if we confider the various Phse- nomena of the Atmofphere, we may obferve, that when Vapours are firlt raifed, they hinder not the tranfparency of the Air, being divided into parts too fmall to caufe any Reflexion in their Superficies. But when in order to com- pofe Drops of Rain they begin to coalefce and conftitute Globules of all intermediate fizes, thofe Globules when they become of a conve- nient lize to refleft fome Colours and tranfmit others, may conititute Clouds of various Co- lours according to their fizes. And I fee not what can be rationally conceived in fo tranfpa- rent a Subftance as Water for the production of thefe Colours, befides the various iizes of its fluid and globular Parcels.

Prop.

[ 229 ]

Prop. VI.

The farts, of Bodies on which their Colours de- fend^ are denfer than the Medium^ which fer^ vades their Interjiices.

THIS will appear' by confidering, that the Colour of a Body depends not only on the Rays which are incident perpendicularly on its parts, but on thofe alfo which are inci- dent at all other Angles. And that according to the 7th Obfervation, a very little variation of obliquity will change the reflefted Colour where the thin Body or fmall Particle is rarer than the ambient Mediurn, infomuch that fuch a fmall Particle will at diverfly oblique Inci- dences refled of all forts of Colours, in lb great a variety that the Colour refulting from them all, confufedly reflected from a heap of fuch Parti- cles, mult rather be a white or grey than any other Colour, or at befl it muft be but a very imperfeft and dirty Colour. Whereas if the thin Body or fmall Particle be much denfer than the ambient Medium, the Colours according to the 19th Obfervation are fo little changed by the variation of obliquity, that the Rays which are refleded lealt obliquely may predominate over the red fo much as to caufe a heap of fuch Particles to appear very intenfely of their Co- lour.

It conduces alfo fomething to the confirma- tion of this Propofition, that, according to the aid Obfervation, the Colours exhibited by the denfer thin Body within the rarer, are more

Q 3 * brisk

[ 230 ]

brisk than thofe exhibited by the rarer within the denfer.

Prop. VII.

The hignefs of the component parts of natural . Bodies may be conjeBured by their Colours.

FOR fince the parts of thefe Bodies by Trop. S' do moll probably exhibit the lame Colours with a Plate of equal thicknefs, provided they have the fame refradive denfity ; and fmce their parts feem for the moft part to have much the fame denfity with Water or Glafs, as by many circumltances is obvious to colled ; to determine the fizes of thofe parts you need only have recourfe to the precedent Tables, in which the thicknefs of Water or Glafs exhibiting any Colour is expreffed. Thus if it be defired to know the diameter of a Cor- pufcle, which being of equal denfity with Glafs ihall refled green of the third Order ; the Num- ber i6i lliews it to be — ^-^ parts of an Inch.

The greateft difficulty is here to know of what order the Colour of any Body is. And for this end we mufl have recourfe to the 4th and 1 8th Obfervations, from whence may be colled- ed thefe particulars.

Scarlets, and other reds, oranges and yel- lows, if they be pure and intenfe are moft pro- bably of the fecond order. Thofe of the firfl and third order alfo may be pretty good, only the yellow of the firit order is faint, and the

orange

t 23r }

orange and red of the third order have a great mixture of violet and blue.

Tiiere may be good greens of the fourth or- der, but the purelt are of the third. And of this order the green of all Vegetables feem to . be, partly by reafon of the intenfenefs of their Colours, and «partly becaufe when they wither fome of them turn to a greenilh yellow, and others to a more perfed yellow or orange, or perhaps to red, pafling firll through all the a- forefaid intermediate Colours. Which Changes feem to be effeded by the exhaling of the moi- fture which may leave the tinging Corpufcles more denfe, and fomething augmented by the accretion of the oily and earrliy part of that moifture. Now the green without doubt is of the fame order with thofe Colours into which it changeth , becaufe the Changes are gradual, and thole Colours, though ufually not very full, yet are often too full and lively to be of the fourth order.

Blues and purples may be either of the fe- cond or third order, but the beil are of the third. Thus the Colour of violets feems to be of that order, becaufe their Syrup by acid Li- quors turns red, and by urinous and alcalizate turns green. For fmce it is of the nature of Acids to diiTolve or attenuate, and of Alcalies to precipitate or incralPate , if the purple Co- lour of the Syrup was of the fecond order, an acid Liquor by attenuating its tinging Cor- pufcles would change it to a red of the tiril or- der, and an Alcali by incralTating them would change it to a green of the fecond order ;

Q 4 which

C 232 ]

which red and green, efpecially the green, feem too imperfed: to be the Colours produced by thefe Changes. But if the faid purple be fup- pofed of the third order, its Change to red of the fecond, and green of the third, may with- out any inconvenience be allow' d.

If there be found any Body of a deeper and lefs reddifli purple than that of the violets, its Colour moil probably is of the fecond order. But yet there being no Body commonly known whole Colour is conftantly more deep than theirs, I have made ufe of their name to denote the deepell and leaft reddifh purples, fuch as manifeltly tranfcend their Colour in purity. .

The blue of the firll order, though very faint and little, may poffibly be the Colour of fome Subftances; and particularly the azure Colour of the Skies feems to be of this order. For all Vapours when they begin to condenfe and co- alesce into fmall parcels, become firil of that bignefs whereby fuch an Azure mull be reliev- ed before they can conltitute Clouds of other Colours. And fo this being the firlt Colour which Vapours begin to relied:, it ought to be the Colour of the finell and moll tranfparent Skies in which Vapours are not arrived to that grolTnefs requifite to relied other Colours, as we find it is by experience.

IVhitenefs^ if moll intenfe and luminous, is that of the firll order, if lefs Itrong and lumi- nous a mixture of the Colours of feveral or- ders. Of this lall kind is the whitenefs of Froth, Paper, Linen, and moll white Sub- flances ; of the former I reckon that of white

Metals

C 233 ]

Metals to be. For whilft the denfefl of Me- tals, Gold, if foliated, is tranfparent, and all Me- tals become tranfparent if dillolved in Men- Ih'uums or vitrified, the opacity of white Me- tals arifeth not from their denfity alone. They being lefs denfe than Gold would be more tranf- parent than it, did not fome other Caufe con- cur with their denfity to make them opake. * And this caufe I take to be fuch a bignefs of their Particles as fits them to refledl the white of the firit order. For if they be of other thick- neiles they may refle6l other Colours, as is ma- nifed by the Colours which appear upon hot Steel in tempering it, and fometimes upon the Surface of melted Metals in the Skin or Scoria which arifes upon them in their cooling. And as the white of the firll order is the llrongelt w^hich can be made by -Plates of tranfparent Subftances, fo it ought to be Ib'onger in the denfer Subftances of Metals than in the rarer of Air, Water and Glafs. Nor do I fee but that metaUic Sublbnces of fuch a thicknefs as may fit them to refled the white of the firlt or- der, may, by reafon of their great denfity (ac- cording to the tenour of the firll of thefe Pro- pofitions } refle6l all the Light incident upon them, and fo be as opake and fplendent as it's pollible for any Body to be. Gold, or Copper mix'd with lefs than half their weight of Silver, or Tin, or Regulus of Antimony, in fufion, or amalgamed with a very little Mercury, become white; which Ihews both that the Particles of vhite Metals have much more Superficies, and fpare fmaller, than thofe of Gold and Copper,

and

C 234 ]

and alfo that they are fo opake as not to fufFcr the Particles of Gold or Copper to fhine through them. Now it is fcarce to be doubted, but that the Colours of Gold and Copper are of the fecond or third order, and therefore the Particles of white Metals cannot be much big- ger than is requifite to make them refled the white of the tirll order. The volatility of Mer- cury argues that they are not much bigger, nor may they be much lefs, left they lofe their opacity, and become either tranfparent as they do when artenuated by vitrification, or by So- lution in Menllruums, or black as they do when ground fmaller, by rubbing Silver, or Tin, or Lead, upon other Subltances to draw black Lines. The firll and only Colour which white Metals take by grinding their Particles fmaller, is black, and therefore their white ought to be that which borders upon the black Spot in the center of the Rings of Colours, that is, the white of the firit order. But if you would hence gather the bignefs of metallic Particles, you mull: allow for their deniity. For were Mercury tranfparent, its denfity is fuch that the Sine of Incidence upon it (by my compu- tation) would be to the Sine of its Refradion, as 71 to -20, or 7 to 2. And therefore the thicknefs of its Particles, that they may exhibit, the fame Colours with thofe of Bubbles of Wa- ter, ought to be lefs than the thicknefs of the Skin of thofe Bubbles in the proportion of ^ to 7. Whence it's poffible that the Particles of Mercury may be as little as the Particles c/

fa-n'e

[ 235 1

fome tranfparent and volatile Fluids, and yet re^ fledl the white of the firft order.

Laftly, for the produdion of i^lack, the Cor- pufcles muft be lefs than any of thofe which ex- hibit Colours. For at all greater lizes there is too much Light refleded to conilitute this Co- lour. But if they be fuppofed a little lefs than is requifite to refleft the white and very faint blue of the firft order, they will, according to the 4th, 8th, 17th and i8th Obiervations, retle<^t fo very little Light as to appear intenfly black, and yet may perhaps varioufly refrad it to and fro within themfelves fo long, until it happen to be ftifled and lofl, by which means they will appear black in all pofitions of the Eye without any tranfparency. And from hence may be under- llood why Fire, and the more fubtle dillolver Pu- trefadion, by dividing the Partitles of Subftan- ces, turn them to black, why fmall quantities of black Subltances impart their Colour very freely and intenfly to other Subltances to which they are applied; the minute Particles ofthefe, by reafon of their very great number, eafily o- verfpreading the grofs Particles of others ; why Gials ground very elaborately with Sand on a Copper Plate, 'till it be well polilh'd, makes the Sand, together with what is worn off from the Glafs and Copper, become very black; why black Subftances do foonelt of all others become hot in the Sun's Light and burn, (which EfFed may proceed partly from the multitude of Refraftions in a little room, and partly from the eafy Commotion of fo very fmall Cor- pufcles ;) and why blacks are ufually a little in-

. clined

[ 23^ ]

clined to a bluilli Colour. For that they are fo may be feen by illuminating white Paper by Light reflefted from black Subltances. For the Paper will ufually appear of a bluifh white ; .and the reafon is, that black borders on the obfcure blue of the firfl order defcribed in the i8th Ob- fervation, and therefore refleds more Rays of that Colour than of any other.

In thefe Defcriptions I have been the more particular, becaufe it is not impoflible but that Microfcopes may at length be improved to the difcovery of the Particles of Bodies on which their Colours depend, if they are not already in fome meafure arrived to that degree of per- fedion. For if thofe Initruments are or can be fo far improved as with fufficient dilHndnefs to reprefent Objeds five or fix hundred times bigger than at a Foot dittance they appear to our naked Eyes, I fliould hope that we might be able to difcover fome of the greateft of thofe • Corpufcles. And by one that would magnify three or four thoufand times perhaps they might all be difcover'd , but thofe which produce blacknefs. In the mean while I fee nothing ma- terial iiT this Difcourfe that may rationally be doubted of, excepting this Pofition. That tranf- parent Corpufcles of the fame thicknefs and denfity with a Plate, do exhibit the fame Co- lour. And this I w^ould have underilood not without fome Latitude, as well becaufe thole Corpufcles may be of irregular Figures, and many Rays muit be obliquely incident on them, and lb have a fliorter way through them than the length of their Diameters, as becaufe the

Itraitnefs

[ 237 ]

ftraitnefs of the Medium pent in on all fides within fuch Corpufcles may a little alter its Mo- tions or other qualities on which' the Reflexion depends. But yet I cannot much fufpeft the kit, becaufe I have obferved of fome fmali Plates of Mufcovy Glafs which were of an even thicknefs, that through a Microfcope they have appeared of the»fame Colour at their edges and corners where the included Medium was ter- minated, which they appeared of in other pla- .ces. However it will add much to our Satif- fadtion, if thofe Corpufcles can be difcover'd with Microfcopes ; which if we fhall at length attain to, I fear it will be the utmoft improve- ment of this Senfe. For it feems impoffible to fee the more fecret and noble Works of Na- ture within the Corpufcles by reafon of their tranfparency. ^c] r;r.

Prop. VIII.

The Caufe of Reflexion is not the impinging of Light on the fblid or impervious parts of Bo- dies^ as is commonly believed.

THIS will appear by the following Confi- derations. Firft, That in the pa^lFage of Light out of Glafs into Air there is a Reflexion as itrong as in its paflage out of Air into Glafs, or rather a little flronger, and by many degrees ftronger than in its paflage out of Glafs into Wa- ter. And it feems not probable that Air ihould have more flrongly reflefting parts than Water or Glafs. But if that iliould poflibly be fuppo- fed, yet it will avail nothing; for the Reflexion is as flrong or itronger when the Air is drawn

awav

C 238 ]

away from the Glafs, (fuppofe in the Air-Pump invented by OttoGuerzc, and improved and made ufefiil by Mr: Boyle J as when it is adjacent to it. Secondly, If Light in its pafTage out of Glafs into Air be incident more obliquely than at an Angle of 40 or 41 Degrees it is wholly refleded, if lefs obliquely it is in great mea- fure tranfmitted. Now it is not to be imagined that Light at one degree of obliquity fhould meet with Pores enough in the Air to tranfmit the greater part of it^ and at another degree of* obliquity lliould meet with nothing but parts to refled it wholly, efpecially coniidering that in its paiTage out of Air into Glafs, how ob* lique foever be its Incidence, it finds Pores e- nough in the Glafs to tranfmit a great part of it. If any Man fuppofe that it is not refleded by the Air, but by the outmofl fuperficial parts of the Glafs , there is itill the fame difficulty : Befides, that fuch a Suppofition is unintelHgi- ble, and will alfo appear to be falfe by applying Water behind fome part of the Glafs inltead of Air. For fo in a convenient obliquity of the Rays, fuppofe of 45- or 46 Degrees, at which they are all refleded where the Air is adjacent to the Glafs, they fhall be in great meafure tranf- mitted where the Water is adjacent to it; which argues, jhat their Reflexion or Tranfmiflion depends on the conititution of the Air and Wa- ter behind the Glafs, and not on the ftriking of the Rays upon the parts of the Glafs. Third- ly, If the Colours made by a Prifm placed at the entrance of a Beam of Light into a darken'd Room be fuccellively call on a fecond Prifm

placed

[ 239 ]

placed at a greater diitance from the former, in fuch -manner that they are all alike incident upon it, the fecond Prifm may be fo incHned to the incident Rays, that thofe which are of a blue Colour (hall be all refleded by it, and yet thofe of a red Colour pretty copiouily tranfmit- ted. Now if the Reflexion be caufcd by the parts of Air or Glafs, I would ask, why at the fame Obliquity of Incidence the blue fhould wholly impinge on thofe parts fo as to be all refleded, and yet the red find Pores enough to be in a grear meafure tranfmitted. Fourth- ly, Where twoGlalfes touch one another, there is no fenlible Reflexion as was declared in the firft Obfervarion ; and yet I fee no reafon why the Rays Ihould not impinge on the parts of Glafs as much when Contiguous to other Glafs as when contiguous to Air.- Fifthly, When the top of a Water-Bubble (in the i7thObfer- vation) by the continual fubfiding and exha- ling of the Water grew very thin, there was fuch a little and almoil infenfible quantity 'of Light refleded from it , that it appeared in- tenfly black ; whereas round about that black Spot, where the Water was thicker, the Refle- xion was fo ftrong as to make the Water feem very white. Nor is it only at the leait thick- nefs of thin Plates or Bubbles, that there is no manifeft Reflexion, but at many other thick- nefles continually greater and greater. For in the 15th Obfervation the Rays of the fame Co- lour were by turns tranfmitted at one thicknefs, and refleded at another thicknefs, for an in- determinate number of Succeflions. And yet

in .4

[ 240 ]

in the Superficies of the thinned Body, where it is of one thicknefs ; there are as many parts for the Rays to impinge on, as where it is of any other thicknefs. Sixthly, If Reflexion were caufed by the parts of refleding Bodies, it would be impoffible for thin Plates or Bub- bles at one and the fame place to refled: the Rays of one Colour and tranfmit thofe of ano- ther, as they do according to the 13th and i5'th Obfervations. For it is not to be imagined that at one place the Rays which for initance exhibit a blue Colour,. fliould have the fortune to dalh upon the parts, and thofe which exhi- bit a red to hit upon the Pores of the Body ; and then at another place, where the Body is either a little thicker, or a httle thinner, that on the contrary the blue fhould hit upon its pores, and the red upon its parts. Laitly, were the Rays of Light refiefted by impinging oil the folid parts of Bodies, their Reflexions from polifli'd Bodies couM not be fo regular as they are. For inpoliihing GLif? with Sand, Putty or Tripoly, it is not to be imagined that thofe Subltances can by grating and fretting the Glafs bring all its leait Particles to an accurate Polifh ; fo that all their Surfaces fliall be truly plain or truly fpherical, and look all the fame way, fo as together to compofe one even Surface. The fmaller the Particles of thofe Sublbnces are, the fmaller will be the Scratches by which they continually fret and wear away the Glafs until it be poliih'd, but be they never fo fmall they can wear away the Glafs no otherwife than by grating and fcratching it, and breaking the

Protu*

[ 241 ]

Protuberances, and therefore polifli it no o- therwife than by bringing its roughnefs to a ve- ry fine Grain, io that the Scratches and Fret- tings of the Surface become too fmall to be vifible. And therefore if Light were reflefted by impinging upon the folid parts of the Glafs, it would be fcatter'd as much by the molt po- lifh'd Glafs as by the rougheft. So then it re- mains a Problem, how Glafs polifli'd by fretting Subftances can refleft Light fo regularly as it does. And this Problem is fcarce otherwife to be folved than by faying, that the Reflexion of a Ray is effeded, not by a Angle point of the refledting Body, but by fome power of the Bo- dy which is evenly diffufed all over its Surface, and by which it ads upon the Ray without im- mediate Contaft. For that the parts of Bodies do ad upon Light at a diltance lliall be ihewn hereafter.

Now if Light be refleded not by impinging on the folid parts of Bodies, but by fome other principle ; it's probable that as many of its Rays as impinge on the folid parts of Bodies are not refleded but ftifled and loft in the Bodies. For otherwife we muft allow two forts of Refle- xions. Should all the Rays be refleded which impinge on the internal parts of clear Water or Cryftal, thofe Subftances would rather have a cloudy Colour than a clear Tranfparency. To make Bodies look black, it's neceflary that ma- ny Rays be Itopp'd, retained- and lolt in them, and it feems not probable that any Rays can be ftopp'd and ftifled in them which do not im- pinge on their parts.

R And

[242 ]

And hence we may underfland that Bodies are much more rare and porous than is com- monly beheved. Water is nineteen times hght- er, and by confequence nineteen times rarer than Gold, and Gold is fo rare as very readily and without the leall: oppofrdon to tranfmit the magnetic Effluvia, and eafily to admit Quick- lilver into its Pores, and to let Water pafs through it. For a concave Sphere of Gold fil- led with Water, and foder'd up, has upon pref- fing the Sphere with great force, let the Water fqueeze through it, and fland all over its out- fide in multitudes of fmall Drops, like Dew, without burfling or cracking the Body of the Gold, as I have been inform'd by an Eye wit- nefs. From all which we may conclude, that Gold has more Pores than folid parts, and by confequence that Water has above forty times more Pores than Parts. And he that fhall find out an Hypothecs, by which Water may be fo rare, and yet not be capable of compreliion by force, may doubt^efs by the fame Hypothefis make Gold and Water, and all other Bodies as much rarer as he pleafes, fo that Light may find a ready paffage through tranfparent Sub- ilances.'

The Magnet ads upon Iron through all denfe Bodies not magnetick nor red hot, without a- ny diminution of its virtue; as for inflance, through Gold, Silver, Lead, Glafs, Water. The gravitating Power of the Sun is tranfmit- ted through the valt Bodies of the Planets with- cut any diminution, fo as to a61 upon all their parts to their very centers with the fame Force

and

[ 24-3 ]

and according to the fame Laws as if the part upon which it ads were not furrounded with the Body of the Planet. The Rays of Light whether they be very fmall Bodies projeded, or only Motion or Force propagated, are mo- ved in right Lines ; and whenever a Ray of Light is by any Obllacle turned out of its redi- linear way, it will never return into the fame redilinear way, unlefs perhaps by very great ac- cident. And yet Light is tranimitted through pellucid folid Bodies in right Lines to very great diilances. How Bodies can have a fuiiicient quantity of Pores for producing thefe Effeds is very difficult to conceive, but perhaps not al- together impoffible. For the Colours of Bodies arife from the Magnitudes of the Particles which refled them, as was explained aboVe. Now if we conceive thefe Particles of Bodies to be fo difpofed amongit themfelves, that the Intervals or empty Spaces between them may be equal in magnitude to them all ; and that thefe Parti- cles may be compofed of other Particles much fmaller, which have as much emjpty Space be- tween ' them as equals all the Magnitudes of thefe fmaller Particles : And that in like man- ner thefe fmaller Particles are again compofed of others much fmaller, all which together are equal to all the Pores or empty Spaces between them ; and fo on perpetually till you come to folid Particles, fucn as have no Pores or empty Spaces within them: And if in any grofs Body there be, for inftance, three fuch degrees of Particles, the leait of which are folid ; this Bo- dy will have fevenjtimes more Pores than folid

R % Parts

C 244 ]

Parts. But if there be four fuch degrees of , Particles, the lealt of which are folid, the Bo- dy will have fifteen times more Pores than folid Parts. If there be five degrees, the Body will have one and thirty times more Pores than folid Parts. If fix degrees, the Body will have fixty and three times more Pores than folid Parts. And fo on perpetually. And there are other ways of conceiving how Bodies may be exceed- ing porous. But what is really their inward Frame is not yet known to us.

Prop. IX.

Bodies refleB and refra£f Light by one and the fame power varioujly exercifed in various Qir- ctimjiances.

^ I ^ H I S appears by feveral Confiderations. \ Firft, Becaufe when Light goes out of Giafs into Air, as obliquely as it can pofTibly do. If its Incidence be made IHll more oblique, it becomes totally refleded. For the power of the Glafs after it has refrafted the Light as ob- liquely as is poflible if the Incidence be. ftill made more oblique, becomes too llrong to let any of its Rays go through, and by confequence caufes total Reflexions. Secondly, Becaufe Light is alternately refledled and tranfmitted by thin Plates of Glafs for many Succeflions ac- cordingly as the thicknefs of the Plate increa- fes in an arithmetical ProgrefTion. For here the thicknefs of the Glafs determines whether that Power by which Glafs ads upon Light ihali caufe it to be rcfleded, or fuffer it to be a tranf-

. [ 245 ]

tranfmitted. And, Thirdly, becaufe thofe Sur- faces of tranfparent Bodies which have the great- efl refrading Power, reflert the greatell quan- tity of Light, as was Ihew'd in the firit Propo- fition.

R o p.

X.

If Light be Jwifter in Bodies than in Vacuo in the proportion of the Sines 'uuhich meafure the Refraction of the Bodies, the Forces of the Bodies to reflet and refraEi Light, are very nearly proportional to the denfities of the fame Bodies, excepting that unBuous and fttl- phureous Bodies refra6i more than others of this fame denfity.

LET AB reprefent the refrading plane Sur- face of any Body, and I C a Ray incident very obliquely upon the Body in C, fo that tlie

Angle ACT may be infinitely little, and let CR be the refrafted Ray. From a given Point B perpendicular to the refrading Surface eredBR meeting with the refrafted Ray C R in R, and if C R reprefent the Motion of the refra6led Ray, and this Motion be diilinguifli'd into two Motions CB and BR, whereof CB is paral-

R 3 kl

[ 246 ]

lei to the refrading Plane, and B R perpendi- cular to it : C B fliall reprelent the Motion of • the incident Ray, and B R the Motion genera- ted-by the Refradion, as Opticians have of late explain'd.