Opticks (1721, 3rd Edition) — part 15 of 18

§^A 10. Is not Flame a Vapour, Fume or Ex- halation heated red hot, that is, fo hot as to Ihine ? For Bodies do not flame without emit- ting a copious Fume, and this Fume burns in the Flame. The Ign^s Fatutts is a V^apour ilii- ning without heat, and is there not the fame difference between this Vapour and Flame, as, between rotten Wood fliining without heat and burning Coals of Fire t In diflilling hot Spirits, if the Head of the Still be taken off, the Va- pour which afcends out of the Still will take fire at the Flame of a Candle, and turn into Flame, and the Flame will run along the Vapour from the Candle to the Still. Some Bodies heated by- Motion or Fermentation, if the heat grow in- tenfe, fume copioufly, and if the heat be great enough the Fumes will fhine and become Flame. Metals in fufion do not flame for want of a co- pious Fume, except Spelter, which fumes co- pioufly, and thereby flames. All flaming Bo- dies, as Oil, Tallow, Wax, Wood, foflil Coals, Pitch, Sulphur, by flaming walte and vanifli in- to burning Smoke, which Smoke, if the Flame be put out, is very thick and vifible, and fome- times fmells fl:rongIyj but in the Flame lofes its

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fmell by burning, and according to the naturb of the Smoke the Flame is of feveral Colours, as that of Sulphur blue, that of Copper open'd with fublimate green, that of Tallow yellow, that of Camphire white. Smoke pafling through Flame cannot but grow red hot, and red hot Smoke can have no other appearance -than that of Flame. When Gun powder takes fire, it goes away into flaming Smoke. For the Char- coal and Sulphur eafily take fire, and fet fire to the Nitre, and the Spirit of the Nitre being thereby rarified into Vapour, rufhes out with Explolion much after the manner that the Va- pour of Water rufhes out of an iEohpile ; the Sulphur alfo being volatile is converted into Vapour, and augments the Explofion. And the acid Vapour of the Sulphur (namely that which diflils under a Bell into Oil of Sulphur,) entring violently into the fix't Body of the Ni- tre, fets loofe the Spirit of the Nitre, and ex- cites a greater Fermentation, whereby the Heat is farther augmented, and the fix'd Body of the Nitre is alfo rarified into Fume, and the Explo- fion is thereby made more vehement and quick. For if Salt of Tartar be mix'd with Gun- pow- der , and that Mixture be warm'd till it takes fire, the Explofion will be more violent and quick than that of Gun-powder alone ; which cannot proceed from any other caufe than the ac^lion of the Vapour of the Gun-powder upon the Salt of Tartar, whereby that Salt is rarihed. The Explofion of Gun powder arifes therefore from the violent a6iion whereby all the Mixture being quickly and vehemently heated, is rarified

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and converted into Fume and Vapour : which Vapour, by the violence of that adion , be- coming fo hot as to fliine, appears in the form of Flame.

^. II. Do not great B.odies conferve their lieat the longeit, their parts heating one ano- ther, and may not great denfe and fix'd Bo- dies, when heated beyond a certain degree, e- mit Light fo copioufly, as by the Emiffion and Re-adtion of its Light, and the Reflexions and Refradions of its Ray w^ithin its Pores to grow flill hotter, till it comes to a certain period of heat, fuch as is that of the Sun ? And are not the Sun and tix'd Stars great Earths vehemently hot, whofe heat is conferved by the greatnefs of the Bodies, and the mutual Adion and Re- aftion between them, and the Light which they emit, and whofe parts are kept from fuming a- way, not only by their fixity, but ah'b By the vafl weight and denfity of the Atmofpheres in- cumbent upon them , and very itrongly com- prefling them, and condenfmg the Vapours and Exhalations which arife from them ? For if Water be made warm in any pellucid VelTel emptied of Air, that Water in the Vacimm will bubble and boil as vehemently as it would in the open Air in a Velfei fet upon the Fire till it conceives a much greater heat. For the weight of the incumbent Atmofphere keeps down the Vapours, and hinders the Water from boiling, until it grow much hotter than is re- quifite to make it boil m 'vacuo. Alfo a mix- ture of Tin and Lead being put upon a red hot Iron in vacm ernics a Fume and Flame, but the

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fame Mixture in the open Air, by reafon of the incumbent Atmofphere, does not fo much as e- mit any Fume which can be perceived by Sight. In hke manner the great weight of the Atmo- phere which lies upon the Globe of the Sun may hinder Bodies there from rifing up and going away from the Sun in the form of Va- pours and Fumes , unlefs by means of a far greater heat than that which on the Surface of our Earth would very eafily turn them into Va- pours and Fumes. And the fame great weight may condenfe thofe Vapours and Exhalations as foon as they ihall at any time begin to afcend from the Sun, and make them prefently fall back again into him, and by that adion increafe his Heat much after the manner that in our Earth the Air increafes the Heat of a culinary Fire. And the fame weight may hinder the Globe of the Sun from being diminifli'd, unlefs by the Emillion of Light, and a very fmall quan- tity of Vapours and Exhalations.

§lu. li. Do not the Rays of Light in falling upon the bottom of the Eye excite Vibrations in the Tunica Retina? Which Vibrations, be- ing propagated along the folid Fibres of the op- tick Nerves into the Brain, caufe the Senfe of feeing. For becaufe denfe Bodies conferve their Heat a long time, and the denfefc Bodies con- ferve their Heat the longeil, the Vibrations of their parts are of a lailing nature, and there- fore may be propagated along folid Fibres of uniform denfe Matter to a great diliance, for conveying into the Brain the imprefiions made upon all the Organs of Senfe. For that ?\1orion

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V^/hlch can continue long in one* and the fame part of a Body, can be propagated a long way from one part to another, fuppofmg the Body homogeneal, fo that the Motion may not be re- fleded, refrafted, interrupted or diforder'd by any unevennefs of the Body.

^ 13 Do not feveral forts of Rays make Vibrations of feveral bigneffes, which according to their bigneffes excite Senfations of feveral Colours much after the manner that the Vibra- tions of the Air, according to their feveral big- neffes excite Senfations of feveral founds ? And particularly do not the mod refrangible Rays excite the Ihortelt Vibrations for making a Sen- fation of deep violet, the leaft refrangible the largelt for making a Senfation of deep red, and the feveral intermediate forts of Rays, Vibra- tions of feveral intermediate bigneffes to make Senfations of the feveral intermediate Colours ?

^. 14. May not the harmony and difcord of Colours arife from the proportions of the Vibra- tions propagated through the Fibres of the op- tick Nerves into the Brain, as the harmony and difcord of Sounds arife from the proportions of the Vibrations of -the Air? For fome Colours, if they be view'd together, are agreeable to one another, as thofe of Gold and Indigo, and others difagree.

^u.i$. Are not the Species ofObjefts feen with both Eyes united where the optick' Nerves meet before they come into the Brain, the Fi- bres on the right fide of both Nerves uniting there, and after union going thence into the Brain in the Nerve which is on the right fide of

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the Head, and the Fibres on the left fide of both Nerves uniting in the fame place , and af- ter union going into the Brain in the Nerve which is on the left fide of the Head, and thefe two Nerves meeting in the Brain in fuch a man- ner that their Fibres make but one entire Spe- cies or Pi(5lure, half of which on the right fide of the Senforium comes from the right fide of. both Eyes through the right fide of both op- tick Nerves to the place where the Nerves meet, and from thence on the right fide of the Head into the Brain, and the other half on the left fide of the Senforium comes in like manner from the left fide of both Eyes. For the op- tick Nerves of fuch Animals as look the fame way with both Eyes (as of Men, Dogs, Sheep, Oxen, &C.J meet before they come into the Brain, but the optick Nerves of fuch Animals as do not look the fame way with both Eyes (as of Filhes and of the Chameleon) do not meet, if I am rightly inform'd.

^. 1 6. When a Man in the dark prefixes either corner of his Eye with his Finger, and turns his Eye away from his Finger, he will fee a Circle of Colours like thofe in the Feather of a Pea- cock's Tail. If the Eye and the Finger remain quiet thefe Colours vanifh in a fecond Minute of Time, but if the Finger be moved with a qua- vering Motion they appear again. Do not thefe Colours arife from fuch Motions excited in the bottom of the Eye by the Preifure and Motion of the Finger, as at other times are excited there by Light for caufing Vifion r And do not the Mptions once excited continue about a Se-

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cond of Time before they ceafe ? And when a Man by a llroke upon his Eye fees a flalh of Light, are not the Uke Motions excited in the Retina by the Itroke ? And when a Coal of Fire moved nimbly in the circumference of a Cir-» cle, makes the whole circumference appear like a Circle of Fire, Is it not becaufe the Motions excited in the bottom of the Eye by the Rays of Light are of a lading nature, and continue till the Coal of Fire in going round returns to its former place ? And confidering the laiting- nefs of the Motions excited, in the bottom of the Eye by Light, are they pot of a vibrating nature ?

^/. 17. If a Stone be thrown into flagnating Water , the Waves excited thereby continue fome time to arife in the place where the Stone fell into the Water, and are propagated from thence in concentrick Circles upon the Surface of the Water to great diftances. And the Vi- brations or Tremors excited in the Air by per- cullion, continue a Httle time to move from the place of percuflion in concentrick Spheres to great diilances. And in like manner, when a Ray of Light falls upon the Surface of any pel- lucid Body, and is there refraded or refleded : may not Waves of Vibrations, or Tremors, be thereby excited in the refrading or refleding Medium at the point of Incidence, and continue to arife there, and to be propagated from thence as long as they continue to arife and be propagated, when they are excited in the bottom of the Eye, by the Preiiure or Motion of the Finger, or by the Light which comes from the Coal of Fire in the

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Experiments above menrion'd ? And are not thefe Vibrations propagated from the point of Inci- dence to great dillances? And do they not o- vertake the Rays of Light, and by overtaking them fucceflively, do they not put them into the Fits of eafy Reflexion and eafyTranfmiffion defcrib'd above ? For if the Rays endeavour to recede from the denfeli part of the Vibration, they may be alternately accelerated and retard- ed by the Vibrations overtaking them.

^. i8. If in two large tall cylindrical Vef- fels of Glafs inverted, two little Thermometers- be fufpended fo as not to touch the VelFels, and the Air be drawn out of one of thefe VefTels, and thefe Veflels thus prepared be carried out of a cold place into a warm 6ne ; the Thermo- meter in vacuo will grow warm as much, and almoft as foon as the Thermometer which is not*/;/ vacuo. And when the VeiTels are carri- ed back into the cold place , the Thermometer in vacuo will grow cold almoft as foon as the other Thermometer. Is not the Heat of the warm Room convey'd through the Vacuum by the Vibrations of a much fubtiler Medium than I Air, which after the Air was drawn out remain^ I ed in the Vacuum ? And is not this Medium the fame with that Medium by which Light i? re- fracted and refleded, and by whofe Vibrations Light communicates Heat to Bodies, and is put into Fits of eafy Reflexion and eafyTranf- miflion? And do 'not the Vibrations of this Me- dium in hot Bodies contribute to the intenfeneis and duration of their Heat.^ And do not hot Bodies communicate their Heat to contiguous

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£old ones, by the Vibrations of this Medium propagated from them into the cold ones? And is not this Medium exceedingly more rare and Inbtile than the Air, and exceedingly more ela- Hick and aflive? And doth it not readily per- vade all Bodies ? And is it not (by its elaitick force) expanded through all the Heavens ?

§n. 19. Doth not the Refradion of Light proceed from the different denfity of this ALiht- real Medium in different places, the Light re- ceding always from the denfer parts of the Me- dium ? And is not the denfity thereof greater in free and open Spaces void of Air and other groiler Bodies, than within the Pores of Wa- ter, Glafs, Cryflal, Gems, and other cornpadi: Bodies? For when Light pafTes through Glafs or Cryltal, and falling very obliquely upon the farther Surface thereof is totally retleded, the total Reflexion ought to proceed rather from the denfity and vigour of the Medium without and beyond the Glafs, than from the rarity and weaknefs thereof

^«. 20. Doth not this i^thereal Medium in pading out of Water, Glafs, Cryllal, and other compaft and denfe Bodies into empty Spaces, grow denfer and denfer by degrees, and by that means refrad the Rays of Light not in a point, but by bending them gradually in curve Lines? And doth not the gradual condenfa- tion of this Medium extend to fome diltance from the Bodies, and thereby caufe the Infle- xions of the Rays of Light,* which pafs by the edges of denfe Bodies, at fome diltance from the Bodies.

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^u. II. Is not this Medium much rarer with- in the denfe Bodies of" the Sun, Stars, Planets and Comets, than in the empty celellial Spaces between them? And in pailing from them to great diftances, doth it not grow denfer and denfer perpetually, and thereby caufe the gra- vity of thofe great Bodies towards one another, and of their parts towards the Bodies ; every Body endeavouring to go from the denfer parts of^he Medium towards the rarer ? For if this Medium be rarer within the Sun's Body than at its Surface, and rarer there than at the hun- dredth part of an Inch from its Body, and ra- r-er there than at the fiftieth part of an Inch from its Body, and rarer there than at the Orb of Saturn ; I fee no reafon why the Increafe of denfity fliould flop any where, and not rather be continued through all di fiances from the Sun to Saturn^ and beyond. And though this In- creafe of denfity may at great dilhmces be ex- ceeding flow, yet if the elaflick force of this Medium be exceeding great, it may fuiiice to impel Bodies from the denfer parts of the Me- dium towards the rarer, with all that power which we call Gravity. And that the elaflick force of this Medium is exceeding great, may be gathered from the fwiftnefs of its Vibrations. Sounds move about 1140 Englifl? Feet in a fe- cond Minute of Time, and in feven or eight Minutes of Time they move about one hundred En^ltjh Miles. Light moves from the Sun to us in about feven or eight Minutes of Time, which dillance is about 70000000 EngliJ]:> Miles, fuppofing the horizontal Parallax of the Sun to

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be about i-l'^ . And the Vibrations orPulfes of this Medium, that they may cauie the alternate Fits of eafy Tranfmiflion and eafy Reflexion, mult be fwifter than Light, and by confequence above 700000 times fwifter than Sounds. And therefore the elaftick force of this Medium, in proportion to its denlity, mult be above 700000 X 700000 (that is, above 490000000000) times greater than the elaftick force of the Air is in proportion to its denfity. . For the Velocities of the Pulles of elauick Mediums are in a fub- dupHcate Ratio of the Elalliciti^s and the Rari- ties of the Mediums taken together.

As Attraction is ftronger in fmall Magnets than in great ones in proportion to their bulk, and Gravity is greater in the Surfaces of fmall Planets than in thofe of great ones in propor- tion to their bulk, and fmall Bodies are agita- ted much more by eledtric attraction than great ones ; fo the fmallnefs of the Rays of Light may contribute very much to the power of the Agent by which they are refrafted. And fo if any one lliould fuppofe that Mther (like our Air) may contain Particles w^hich endeavour to recede from one another (for I do not know what this Mtber is) and that its Particles are exceedingly fmaller than thofe of Air, or even than thole of Light : The exceeding fmallnefs of its Particles may contribute to the greatnefs of the force by which thofe Particles may re- cede from one another, and thereby make that Medium exceedingly more rare and elaltick than Air, and by confequence exceedingly lefs able to rcfiit the motions of Projectiles, and

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exceedingly more able to prefs upon grofs Bo- dies, by endeavouring to expand it felf.

^i. iz. May not Planets and Comets, and all grots Bodies, perform their Motions more free- ly, and with lefs refiftance in this i^thereal Me- dium than in any Fluid, which fills all Space ade- quately without leaving any Pores, and by confe- quenceis much denier thanQuick-filver or Gold ? And may not its reil (lance be fo fmall, as to be inconfiderable? Forinltance; If this ^/^Z^^r (for fo I will call it) fhould be fuppofed 700000 times more elallick than our Air, and above 700000 times more rare ; its refiitance would be above 600000000 times lefs than that of Wa- ter. And fo fmall a refiliance would fcarcc make any fenfible alteration in the Motions of the Planets in ten thoufand Years. If any one would ask how a Medium can be fo rare, let him tell me how the Air, in the upper parts of the Atmofphere, can be above an hundred thou- fand thoufand times rarer than Gold. Let him alfo tell me, how an eledrick Body can by Fri- ction emit an Exhalation fo rare and fubtle, and yet fo potent , as by its Emiflion to caufe no fenfible Diminution of the weight of the ele- drick Body, and to be expanded through a Sphere, whofe Diameter is above two Feet, and yet to be able to agitate and carry up Leaf Cop- per, or Leaf Gold, at the diltance of above a Foot from the eledrick Body ? And how the Effluvia of a Magnet can be fo rare and fubtiie, as to pafs though a Plate of Glafs without any Refiftance or Diminution of their Force, and yet fo potent as to turn a magnetick Needle be- yond the Glafs ?

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§iL 13. Is not Vifion perform'd chiefly by the Vibrations of this Medium, excited in the bot- tom of the Eye by the Rays of Light, and pro- pagated through the folid, pellucid and uniform Capillamenta of the optick Nerves into the place of Senfation ? And is not Hearing perform'd by the Vibrations either of this or fome other Medium, excited in the auditory Nerves by the Tremors of the Air, and propagated through the folid, pellucid and uniform Capillamenta of thofe Nerves into the place of Senfation ? And fp of the other Senfes.

^^.14. Is not Animal Motion performed by the Vibrations of this Medium, excited in the Brain by the power of the Will, and propaga- ted from thence through the folid, pellucid and uniform Capillamenta of the Nerves into the Mufcles, for contrafting and dilating them ? I fuppofe that the Capillamenta of the Nerves are each of them folid and uniform, that the vibra- ting Motion of the i^thereal Medium may be propagated along them from one end to the o- ther uniformly, and without interruption: For Obllrudions in the Nerves create Paliies. And that they may be fufficiently uniform, I fuppofe them to be pellucid when view'd fingly, tho' the Reflexions in their cylindrical Surfaces may make the whole Nerve (compofed of many Ca- pillamenta) appear opake and white. For opaci- ty arifes from reflefting Surfaces, fuch as may di- flurb and interrupt the Motions of this Medium.

^u.^S- Are there not other original Proper- ties of the Rays of Light, befides thofe alrea- dy defcribed? An inftance of anpther original

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property we have in the Refraftion of Ifland Cryltal, defcribed firft by Erafmus Bartholine^ and afterwards more exadly by Hugenius^ in his Book T>e la Lumiere. This Cryflal is a pel- lucid fiflile Stone, clear as Water or Cryftal of the Rock, and without Colour ; enduring a red Heat without lofing its tranfparency, and in a very flrong Heat calcining without Fufion, Steep'd a Day or two in Water, it lofes its na- tural Polifh. Being rubb'd on Cloth, it attrads pieces-of Straws and other light things, like Am-r bar or Glafs; and with Aquafortis it makes an Ebullition. It feems to be a fort of Talk, and is found in form of an oblique Parallelopiped, with fix parallelogram Sides and eight folid An- gles. The obtufe Angles of the Parallelograms are each of them 1 01 Degrees and 51 Minutes; the acute ones 78 Degrees and 8 Minutes. Two of the folid Angles oppofite to one another, as C and E, are compafled each of them with three of thefe obtufe f„l scUt''""' Angles, and each of the other fix with one obtufe and two acute ones. It cleaves eafily in Planes parallel to any of its Sides, and not in any other Planes. It cleaves with a gloffy polite Surface not perfedly plane, but with fome little unevennefs. It is eafily fcratch'd^ and by reafon of its foftnefs it takes a Polifli very ditficultly. It poliilies better up- on poliQi'd llooking-glafs than upon Metal, and perhaps JDetter upon Pitch, Leather or Parch- ment. Afterw^ards it muit be rubb'd with a little Oil ^r White of an Egg, to fill up^^its Scratches ;' whereby it will become very tranf-

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parent and polite. But for feveral Experiments, it is not neceffary to polifh it. If a -piece of this cryftalline Stone bt laid upon a Book, every Letter of the Book feen through it will appear double, by means of a double Refradion. And if any beam of Light falls either perpendicularly, or in any oblique Angle upon any Surface of this Cryftal, it becomes divided into two beams by means of the fame double Refradion. Which beams are of the fame Colour with the incident beam of Light, and feem equal to one another in the quantity of their Light, or very nearly equal. One of thefe Refradions is perform 'd by the ufual Rule of Opticks, the Sine of Inci- dence out of Air into this Cryltal being to the Sine of Refradion, as five to three. The other Refraftion, which may be called the unufual Refradtion, is perform'd by the following Rule.

Let A D B C reprefent the refrading Surface

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of the Cryftal, C the biggeft folid Angle at that Surface, GEHF the oppofite Surface, and CK a perpendicular on that Surface. This perpen- dicular makes with the edge of the Cryital C F, an Angle of 19 Degr. 3^ Join KF, and in it take K L, fo that the Angle K C L be 6 Degr. 40'. and the Angle LC F ii Degr. 13'. And if S T reprefent any beam of Light incident at T in any Angle upon the refrading Surface ADBC, let T V be the refrafted beam determin'd by the given Proportion of the Sines 5 to 3, accord- ing to the ufual Rule of Opticks. Draw V X parallel and equal to KL. Draw it the fame way from V in which L Ueth from K; and joining T X, this line T X lliall be the other re- fracted beam carried from T to X, by the un- ufual RefraAion.

If therefore the incident beam ST be per- pendicular to the refrading Surface, the two beams TV and TX, into which it ihall be- come divided, fhall be parallel to the lines CK and C L ; one of thofe beams going through the Cryital perpendicularly, as it ought to do by the ufual Laws of Opticks, and the other T X by an unufual Refraction diverging from the perpendicular, and making with it an An- gle VTX of about 67 Degrees, "as is found by experience. And hence, the Plane VTX, and fuch like Planes which are parallel to the Plane C F K, may be called the Planes of perpendicu- lar Refradion. And the Coail towards which the lines K L and V X are drawn, may be call'd the Coad of an unufual Refradion.

In like manner Cryilal of the Rock has a

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double Refradion: But the differences of the two Refraftions is not fo great and manifeit as in liland Cryftal.

When the beam S T incident on Ifland Cry- flal, is divided into two beams TVandTX and thefe two beams arrive at the farther Sur- face of the Glafs ; the beam T V, which was refraded at the firlt Surface after the ufual man- ner, Ihall be again refraded entirely after the ufual manner at the fecond Surface ; and the beam T X, which was refrafted after the unu- fual manner in the firil Surface, fhall be again refradted entirely after the unufual manner in the fecond Surface ; fo that both thefe beams fliall emerge out of the fecond Surface in lines parallel to the firft incident beam ST.

And if two pieces of Ifland Cryltal be pla-^ ced one after another, in fuch manner that all the Surfaces of the latter be parallel to all the correfponding Surfaces of the former : The Rays which are refraded after the ufual man- ner in the firit Surface of the firft Cryftal fliall be refrafted after the ufual manner in all the following Surfaces ; and the Rays which are re- frafted after the unufual manner in the firft Sur- face, fhall be refraded after the unufual manner in all the following Surfaces. And the fame thing happens, 'though the Surfaces of the Cry- ftals be any ways inclined to one another, pro- vided that their Planes of perpendicular Refra^ ftion be parallel to one another.

And therefore there is an original difference in the Rays of Light, by means of which fome Rays, are in this Experiment conftantly refrad-

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ed after the ufual manner, and others coriflanf- ly after the unufual manner : For if the diffe- rence be not original, but arifes from new Mo- difications imprefs'd on the Rays at their firfl Refradion, it would be alter'd by new Modifi- cations in the three following Refraftions; whereas it fuffers no alteration, but is conftant, and has the fame efFed upon the Rays in all the Refradions. The unufual Refradion is there- fore perform'd by an original property of the Rays. And it remains to be enquired, whether the Rays have not more original Properties than are yet difcover'd.

^i. z6. Have not the Rays of Light feveral fides, endued with feveral original Properties ? For if the Planes of perpendicular Refradion of the fecond Cryftal, be at right Angles with the Planes of perpendicular Refradion- of the firft Crydal, the Rays which are refraded after the ufual manner in palling through the firft Cryftal, will be all of them refraded after the unufual manner in pafling through the fecond Cryftal; and the Rays which are refraded af- ter the unufual manner in palling through the iirft Cryltal, will be all of them refraded after the ufual manner in paifing through the fecond Cryltal. And therefore there are not two forts of Rays differing in their nature from one ano- ther, one of which is conltantly and in all Po- fitions refraded after the ufual manner, and the other conllantly and in all Pofitions after the unufual manner. The difference between the two forts of Rays in the Fxperiment mention'd in the 25th Queition, was only in the Pofitions

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of the Sides of the Rays to the Planes of per- pendicular Refradion. For one and the fame Ray is here refracted fometimes after the ufual, and fometimes after the unufual manner, ac- cording to the Pofition which its Sides have to the Cryftals. If the fides of the Ray are pofi- ted the fame way to both Cryftals, it is refraft- ed after the fame manner in them both : But if that fide of the Ray which looks towards the Coaft of the unufual Refraftion of the firit Cryftal, be 90 Degrees from that fide of the fame Ray which looks towards the Coaft of the unufual Refra6lion ofthefecond Cryftal, (which may be effefted by varying the Pofition of the fecond Cryftal to the firft, and by confequence to the Rays of Light) the Ray Ihall be refrafted after feveral manners in the feveral Cryftals. There is nothing more required to determine whether the Rays of Light which fall upon the fecond Cryftal, ihall be refrafted after the ufual or after the unufual manner, but to turn about this Cryftal, fo that the Coaft of this Cryftal's unufual Refraftion may be on this or on that lide of the Ray. And therefore every Ray may be confider'd as having four Sides or Quarters, two of which oppolite to one another incline the Ray to be refrafted after the unufual man- ner , as often as either of them are turn'd to- wards the Coaft of unufual Refradion ; and the other two, whenever either of them are turn'd towards the Coaft of unufual Refraftion, do not incline it to be otherwife refraded than after the ufual manner. The two firft may there- fore be call'd the Sides of unufual Refraction.

And

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And fince thefe Difpofitions were in the Rays before their Incidence on the fecond, third and fourth Surfaces of the two Cryftals, and fufFer- ed no alteration (fo far as appears) by the Re- fraction of the Rays in their parfage through thofe Surfaces, and the Rays were refrafted by the fame Laws in all the four Surfaces ; it ap- pears that thofe Difpofitions were in the Rays originally, and fuffer'd no alteration by the firft Refradion, and that by means of thofe Difpofi- tions the Rays were refraded at their Incidence on the firil Surface of the firft Cryftal, fome of them after the ufual, and fome of them after the unufual manner , accordingly as their Sides of unufual Refradion were then turn'd towards the Coalt of the unufual Refradion of that Cry- ftal, or fideways from it.

Every Ray of Light has therefore two oppo- fite Sides, originally endued with a Property on which the unufual Refraftion depends, and the other two oppofite Sides not endued with that Property. And it remains to be enquired, whether there are not more Properties of Light by which the Sides of the Rays differ, and are diftinguiili'd from one another.

In explaining the difference of the Sides of the Rays abovemention'd, I have fuppofed that the Rays fall perpendicularly on the firft Cry- ftal. But if they fall obliquely on it, the Suc- cefs is the fame. Thofe Rays which are refrad:- ed after the ufual manner in the firft Cryftal, will be refraded after the unufual manner in the fecond Cryftal, fuppofmg the Planes of per- pendicular Refradion to be at right Angles with

one

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one another, as above ; and on the contrary.

If the Planes of the perpendicular Refradiori of the two Cryltals be neither parallel nor per-' pendicular to one another, but contain an acute Angle : The two beams of Light which emerge out of the firft Cryltal, will be each of them di- vided into two more at their Incidence on the fecond Cryftal. For in this cafe the Rays in each of the two Beams will fome of them have their Sides of unufual Refraftion, and fome of them their other Sides turn'd toward the Coaft of the unufual Refradtion of the fecond Cry- ftal.

^u. 27. Are not all Hypothefes erroneous which have hitherto been invented for explain- ing the Phaenomena of Light, by new Modifica- tions of the Rays ? For thofe Phaenomena de- pend not upon new Modifications, as has been fuppofed, but upon the original and unchange- able Properties of the Rays.

^u. i8. Are not all Hypothefes erroneous, in which Light is fuppofed to confilt in Pref- fion or Motion, propagated though a fluid Me- dium? For in all thefe Hypothefes, the Phaeno- mena of Light have •been hitherto explain'd by fuppofing that they arife from new Modifica* tions of the Rays ; which is an erroneous Sup-* pofition.

If Light confifted only in Preffion propaga- ted without aftual Motion, it would not be a- ble to agitate and heat the Bodies which refraft and refled it. If it confifted in Motion propa- gated to all diftances in an inftant, it would re- quire an infinite force every moment, in every

ihining

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fhining Particle, to generate that Motion. And if it confilted in Preflion or Motion, propaga- ted either in an inllant or in time, it would bend into the Shadow. For Preflion or Motion cannot be propagated in a Fluid in right Lines beyond an Oblbcle which ilops part of the Mo- tion, but will bend and fpread every way into the quiefcent Medium which lies beyond the Obllacle. Gravity tends downwards, but the PrefTure of Water arifmg from Gravity tends every way with equal force, and is propagated as readily, and with as much force, fideways as downwards, and through crooked palfages as through Itrait ones. The Waves on the Surface of flagnating Water, paffing by the fides of a broad Obftacle which Hops part of them, bend afterwards and dilate themfelves gradually into the quiet Water behind the Obllacle. The Waves, Pulfes or Vibrations of the Air, where- in Sounds confitt, bend manifellly, though not fo much as the Waves of Water, For a Bell or a Canon may be heard beyond a Hill which intercepts the fight of the founding Body, and Sounds are propagated as readily through crook- ed Pipes as through ftreight ones. But Light is never known to follow crooked Pallages nor to bend into the Shadow. For the fix'd Stars by the Interpofition of any of the Planets ceafe to be feen. And fo do the Parts of the Sun by the Interpofition of the Moon, Mercury of Venus. The Rays which pafs very near to the edges of any Body, are bent a Uttle by the adion of the Body, as we fliew'd above; but this bending is not towards but from the Shadow^

Z and

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and is performed only in the pafTage of the Ray by the Body, and at a very fmall diftance from it. So foon as the Ray is pall the Body, it goes right on.