Opticks: or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light

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When I made the foregoing Observations, I designed to repeat most of them with more care and exactness, and to make some new ones for determining the manner how the rays of light are bent in their passage by bodies, for making the fringes of colours with the dark lines between them. But I was then interrupted, and cannot now think of taking these things into further consideration. And since I have not finished this part of my design, I shall conclude with proposing only some queries, in order to a further search to be made by others.

QUERIE 1. Do not bodies act upon light at a distance, and by their action bend its rays; and is not this action (caeteris paribus) strongest at the least distance?
 

QUERIE 2. Do not the rays which differ in refrangibility differ also in flexibility; and are they not by their different inflexions separated from one another, so as after separation to make the colours in the three fringes above described? And after what manner are they inflected to make those fringes?

QUERIE 3. Are not the rays of light, in passing by the edges and sides of bodies, bent several times backwards and forwards, with a motion like that of an eel? And do not the three fringes of coloured light above mentioned arise from three such bendings?

QUERIE 4. Do not the rays of light which fall upon bodies, and are reflected or refracted, begin to bend before they arrive at the bodies; and are they not reflected, refracted, and inflected, by one and the same principle, acting variously in various circumstances?

QUERIE 5. Do not bodies and light act mutually upon one another; that is to say, bodies upon light in emitting, reflecting, refracting and inflecting it, and light upon bodies for heating them, and putting their parts into a vibrating motion wherein heat consists?

QUERIE 6. Do not black bodies conceive heat more easily from light than those of other colours do, by reason that the light falling on them is not reflected outwards, but enters the bodies, and is often reflected and refracted within them, until it be stifled and lost?

QUERIE 7. Is not the strength and vigor of the action between light and sulphureous bodies observed above, one reason why sulphureous bodies take fire more readily, and burn more vehemently than other bodies do?

QUERIE 8. Do not all fixed bodies, when heated beyond a certain degree, emit light and shine; and is not this emission performed by the vibrating motions of their parts? And do not all bodies which abound with terrestrial parts, and especially with sulphureous ones, emit light as often as those parts are sufficiently agitated; whether that agitation be made by heat, or by friction, or percussion, or putrefaction, or by any vital motion, or any other cause? As for instance; sea-water in a raging storm; quick-silver agitated in vacuo; the back of a cat, or neck of a horse, obliquely struck or rubbed in a dark place; wood, flesh and fish while they putrefy; vapours arising from putrefied waters, usually called ignes fatui; stacks of moist hay or corn growing hot by fermentation; glow-worms and the eyes of some animals by vital motions; the vulgar phosphorus agitated by the attrition of any body, or by the acid particles of the air; amber and some diamonds by striking, pressing or rubbing them; scrapings of steel struck off with a flint; iron hammered very nimbly till it become so hot as to kindle sulphur thrown upon it; the axletrees of chariots taking fire by the rapid rotation of the wheels; and some liquors mixed with one another whose particles come together with an impetus, as oil of vitriol distilled from its weight of nitre, and then mixed with twice its weight of oil of anniseeds. So also a globe of glass about 8 or 10 inches in diameter, being put into a frame where it may be swiftly turned round its axis, will in turning shine where it rubs against the palm of one’s hand applied to it. And if at the same time a piece of white paper or white cloth, or the end of one’s finger be held at the distance of about a quarter of an inch or half an inch from that part of the glass where it is most in motion, the electric vapour which is excited by the friction of the glass against the hand will (by dashing against the white paper, cloth or finger) be put into such an agitation as to emit light, and make the white paper, cloth, or finger appear lucid like a glow-worm; and in rushing out of the glass will sometimes push against the finger so as to be felt. And the same things have been found by rubbing a long and large cylinder or glass or amber with a paper held in one’s hand, and continuing the friction till the glass grew warm.

QUERIE 9. Is not fire a body heated so hot as to emit light copiously? For what else is a red hot iron than fire? And what else is a burning coal than red hot wood?

QUERIE 10. Is not flame a vapour, fume or exhalation heated red hot; that is, so hot as to shine? For bodies do not flame without emitting a copious fume, and this fume burns in the flame. The ignis fatuus is a vapour shining without heat, and is there not the same difference between this vapour and flame as between rotten wood shining without heat and burning coals of fire? In distilling hot spirits, if the head of the still be taken off, the vapour which ascends 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 intense, fume copiously, and if the heat be great enough the fumes will shine and become flame. Metals in fusion do not flame for want of a copious fume, except spelter, which fumes copiously, and thereby flames. All flaming bodies, as oil, tallow, wax, wood, fossil coals, pitch, sulphur, by flaming waste and vanish into burning smoke, which smoke, if the flame be put out, is very thick and visible, and sometimes smells strongly, but in the flame loses its smell by burning, and according to the nature of the smoke the flame is of several colours, as that of sulphur blue, that of copper opened with sublimate green, that of tallow yellow, that of camphor white. Smoke passing through flame cannot but grow red-hot, and red-hot smoke can have no other appearance than that of flame. When gunpowder takes fire, it goes away into flaming smoke. For the charcoal and sulphur easily take fire, and set fire to the nitre, and the spirit of the nitre being thereby rarified into vapour, rushes out with explosion much after the manner that the vapour of water rushes out of an aeolipile; the sulphur also being volatile is converted into vapour, and augments the explosion. And the acid vapour of the sulphur (namely, that which distils under a bell into oil of sulphur) entering violently into the fixed body of the nitre, sets loose the spirit of the nitre, and excites a great fermentation whereby the heat is further augmented, and the fixed body of the nitre is also rarified into fume, and the explosion is thereby made more vehement and quick. For if salt of tartar be mixed with gunpowder, and that mixture be warmed till it takes fire, the explosion will be more violent and quick than that of gunpowder alone; which cannot proceed from any other cause than the action of the vapour of the gunpowder upon the salt of tartar, whereby that salt is rarified. The explosion of gunpowder arises, therefore, from the violent action whereby all the mixture, being quickly and vehemently heated, is rarified and converted into fume and vapour: which vapour, by the violence of that action, becoming so hot as to shine, appears in the form of flame.

QUERIE 11. Do not great bodies conserve their heat the longest, their parts heating one another, and may not great dense and fixed bodies, when heated beyond a certain degree, emit light so copiously, as by the emission and reaction of its light, and the reflexions and refractions of its rays within its pores to grow still hotter, till it comes to a certain period of heat, such as is that of the Sun? And are not the Sun and fixed stars great earths vehemently hot, whose heat is conserved by the greatness of the bodies, and the mutual action and reaction between them, and the light which they emit, and whose parts are kept from fuming away, not only by their fixity, but also by the vast weight and density of the atmospheres incumbent upon them; and very strongly compressing them, and condensing the vapours and exhalations which arise from them? For if water be made warm in any pellucid vessel emptied of air, that water in the vacuum will bubble and boil as vehemently as it would in the open air in a vessel set upon the fire till it conceives a much greater heat. For the weight of the incumbent atmosphere keeps down the vapours, and hinders the water from boiling, until it grow much hotter than is requisite to make it boil in vacuo. Also a mixture of tin and lead being put upon a red-hot iron in vacuo emits a fume and flame, but the same mixture in the open air, by reason of the incumbent atmosphere, does not so much as emit any fume which can be perceived by sight. In like manner the great weight of the atmosphere which lies upon the globe of the Sun may hinder bodies there from rising up and going away from the Sun in the form of vapours and fumes, unless by means of a far greater heat than that which on the surface of our Earth would very easily turn them into vapours and fumes. And the same great weight may condense those vapours and exhalations as soon as they shall at any time begin to ascend from the Sun, and make them presently fall back again into him, and by that action increase his heat much after the manner that in our Earth the air increases the heat of a culinary fire. And the same weight may hinder the globe of the Sun from being diminished, unless by the emission of light, and a very small quantity of vapours and exhalations.

QUERIE 12. Do not the rays of light in falling upon the bottom of the eye excite vibrations in the tunica retina? Which vibrations, being propagated along the solid fibres of the optic nerves into the brain, cause the sense of seeing? For because dense bodies conserve their heat a long time, and the densest bodies conserve their heat the longest, the vibrations of their parts are of a lasting nature, and therefore may be propagated along solid fibres of uniform dense matter to a great distance, for conveying into the brain the impressions made upon all the organs of sense. For that motion which can continue long in one and the same part of a body, can be propagated a long way from one part to another, supposing the body homogeneal, so that the motion may not be reflected, refracted, interrupted or disordered by any unevenness of the body.

QUERIE 13. Do not several sorts of rays make vibrations of several bignesses, which according to their bignesses excite sensations of several colours, much after the manner that the vibrations of the air, according to their several bignesses excite sensations of several sounds? And particularly do not the most refrangible rays excite the shortest vibrations for making a sensation of deep violet, the least refrangible the largest for making a sensation of deep red, and the several intermediate sorts of rays, vibrations of several intermediate bignesses to make sensations of the several intermediate colours?

QUERIE 14. May not the harmony and discord of colours arise from the proportions of the vibrations propagated through the fibres of the optic nerves into the brain, as the harmony and discord of sounds arise from the proportions of the vibrations of the air? For some colours, if they be viewed together, are agreeable to one another, as those of gold and indigo, and others disagree.

QUERIE 15. Are not the species of objects seen with both eyes united where the optic nerves meet before they come into the brain, the fibres on the right side of both nerves uniting there, and after union going thence into the brain in the nerve which is on the right side of the head, and the fibres on the left side of both nerves uniting in the same place, and after union going into the brain in the nerve which is on the left side of the head, and these two nerves meeting in the brain in such a manner that their fibres make but one entire species or picture, half of which on the right side of the sensorium comes from the right side of both eyes through the right side of both optic nerves to the place where the nerves meet, and from thence on the right side of the head into the brain, and the other half on the left side of the sensorium comes in like manner from the left side of both eyes? For the optic nerves of such animals as look the same way with both eyes (as of men, dogs, sheep, oxen, &c.) meet before they come into the brain, but the optic nerves of such animals as do not look the same way with both eyes (as of fishes, and of the chameleon) do not meet, if I am rightly informed.

QUERIE 16. When a man in the dark presses either corner of his eye with his finger, and turns his eye away from his finger, he will see a circle of colours like those in the feather of a peacock’s tail. If the eye and the finger remain quiet these colours vanish in a second minute of time, but if the finger be moved with a quavering motion they appear again. Do not these colours arise from such motions excited in the bottom of the eye by the pressure and motion of the finger as at other times are excited there by light for causing vision? And do not the motions once excited continue about a second of time before they cease? And when a man by a stroke upon his eye sees a flash of light, are not the like motions excited in the retina by the stroke? And when a coal of fire, moved nimbly in the circumference of a circle, makes the whole circumference appear like a circle of fire, is it not because the motions excited in the bottom of the eye by the rays of light are of a lasting nature, and continue till the coal of fire in going round returns to its former place? And considering the lastingness of the motions excited in the bottom of the eye by light, are they not of a vibrating nature?

QUERIE 17. If a stone be thrown into stagnating water, the waves excited thereby continue some time to arise in the place where the stone fell into the water, and are propagated from thence in concentric circles upon the surface of the water to great distances. And the vibrations or tremors excited in the air by percussion continue a little time to move from the place of percussion in concentric spheres to great distances. And in like manner, when a ray of light falls upon the surface of any pellucid body, and is there refracted or reflected, may not waves of vibrations, or tremors, be thereby excited in the refracting or reflecting medium at the point of incidence, and continue to arise there, and to be propagated from thence as long as they continue to arise and be propagated, when they are excited in the bottom of the eye by the pressure or motion of the finger or by the light which comes from the coal of fire in the experiments above mentioned? And are not these vibrations propagated from the point of incidence to great distances? And do they not overtake the rays of light, and, by overtaking them successively, do they not put them into the fits of easy reflexion and easy transmission described above? For if the rays endeavour to recede from the densest part of the vibration, they may be alternately accelerated and retarded by the vibrations overtaking them.

QUERIE 18. If in two large tall cylindrical vessels of glass inverted, two little thermometers be suspended so as not to touch the vessels, and the air be drawn out of one of these vessels, and these vessels thus prepared be carried out of a cold place into a warm one, the thermometer in vacuo will grow warm as much, and almost as soon, as the thermometer which is not in vacuo. And when the vessels are carried back into the cold place, the thermometer in vacuo will grow cold almost as soon as the other thermometer. Is not the heat of the warm room conveyed through the vacuum by the vibrations of a much subtiler medium than air, which after the air was drawn out remained in the vacuum? And is not this medium the same with that medium by which light is refracted and reflected, and by whose vibrations light communicates heat to bodies, and is put into fits of easy reflexion and easy transmission? And do not the vibrations of this medium in hot bodies contribute to the intenseness and duration of their heat? And do not hot bodies communicate their heat to contiguous cold ones, by the vibrations of this medium propagated from them into the cold ones? And is not this medium exceedingly more rare and subtile than the air, and exceedingly more elastic and active? And doth it not readily pervade all bodies? And is it not (by its elastic force) expanded through all the heavens?

QUERIE 19. Doth not the refraction of light proceed from the different density of this aethereal medium in different places, the light receding always from the denser parts of the medium? And is not the density thereof greater in free and open spaces void of air and other grosser bodies, than within the pores of water, glass, crystal, gems, and other compact bodies? For when light passes through glass or crystal, and falling very obliquely upon the farther surface thereof is totally reflected, the total reflexion ought to proceed rather from the density and vigour of the medium without and beyond the glass, than from the rarity and weakness thereof.

QUERIE 20. Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies into empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curved lines? And doth not the gradual condensation of this medium extend to some distance from the bodies, and thereby cause the inflexions of the rays of light, which pass by the edges of dense bodies, at some distance from the bodies?

QUERIE 21. Is not this medium much rarer within the dense bodies of the Sun, stars, planets and comets, than in the empty celestial spaces between them? And in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the 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 hundredth part of an inch from its body, and rarer there than at the fiftieth part of an inch from its body, and rarer there than at the orb of Saturn, I see no reason why the increase of density should stop anywhere, and not rather be continued through all distances from the Sun to Saturn, and beyond. And though this increase of density may at great distances be exceeding slow, yet if the elastic force of this medium be exceeding great, it may suffice to impel bodies from the denser parts of the medium towards the rarer, with all that power which we call gravity. And that the elastic force of this medium is exceeding great, may be gathered from the swiftness of its vibrations. Sounds move about 1,140 English feet in a second minute of time, and in seven or eight minutes of time they move about one hundred English miles. Light moves from the Sun to us in about seven or eight minutes of time, which distance is about 7,000,000 English miles, supposing the horizontal parallax of the Sun to be about 12´´. And the vibrations or pulses of this medium, that they may cause the alternate fits of easy transmission and easy reflexion, must be swifter than light, and by consequence above 700,000 times swifter than sounds. And, therefore, the elastic force of this medium, in proportion to its density, must be above 700,000 • 700,000 (that is, above 490,000,000,000) times greater than the elastic force of the air is in proportion to its density. For the velocities of the pulses of elastic mediums are in a subduplicate ratio of the elasticities and the rarities of the mediums taken together.
As attraction is stronger in small magnets than in great ones in proportion to their bulk, and gravity is greater in the surfaces of small planets than in those of great ones in proportion to their bulk, and small bodies are agitated much more by electric attraction than great ones; so the smallness of the rays of light may contribute very much to the power of the agent by which they are refracted. And so if any one should suppose that aether (like our air) may contain particles which endeavour to recede from one another (for I do not know what this aether is) and that its particles are exceedingly smaller than those of air, or even than those of light: the exceeding smallness of its particles may contribute to the greatness of the force by which those particles may recede from one another, and thereby make that medium exceedingly more rare and elastic than air, and by consequence exceedingly less able to resist the motions of projectiles, and exceedingly more able to press upon gross bodies, by endeavouring to expand itself.

QUERIE 22. May not planets and comets, and all gross bodies, perform their motions more freely, and with less resistance in this aethereal medium than in any fluid, which fills all space adequately without leaving any pores, and by consequence is much denser than quick-silver or gold? And may not its resistance be so small, as to be inconsiderable? For instance: if this aether (for so I will call it) should be supposed 700,000 times more elastic than our air, and above 700,000 times more rare, its resistance would be above 600,000,000 times less than that of water. And so small a resistance would scarce make any sensible alteration in the motions of the planets in ten thousand years. If any one would ask how a medium can be so rare, let him tell me how the air, in the upper parts of the atmosphere, can be above a hundred thousand thousand times rarer than gold. Let him also tell me how an electric body can by friction emit an exhalation so rare and subtile, and yet so potent, as by its emission to cause no sensible diminution of the weight of the electric body, and to be expanded through a sphere, whose diameter is above two feet, and yet to be able to agitate and carry up leaf copper, or leaf gold, at the distance of above a foot from the electric body? And how the effluvia of a magnet can be so rare and subtile as to pass through a plate of glass without any resistance or diminution of their force, and yet so potent as to turn a magnetic needle beyond the glass?

QUERIE 23. Is not vision performed chiefly by the vibrations of this medium, excited in the bottom of the eye by the rays of light, and propagated through the solid, pellucid and uniform capillamenta of the optic nerves into the place of sensation? And is not hearing performed by the vibrations either of this or some other medium, excited in the auditory nerves by the tremors of the air, and propagated through the solid, pellucid and uniform capillamenta of those nerves into the place of sensation? And so of the other senses.

QUERIE 24. Is not animal motion performed by the vibrations of this medium, excited in the brain by the power of the will, and propagated from thence through the solid, pellucid and uniform capillamenta of the nerves into the muscles, for contracting and dilating them? I suppose that the capillamenta of the nerves are each of them solid and uniform, that the vibrating motion of the aethereal medium may be propagated along them from one end to the other uniformly, and without interruption, for obstructions in the nerves create palsies. And that they may be sufficiently uniform, I suppose them to be pellucid when viewed singly, tho’ the reflexions in their cylindrical surfaces may make the whole nerve (composed of many capillamenta) appear opaque and white. For opacity arises from reflecting surfaces; such as may disturb and interrupt the motions of this medium.

QUERIE 25. Are there not other original properties of the rays of light, besides those already described? An instance of another original property we have in the refraction of island crystal, described first by Erasmus Bartholinus, and afterwards more exactly by Huygens, in his book De la Lumiëre. This crystal is a pellucid, fissile stone, clear as water or crystal of the rock, and without colour; enduring a red heat without losing its transparency, and in a very strong heat calcining without fusion. Steeped a day or two in water, it loses its natural polish. Being rubbed on cloth, it attracts pieces of straws and other light things, like amber or glass; and with aqua fortis it makes an ebullition. It seems to be a sort of talc, and is found in form of an oblique parallelepiped, with six parallelogram sides and eight solid angles. The obtuse angles of the parallelograms are each of them 101 degrees and 52 minutes; the acute ones 78 degrees and 8 minutes. Two of the solid angles opposite to one another, as C and E, are compassed each of them with three of these obtuse angles, and each of the other six with one obtuse and two acute ones.

It cleaves easily in planes parallel to any of its sides, and not in any other planes. It cleaves with a glossy polite surface not perfectly plane, but with some little unevenness. It is easily scratched, and by reason of its softness it takes a polish very difficultly. It polishes better upon polished looking-glass than upon metal, and perhaps better upon pitch, leather or parchment. Afterwards it must be rubbed with a little oil or white of an egg to fill up its scratches; whereby it will become very transparent and polite. But for several experiments it is not necessary to polish it. If a piece of this crystalline stone be laid upon a book, every letter of the book seen through it will appear double, by means of a double refraction. And if any beam of light falls either perpendicularly, or in any oblique angle upon any surface of this crystal, it becomes divided into two beams by means of the same double refraction. Which beams are of the same colour with the incident beam of light, and seem equal to one another in the quantity of their light, or very nearly equal. One of these refractions is performed by the usual rule of Optics, the sine of incidence out of air into this crystal being to the sine of refraction as five to three. The other refraction, which may be called the unusual refraction, is performed by the following rule:
Let ADBC represent the refracting surface of the crystal, C the biggest solid angle at that surface, GEHF the opposite surface, and CK a perpendicular on that surface. This perpendicular makes with the edge of the crystal CF, an angle of 19 degrees 3´. Join KF, and in it take KL, so that the angle KCL be 6 degrees 40´ and the angle LCF 12 degrees 23´. And if ST represent any beam of light incident at T in any angle upon the refracting surface ADBC, let TV be the refracted beam determined by the given portion of the sines 5 to 3, according to the usual rule of Optics. Draw VX parallel and equal to KL. Draw it the same way from V in which L lieth from K; and joining TX, this line TX shall be the other refracted beam carried from T to X, by the unusual refraction.
If, therefore, the incident beam ST be perpendicular to the refracting surface, the two beams TV and TX, into which it shall become divided, shall be parallel to the lines CK and CL; one of those beams going through the crystal perpendicularly, as it ought to do by the usual laws of Optics, and the other TX by an unusual refraction diverging from the perpendicular, and making with it an angle VTX of about 6 2/3 degrees, as is found by experience. And hence, the plane VTX, and such like planes which are parallel to the plane CFK, may be called the planes of perpendicular refraction. And the coast towards which the lines KL and VX are drawn, may be called the coast of unusual refraction.
In like manner, crystal of the rock has a double refraction; but the difference of the two refractions is not so great and manifest as in island crystal.
When the beam ST incident on island crystal is divided into two beams TV and TX, and these two beams arrive at the farther surface of the glass, the beam TV, which was refracted at the first surface after the usual manner, shall be again refracted entirely after the usual manner at the second surface; and the beam TX, which was refracted after the unusual manner in the first surface, shall be again refracted entirely after the unusual manner in the second surface; so that both these beams shall emerge out of the second surface in lines parallel to the first incident beam ST.
And if two pieces of island crystal be placed one after another, in such manner that all the surfaces of the latter be parallel to all the corresponding surfaces of the former, the rays which are refracted after the usual manner in the first surface of the first crystal, shall be refracted after the usual manner in all the following surfaces; and the rays which are refracted after the unusual manner in the first surface shall be refracted after the unusual manner in all the following surfaces. And the same thing happens, though the surfaces of the crystals be any ways inclined to one another, provided that their planes of perpendicular refraction be parallel to one another.
And, therefore, there is an original difference in the rays of light, by means of which some rays are in this experiment constantly refracted after the usual manner, and others constantly after the unusual manner; for if the difference be not original, but arises from new modifications impressed on the rays at their first refraction, it would be altered by new modifications in the three following refractions; whereas it suffers no alteration, but is constant, and has the same effect upon the rays in all the refractions. The unusual refraction is, therefore, performed by an original property of the rays. And it remains to be enquired whether the rays have not more original properties than are yet discovered.

QUERIE 26. Have not the rays of light several sides, endued with several original properties? For if the planes of perpendicular refraction of the second crystal be at right angles with the planes of perpendicular refraction of the first crystal, the rays which are refracted after the usual manner in passing through the first crystal will be all of them refracted after the unusual manner in passing through the second crystal; and the rays which are refracted after the unusual manner in passing through the first crystal will be all of them refracted after the usual manner in passing through the second crystal. And, therefore, there are not two sorts of rays differing in their nature from one another, one of which is constantly and in all positions refracted after the usual manner, and the other constantly and in all positions after the unusual manner. The difference between the two sorts of rays, in the experiment mentioned in the 25th Question, was only in the positions of the sides of the rays to the planes of perpendicular refraction. For one and the same ray is here refracted, sometimes after the usual, and sometimes after the unusual manner, according to the position which its sides have to the crystals. If the sides of the ray are posited the same way to both crystals, it is refracted after the same manner in them both; but if that side of the ray which looks towards the coast of the unusual refraction of the first crystal be 90 degrees from that side of the same ray which looks toward the coast of the unusual refraction of the second crystal (which may be effected by varying the position of the second crystal to the first, and by consequence to the rays of light), the ray shall be refracted after several manners in the several crystals. There is nothing more required to determine whether the rays of light which fall upon the second crystal shall be refracted after the usual or after the unusual manner, but to turn about this crystal, so that the coast of this crystal’s unusual refraction may be on this or on that side of the ray. And, therefore, every ray may be considered as having four sides or quarters, two of which opposite to one another incline the ray to be refracted after the unusual manner, as often as either of them are turned towards the coast of unusual refraction; and the other two, whenever either of them are turned towards the coast of unusual refraction, do not incline it to be otherwise refracted than after the usual manner. The two first may, therefore, be called the sides of unusual refraction. And since these dispositions were in the rays before their incidence on the second, third, and fourth surfaces of the two crystals, and suffered no alteration (so far as appears) by the refraction of the rays in their passage through those surfaces, and the rays were refracted by the same laws in all the four surfaces, it appears that those dispositions were in the rays originally, and suffered no alteration by the first refraction, and that by means of those dispositions the rays were refracted at their incidence on the first surface of the first crystal, some of them after the usual, and some of them after the unusual manner, accordingly as their sides of unusual refraction were then turned towards the coast of the unusual refraction of that crystal, or sideways from it.
Every ray of light has, therefore, two opposite sides, originally endued with a property on which the unusual refraction depends, and the other two opposite 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 distinguished from one another.
In explaining the difference of the sides of the rays above mentioned, I have supposed that the rays fall perpendicularly on the first crystal. But if they fall obliquely on it, the success is the same. Those rays which are refracted after the usual manner in the first crystal will be refracted after the unusual manner in the second crystal, supposing the planes of perpendicular refraction to be at right angles with one another, as above; and on the contrary.
If the planes of the perpendicular refraction of the two crystals be neither parallel nor perpendicular to one another, but contain an acute angle, the two beams of light which emerge out of the first crystal will be each of them divided into two more at their incidence on the second crystal. For in this case the rays in each of the two beams will some of them have their sides of unusual refraction, and some of them their other sides turned towards the coast of the unusual refraction of the second crystal.

QUERIE 27. Are not all hypotheses erroneous which have hitherto been invented for explaining the phenomena of light, by new modifications of the rays? For those phenomena depend not upon new modifications, as has been supposed, but upon the original and unchangeable properties of the rays.

QUERIE 28. Are not all hypotheses erroneous in which light is supposed to consist in pression or motion, propagated through a fluid medium? For in all these hypotheses the phenomena of light have been hitherto explained by supposing that they arise from new modifications of the rays; which is an erroneous supposition.
If light consisted only in pression propagated without actual motion, it would not be able to agitate and heat the bodies which refract and reflect it. If it consisted in motion propagated to all distances in an instant, it would require an infinite force every moment, in every shining particle, to generate that motion. And if it consisted in pression or motion, propagated either in an instant or in time, it would bend into the shadow. For pression or motion cannot be propagated in a fluid in right lines, beyond an obstacle which stops part of the motion, but will bend and spread every way into the quiescent medium which lies beyond the obstacle. Gravity tends downwards, but the pressure of water arising from gravity tends every way with equal force, and is propagated as readily, and with as much force sideways as downwards, and through crooked passages as through straight ones. The waves on the surface of stagnating water, passing by the sides of a broad obstacle which stops part of them, bend afterwards and dilate themselves gradually into the quiet water behind the obstacle. The waves, pulses or vibrations of the air, wherein sounds consist, bend manifestly, though not so much as the waves of water. For a bell or a cannon may be heard beyond a hill which intercepts the sight of the sounding body, and sounds are propagated as readily through crooked pipes as through straight ones. But light is never known to follow crooked passages nor to bend into the shadow. For the fixed stars by the interposition of any of the planets cease to be seen. And so do the parts of the sun by the interposition of the Moon, Mercury or Venus. The rays which pass very near to the edges of any body are bent a little by the action of the body, as we shewed above; but this bending is not towards but from the shadow, and is performed only in the passage of the ray by the body, and at a very small distance from it. So soon as the ray is past the body, it goes right on.
To explain the unusual refraction of island crystal by pression or motion propagated, has not hitherto been attempted (to my knowledge) except by Huygens, who for that end supposed two several vibrating mediums within that crystal. But when he tried the refractions in two successive pieces of that crystal, and found them such as is mentioned above, he confessed himself at a loss for explaining them. For pressions or motions, propagated from a shining body through an uniform medium, must be on all sides alike; whereas by those experiments it appears that the rays of light have different properties in their different sides. He suspected that the pulses of aether in passing through the first crystal might receive certain new modifications, which might determine them to be propagated in this or that medium within the second crystal, according to the position of that crystal. But what modifications those might be he could not say, nor think of anything satisfactory in that point. And if he had known that the unusual refraction depends not on new modifications, but on the original and unchangeable dispositions of the rays, he would have found it as difficult to explain how those dispositions, which he supposed to be impressed on the rays by the first crystal, could be in them before their incidence on that crystal, and, in general, how all rays emitted by shining bodies can have those dispositions in them from the beginning. To me, at least, this seems inexplicable, if light be nothing else than pression or motion propagated through aether.
And it is as difficult to explain by these hypotheses how rays can be alternately in fits of easy reflexion and easy transmission, unless perhaps one might suppose that there are in all space two aethereal vibrating mediums, and that the vibrations of one of them constitute light, and the vibrations of the other are swifter, and as often as they overtake the vibrations of the first, put them into those fits. But how two aethers can be diffused through all space, one of which acts upon the other, and by consequence is reacted upon, without retarding, shattering, dispersing and confounding one another’s motions, is inconceivable. And against filling the heavens with fluid mediums, unless they be exceeding rare, a great objection arises from the regular and very lasting motions of the planets and comets in all manner of courses through the heavens. For thence it is manifest that the heavens are void of all sensible resistance, and by consequence of all sensible matter.
For the resisting power of fluid mediums arises partly from the attrition of the parts of the medium, and partly from the vis inertiae of the matter. That part of the resistance of a spherical body which arises from the attrition of the parts of the medium is very nearly as the diameter, or, at the most, as the factum of the diameter, and the velocity of the spherical body together. And that part of the resistance which arises from the vis inertice of the matter is as the square of that factum. And by this difference the two sorts of resistance may be distinguished from one another in any medium; and these being distinguished, it will be found that almost all the resistance of bodies of a competent magnitude moving in air, water, quick-silver, and such like fluids with a competent velocity, arises from the vis inertiae of the parts of the fluid.
Now, that part of the resisting power of any medium which arises from the tenacity, friction or attrition of the parts of the medium, may be diminished by dividing the matter into smaller parts, and making the parts more smooth and slippery; but that part of the resistance which arises from the vis inertiae is proportional to the density of the matter, and cannot be diminished by dividing the matter into smaller parts, nor by any other means than by decreasing the density of the medium. And for these reasons the density of fluid mediums is very nearly proportional to their resistance. Liquors which differ not much in density as water, spirit of wine, spirit of turpentine, hot oil, differ not much in resistance. Water is thirteen or fourteen times lighter than quick-silver and by consequence thirteen or fourteen times rarer, and its resistance is less than that of quick-silver in the same proportion, or thereabouts, as I have found by experiments made with pendulums. The open air in which we breathe is eight or nine hundred times lighter than water, and by consequence eight or nine hundred times rarer, and accordingly its resistance is less than that of water in the same proportion, or thereabouts, as I have also found by experiments made with pendulums. And in thinner air the resistance is still less, and at length, by rarefying the air, becomes insensible. For small feathers falling in the open air meet with great resistance, but in a tall glass well emptied of air, they fall as fast as lead or gold, as I have seen tried several times. Whence the resistance seems still to decrease in proportion to the density of the fluid. For I do not find by any experiments that bodies moving in quick-silver, water, or air meet with any other sensible resistance than what arises from the density and tenacity of those sensible fluids, as they would do if the pores of those fluids, and all other spaces, were filled with a dense and subtile fluid. Now, if the resistance in a vessel well emptied of air was but a hundred times less than in the open air, it would be about a million of times less than in quick-silver. But it seems to be much less in such a vessel, and still much less in the heavens, at the height of three or four hundred miles from the Earth, or above. For Mr. Boyle has shewed that air may be rarified above ten thousand times in vessels of glass; and the heavens are much emptier of air than any vacuum we can make below. For since the air is compressed by the weight of the incumbent atmosphere, and the density of air is proportional to the force compressing it, it follows by computation that, at the height of about seven and a half English miles from the Earth, the air is four times rarer than at the surface of the Earth; and at the height of 15 miles it is sixteen times rarer than that at the surface of the Earth; and at the height of 22.5, 30, or 38 miles, it is respectively 64, 256, or 1,024 times rarer, or thereabouts; and at the height of 76, 152, 228 miles, it is about 1,000,000, 1,000,000,000,000, or 1,000,000,000,000,000,000 times rarer; and so on.
Heat promotes fluidity very much by diminishing the tenacity of bodies. It makes many bodies fluid which are not fluid in cold, and increases the fluidity of tenacious liquids, as of oil, balsam, and honey, and thereby decreases their resistance. But it decreases not the resistance of water considerably, as it would do if any considerable part of the resistance of water arose from the attrition or tenacity of its parts. And, therefore, the resistance of water arises principally and almost entirely from the vis inertiae of its matter; and by consequence, if the heavens were as dense as water, they would not have much less resistance than water; if as dense as quick-silver, they would not have much less resistance than quick-silver; if absolutely dense, or full of matter without any vacuum, let the matter be never so subtile and fluid, they would have a greater resistance than quick-silver. A solid globe in such a medium would lose above half its motion in moving three times the length of its diameter, and a globe not solid (such as are the planets), would be retarded sooner. And, therefore, to make way for the regular and lasting motions of the planets and comets, it’s necessary to empty the heavens of all matter, except perhaps some very thin vapours, steams, or effluvia, arising from the atmospheres of the Earth, planets, and comets, and from such an exceedingly rare aethereal medium as we described above. A dense fluid can be of no use for explaining the phenomena of Nature, the motions of the planets and comets being better explained without it. It serves only to disturb and retard the motions of those great bodies, and make the frame of Nature languish; and in the pores of bodies it serves only to stop the vibrating motions of their parts, wherein their heat and activity consists. And as it is of no use, and hinders the operations of Nature, and makes her languish, so there is no evidence for its existence; and, therefore, it ought to be rejected. And if it be rejected, the hypotheses that light consists in pression or motion, propagated through such a medium, are rejected with it.
And, for rejecting such a medium, we have the authority of those the oldest and most celebrated philosophers of Greece and Phoenicia, who made a vacuum, and atoms, and the gravity of atoms, the first principles of their philosophy; tacitly attributing gravity to some other cause than dense matter. Later philosophers banish the consideration of such a cause out of natural philosophy, feigning hypotheses for explaining all things mechanically, and referring other causes to metaphysics; whereas the main business of natural philosophy is to argue from phenomena without feigning hypotheses, and to deduce causes from effects, till we come to the very first cause, which certainly is not mechanical; and not only to unfold the mechanism of the world, but chiefly to resolve these and such like questions. What is there in places almost empty of matter, and whence is it that the Sun and planets gravitate towards one another, without dense matter between them? Whence is it that Nature doth nothing in vain; and whence arises all that order and beauty which we see in the world? To what end are comets, and whence is it that planets move all one and the same way in orbs concentric, while comets move all manner of ways in orbs very eccentric; and what hinders the fixed stars from falling upon one another? How came the bodies of animals to be contrived with so much art, and for what ends were their several parts? Was the eye contrived without skill in Optics, and the ear without knowledge of sounds? How do the motions of the body follow from the will, and whence is the instinct in animals? Is not the sensory of animals that place to which the sensitive substance is present, and into which the sensible species of things are carried through the nerves and brain, that there they may be perceived by their immediate presence to that substance? And these things being rightly dispatched, does it not appear from phenomena that there is a Being incorporeal, living, intelligent, omnipresent, who in infinite space (as it were in his sensory) sees the things themselves intimately, and throughly perceives them, and comprehends them wholly by their immediate presence to himself? Of which things the images only carried through the organs of sense into our little sensoriums are there seen and beheld by that which in us perceives and thinks. And though every true step made in this philosophy brings us not immediately to the knowledge of the First Cause, yet it brings us nearer to it, and on that account is to be highly valued.

QUERIE 29. Are not the rays of light very small bodies emitted from shining substances? For such bodies will pass through uniform mediums in right lines without bending into the shadow, which is the nature of the rays of light. They will also be capable of several properties, and be able to conserve their properties unchanged in passing through several mediums, which is another condition of the rays of light. Pellucid substances act upon the rays of light at a distance in refracting, reflecting, and inflecting them, and the rays mutually agitate the parts of those substances at a distance for heating them; and this action and reaction at a distance very much resembles an attractive force between bodies. If refraction be performed by attraction of the rays, the sines of incidence must be to the sines of refraction in a given proportion, as we shewed in our principles of philosophy. And this rule is true by experience. The rays of light in going out of glass into a vacuum, are bent towards the glass; and if they fall too obliquely on the vacuum, they are bent backwards into the glass, and totally reflected; and this reflexion cannot be ascribed to the resistance of an absolute vacuum, but must be caused by the power of the glass attracting the rays at their going out of it into the vacuum, and bringing them back. For if the farther surface of the glass be moistened with water or clear oil, or liquid and clear honey, the rays which would otherwise be reflected will go into the water, oil, or honey; and, therefore, are not reflected before they arrive at the farther surface of the glass, and begin to go out of it. If they go out of it into the water, oil, or honey, they go on, because the attraction of the glass is almost balanced and rendered ineffectual by the contrary attraction of the liquor. But if they go out of it into a vacuum which has no attraction to balance that of the glass, the attraction of the glass either bends and refracts them, or brings them back and reflects them. And this is still more evident by laying together two prisms of glass, or two object-glasses of very long telescopes, the one plane, the other a little convex, and so compressing them that they do not fully touch, nor are too far asunder. For the light which falls upon the farther surface of the first glass where the interval between the glasses is not above the ten hundred thousandth part of an inch, will go through that surface, and through the air or vacuum between the glasses, and enter into the second glass, as was explained in the first, fourth, and eighth Observations of the first part of the second book. But, if the second glass be taken away, the light which goes out of the second surface of the first glass into the air or vacuum, will not go on forwards, but turns back into the first glass, and is reflected; and, therefore, it is drawn back by the power of the first glass, there being nothing else to turn it back. Nothing more is requisite for producing all the variety of colours, and degrees of refrangibility, than that the rays of light be bodies of different sizes, the least of which may take violet the weakest and darkest of the colours, and be more easily diverted by refracting surfaces from the right course; and the rest, as they are bigger and bigger, may make the stronger and more lucid colours (blue, green, yellow, and red) and be more and more difficultly diverted. Nothing more is requisite for putting the rays of light into fits of easy reflexion and easy transmission, than that they be small bodies which by their attractive powers, or some other force, stir up vibrations in what they act upon, which vibrations, being swifter than the rays, overtake them successively, and agitate them so as by turns to increase and decrease their velocities, and thereby put them into those fits. And, lastly, the unusual refraction of island crystal looks very much as if it were performed by some kind of attractive virtue lodged in certain sides both of the rays, and of the particles of the crystal. For were it not for some kind of disposition or virtue lodged in some sides of the particles of the crystal, and not in their other sides, and which inclines and bends the rays towards the coast of unusual refraction, the rays which fall perpendicularly on the crystal would not be refracted towards that coast rather than towards any other coast, both at their incidence and at their emergence, so as to emerge perpendicularly by a contrary situation of the coast of unusual refraction at the second surface; the crystal acting upon the rays after they have passed through it, and are emerging into the air; or, if you please, into a vacuum. And since the crystal by this disposition or virtue does not act upon the rays, unless when one of their sides of unusual refraction looks towards that coast, this argues a virtue or disposition in those sides of the rays which answers to, and sympathizes with, that virtue or disposition of the crystal as the poles of two magnets answer to one another. And as magnetism may be intended and remitted, and is found only in the magnet and in iron, so this virtue of refracting the perpendicular rays is greater in island crystal, less in crystal of the rock, and is not yet found in other bodies. I do not say that this virtue is magnetical: it seems to be of another kind. I only say that whatever it be, it’s difficult to conceive how the rays of light, unless they be bodies, can have a permanent virtue in two of their sides which is not in their other sides, and this without any regard to their position to the space or medium through which they pass.
What I mean in this Question by a vacuum, and by the attractions of the rays of light towards glass or crystal, may be understood by what was said in the l8th, l9th, and 20th Questions.

QUERY 30. Are not gross bodies and light convertible into one another, and may not bodies receive much of their activity from the particles of light which enter their composition? For all fixed bodies being heated emit light so long as they continue sufficiently hot, and light mutually stops in bodies as often as its rays strike upon their parts, as we shewed above. I know no body less apt to shine than water; and yet water, by frequent distillations, changes into fixed earth, as Mr. Boyle has tried, and then this earth being enabled to endure a sufficient heat, shines by heat like other bodies.
The changing of bodies into light, and light into bodies, is very conformable to the course of Nature, which seems delighted with transmutations. Water, which is a very fluid, tasteless salt she changes by heat into vapour, which is a sort of air, and by cold into ice, which is a hard, pellucid, brittle, fusible stone; and this stone returns into water by heat, and vapour returns into water by cold. Earth by heat becomes fire, and by cold returns into earth. Dense bodies by fermentation rarefy into several sorts of air, and this air by fermentation, and sometimes without it, returns into dense bodies. Mercury appears sometimes in the form of a fluid metal, sometimes in the form of a hard brittle metal, sometimes in the form of a corrosive pellucid salt called sublimate, sometimes in the form of a tasteless, pellucid, volatile white earth called Mercurius dulcis; or in that of a red opaque volatile earth called Cinnabar; or in that of a red or white precipitate, or in that of a fluid salt; and in distillation it turns into vapour, and being agitated in vacuo, it shines like fire. And after all these changes it returns again into its first form of mercury. Eggs grow from insensible magnitudes, and change into animals; tadpoles into frogs; and worms into flies. All birds, beasts and fishes, insects, trees, and other vegetables, with their several parts, grow out of water and watery tinctures and salts, and by putrefaction return again into watery substances. And water standing a few days in the open air, yields a tincture, which (like that of malt) by standing longer yields a sediment and a spirit, but before putrefaction is fit nourishment for animals and vegetables. And among such various and strange transmutations, why may not Nature change bodies into light, and light into bodies?