11. As for the explication of the yearly precession of the equinoctial points, we must remember that, as I have already shown, the annual motion of the earth is not in the circumference of a circle, but of an ellipsis, or a line not considerably different from that of an ellipsis. In the first place, therefore, this elliptical line is to be described.

Let the ecliptic ♎ ♑ ♈ ♋ (in fig. 5) be divided into four equal parts by the two strait lines a b and ♑ ♋, cutting one another at right angles in the centre c. And taking the arch b d of two degrees and sixteen minutes, let the strait line d e be drawn parallel to a b, and cutting ♑ ♋ in f; which being done, the eccentricity of the earth will be c f. Seeing therefore the annual motion of the earth is in the circumference of an ellipsis, of which ♑ ♋ is the greater axis, a b cannot be the lesser axis; for a b and ♑ ♋ are equal. Wherefore the earth passing through a and b, will either pass above ♑, as through g, or passing through ♑, will fall between c and a; it is no matter which. Let it pass therefore through g; and let g l be taken equal to the strait line ♑ ♋; and dividing g l equally in i, g i will be equal to ♑ f, and i l equal to f ♋; and consequently the point i will cut the eccentricity c f into two equal parts; and taking i h equal to i f, h i will be the whole eccentricity. If now a strait line, namely, the line ♎ i ♈, be drawn through i parallel to the strait lines a b and e d, the way of the sun in summer, namely, the arch ♎ g ♈, will be greater than his way in winter, by 8¼ degrees. Wherefore the true equinoxes will be in the strait line ♎ i ♈; and therefore the ellipsis of the earth's annual motion will not pass through a, g, b, and l; but through ♎, g, ♈ and l. Wherefore the annual motion of the earth is in the ellipsis ♎ g ♈ l; and cannot be, the eccentricity being salved, in any other line. And this perhaps is the reason, why Kepler, against the opinion of all the astronomers of former time, thought fit to bisect the eccentricity of the earth, or, according to the ancients, of the sun, not by diminishing the quantity of the same eccentricity, (because the true measure of that quantity is the difference by which the summer arch exceeds the winter arch), but by taking for the centre of the ecliptic of the great orb the point c nearer to f, and so placing the whole great orb as much nearer to the ecliptic of the fixed stars towards ♋, as is the distance between c and i. For seeing the whole great orb is but as a point in respect of the immense distance of the fixed stars, the two strait lines ♎ ♈ and a b, being produced both ways to the beginnings of Aries and Libra, will fall upon the same points of the sphere of the fixed stars. Let therefore the diameter of the earth m n be in the plane of the earth's annual motion. If now the earth be moved by the sun's simple motion in the circumference of the ecliptic about the centre i, this diameter will be kept always parallel to itself and to the strait line g l. But seeing the earth is moved in the circumference of an ellipsis without the ecliptic, the point n, whilst it passeth through ♎ ♑ ♈, will go in a lesser circumference than the point m; and consequently, as soon as ever it begins to be moved, it will lose its parallelism with the strait line ♑ ♋; so that m n produced will at last cut the strait line g l produced. And contrarily, as soon as m n is past ♈, the earth making its way in the internal elliptical line ♈ l ♎, the same m n produced towards m, will cut l g produced. And when the earth hath almost finished its whole circumference, the same m n shall again make a right angle with a line drawn from the centre i, a little short of the point from which the earth began its motion. And there the next year shall be one of the equinoctial points, namely, near the end of ♍; the other shall be opposite to it near the end of ♓. And thus the points in which the days and nights are made equal do every year fall back; but with so slow a motion, that, in a whole year, it makes but 51 first minutes. And this relapse being contrary to the order of the signs, is commonly called the precession of the equinoxes. Of which I have from my former suppositions deduced a possible cause; which was to be done.

According to what I have said concerning the cause of the eccentricity of the earth; and according to Kepler, who for the cause thereof supposeth one part of the earth to be affected to the sun, the other part to be disaffected; the apogæum and perigæum of the sun should be moved every year in the same order, and with the same velocity, with which the equinoctial points are moved; and their distance from them should always be the quadrant of a circle; which seems to be otherwise. For astronomers say, that the equinoxes are now, the one about 28 degrees gone back from the first star of Aries, the other as much from the beginning of Libra; so that the apogæum of the sun or the aphelium of the earth ought to be about the 28th degree of Cancer. But it is reckoned to be in the 7th degree. Seeing, therefore, we have not sufficient evidence of the ὁτί (that so it is,) it is in vain to seek for the διότι (why it is so.) Wherefore, as long as the motion of the apogæum is not observable by reason of the slowness thereof, and as long as it remains doubtful whether their distance from the equinoctial points be more or less than a quadrant precisely; so long it may be lawful for me to think they proceed both of them with equal velocity.

Also, I do not at all meddle with the causes of the eccentricities of Saturn, Jupiter, Mars, and Mercury. Nevertheless, seeing the eccentricity of the earth may, as I have shewn, be caused by the unlike constitution of the several parts of the earth which are alternately turned towards the sun, it is credible also, that like effects may be produced in these other planets from their having their superficies of unlike parts.

And this is all I shall say concerning Sidereal Philosophy. And, though the causes I have here supposed be not the true causes of these phenomena, yet I have demonstrated that they are sufficient to produce them, according to what I at first propounded.

Vol. 1. Lat. & Eng.
C. XXVI.
Fig. 1-5

CHAPTER XXVII.
OF LIGHT, HEAT, AND OF COLOURS.

1. Of the immense magnitude of some bodies, and the unspeakable littleness of others.—2. Of the cause of the light of the sun.—3. How light heateth.—4. The generation of fire from the sun.—5. The generation of fire from collision.—6. The cause of light in glow-worms, rotten wood, and the Bolognan stone.—7. The cause of light in the concussion of sea water.—8. The cause of flame, sparks, and colliquation.—9. The cause why wet hay sometimes burns of its own accord; also the cause of lightning.—10. The cause of the force of gunpowder; and what is to be ascribed to the coals, what to the brimstone, and what to the nitre.—11. How heat is caused by attrition.—12. The distinction of light into first, second, &c.—13. The causes of the colours we see in looking through a prisma of glass, namely, of red, yellow, blue, and violet colour.—14. Why the moon and the stars appear redder in the horizon than in the midst of the heaven.—15. The cause of whiteness.—16. The cause of blackness.

1. Besides the stars, of which I have spoken in the last chapter, whatsoever other bodies there be in the world, they may be all comprehended under the name of intersidereal bodies. And these I have already supposed to be either the most fluid æther, or such bodies whose parts have some degree of cohesion. Now, these differ from one another in their several consistencies, magnitudes, motions, and figures. In consistency, I suppose some bodies to be harder, others softer through all the several degrees of tenacity. In magnitude, some to be greater, others less, and many unspeakably little. For we must remember that, by the understanding, quantity is divisible into divisibles perpetually. And, therefore, if a man could do as much with his hands as he can with his understanding, he would be able to take from any given magnitude a part which should be less than any other magnitude given. But the Omnipotent Creator of the world can actually from a part of any thing take another part, as far as we by our understanding can conceive the same to be divisible. Wherefore there is no impossible smallness of bodies. And what hinders but that we may think this likely? For we know there are some living creatures so small that we can scarce see their whole bodies. Yet even these have their young ones; their little veins and other vessels, and their eyes so small as that no microscope can make them visible. So that we cannot suppose any magnitude so little, but that our very supposition is actually exceeded by nature. Besides, there are now such microscopes commonly made, that the things we see with them appear a hundred thousand times bigger than they would do if we looked upon them with our bare eyes. Nor is there any doubt but that by augmenting the power of these microscopes (for it may be augmented as long as neither matter nor the hands of workmen are wanting) every one of those hundred thousandth parts might yet appear a hundred thousand times greater than they did before. Neither is the smallness of some bodies to be more admired than the vast greatness of others. For it belongs to the same Infinite Power, as well to augment infinitely as infinitely to diminish. To make the great orb, namely, that whose radius reacheth from the earth to the sun, but as a point in respect of the distance between the sun and the fixed stars; and, on the contrary, to make a body so little, as to be in the same proportion less than any other visible body, proceeds equally from one and the same Author of Nature. But this of the immense distance of the fixed stars, which for a long time was accounted an incredible thing, is now believed by almost all the learned. Why then should not that other, of the smallness of some bodies, become credible at some time or other? For the Majesty of God appears no less in small things than in great; and as it exceedeth human sense in the immense greatness of the universe, so also it doth in the smallness of the parts thereof. Nor are the first elements of compositions, nor the first beginnings of actions, nor the first moments of times more credible, than that which is now believed of the vast distance of the fixed stars.

Some things are acknowledged by mortal men to be very great, though finite, as seeing them to be such. They acknowledge also that some things, which they do not see, may be of infinite magnitude. But they are not presently nor without great study persuaded, that there is any mean between infinite and the greatest of those things which either they see or imagine. Nevertheless, when after meditation and contemplation many things which we wondered at before are now grown more familiar to us, we then believe them, and transfer our admiration from the creatures to the Creator. But how little soever some bodies may be, yet I will not suppose their quantity to be less than is requisite for the salving of the phenomena. And in like manner I shall suppose their motion, namely, their velocity and slowness, and the variety of their figures, to be only such as the explication of their natural causes requires. And lastly, I suppose, that the parts of the pure æther, as if it were the first matter, have no motion at all but what they receive from bodies which float in them, and are not themselves fluid.

2. Having laid these grounds, let us come to speak of causes; and in the first place let us inquire what may be the cause of the light of the sun. Seeing, therefore, the body of the sun doth by its simple circular motion thrust away the ambient ethereal substance sometimes one way sometimes another, so that those parts, which are next the sun, being moved by it, do propagate that motion to the next remote parts, and these to the next, and so on continually; it must needs be that, notwithstanding any distance, the foremost part of the eye will at last be pressed; and by the pressure of that part, the motion will be propagated to the innermost part of the organ of sight, namely, to the heart; and from the reaction of the heart, there will proceed an endeavour back by the same way, ending in the endeavour outwards of the coat of the eye, called the retina. But this endeavour outwards, as has been defined in chapter XXV, is the thing which is called light, or the phantasm of a lucid body. For it is by reason of this phantasm that an object is called lucid. Wherefore we have a possible cause of the light of the sun; which I undertook to find.

3. The generation of the light of the sun is accompanied with the generation of heat. Now every man knows what heat is in himself, by feeling it when he grows hot; but what it is in other things, he knows only by ratiocination. For it is one thing to grow hot, and another thing to heat or make hot. And therefore though we perceive that the fire or the sun heateth, yet we do not perceive that it is itself hot. That other living creatures, whilst they make other things hot, are hot themselves, we infer by reasoning from the like sense in ourselves. But this is not a necessary inference. For though it may truly be said of living creatures, that they heat, therefore they are themselves hot; yet it cannot from hence be truly inferred that fire heateth, therefore it is itself hot; no more than this, fire causeth pain, therefore it is itself in pain. Wherefore, that is only and properly called hot, which when we feel we are necessarily hot.

Now when we grow hot, we find that our spirits and blood, and whatsoever is fluid within us, is called out from the internal to the external parts of our bodies, more or less, according to the degree of the heat; and that our skin swelleth. He, therefore, that can give a possible cause of this evocation and swelling, and such as agrees with the rest of the phenomena of heat, may be thought to have given the cause of the heat of the sun.

It hath been shown, in the 5th article of chapter XXI, that the fluid medium, which we call the air, is so moved by the simple circular motion of the sun, as that all its parts, even the least, do perpetually change places with one another; which change of places is that which there I called fermentation. From this fermentation of the air, I have, in the 8th article of the last chapter, demonstrated that the water may be drawn up into the clouds.

And I shall now show that the fluid parts may, in like manner, by the same fermentation, be drawn out from the internal to the external parts of our bodies. For seeing that wheresoever the fluid medium is contiguous to the body of any living creature, there the parts of that medium are, by perpetual change of place, separated from one another; the contiguous parts of the living creature must, of necessity, endeavour to enter into the spaces of the separated parts. For otherwise those parts, supposing there is no vacuum, would have no place to go into. And therefore that, which is most fluid and separable in the parts of the living creature which are contiguous to the medium, will go first out; and into the place thereof will succeed such other parts as can most easily transpire through the pores of the skin. And from hence it is necessary that the rest of the parts, which are not separated, must altogether be moved outwards, for the keeping of all places full. But this motion outwards of all parts together must, of necessity, press those parts of the ambient air which are ready to leave their places; and therefore all the parts of the body, endeavouring at once that way, make the body swell. Wherefore a possible cause is given of heat from the sun; which was to be done.

4. We have now seen how light and heat are generated; heat by the simple motion of the medium, making the parts perpetually change places with one another; and light by this, that by the same simple motion action is propagated in a strait line. But when a body hath its parts so moved, that it sensibly both heats and shines at the same time, then it is that we say fire is generated.

Now by fire I do not understand a body distinct from matter combustible or glowing, as wood or iron, but the matter itself, not simply and always, but then only when it shineth and heateth. He, therefore, that renders a cause possible and agreeable to the rest of the phenomena, namely, whence, and from what action, both the shining and heating proceed, may be thought to have given a possible cause of the generation of fire.

Let, therefore, A B C (in the first figure) be a sphere, or the portion of a sphere, whose centre is D; and let it be transparent and homogeneous, as crystal, glass, or water, and objected to the sun. Wherefore, the foremost part A B C will, by the simple motion of the sun, by which it thrusts forwards the medium, be wrought upon by the sunbeams in the strait lines E A, F B, and G C; which strait lines may, in respect of the great distance of the sun, be taken for parallels. And seeing the medium within the sphere is thicker than the medium without it, those beams will be refracted towards the perpendiculars. Let the strait lines E A and G C be produced till they cut the sphere in H and I; and drawing the perpendiculars A D and C D, the refracted beams E A and G C will of necessity fall, the one between A H and A D, the other between C I and C D. Let those refracted beams be A K and C L. And again, let the lines D K M and D L N be drawn perpendicular to the sphere; and let A K and C L be produced till they meet with the strait line B D produced in O. Seeing, therefore, the medium within the sphere is thicker than that without it, the refracted line A K will recede further from the perpendicular K M than K O will recede from the same. Wherefore K O will fall between the refracted line and the perpendicular. Let, therefore, the refracted line be K P, cutting F O in P; and for the same reason the strait line L P will be the refracted line of the strait line C L. Wherefore, seeing the beams are nothing else but the ways in which the motion is propagated, the motion about P will be so much more vehement than the motion about A B C, by how much the base of the portion A B C is greater than the base of a like portion in the sphere, whose centre is P, and whose magnitude is equal to that of the little circle about P, which comprehendeth all the beams that are propagated from A B C; and this sphere being much less than the sphere A B C, the parts of the medium, that is, of the air about P, will change places with one another with much greater celerity than those about A B C. If, therefore, any matter combustible, that is to say, such as may be easily dissipated, be placed in P, the parts of that matter, if the proportion be great enough between A C and a like portion of the little circle about P, will be freed from their mutual cohesion, and being separated will acquire simple motion. But vehement simple motion generates in the beholder a phantasm of lucid and hot, as I have before demonstrated of the simple motion of the sun; and therefore the combustible matter which is placed in P will be made lucid and hot, that is to say, will be fire. Wherefore I have rendered a possible cause of fire; which was to be done.

5. From the manner by which the sun generateth fire, it is easy to explain the manner by which fire may be generated by the collision of two flints. For by that collision some of those particles of which the stone is compacted, are violently separated and thrown off; and being withal swiftly turned round, the eye is moved by them, as it is in the generation of light by the sun. Wherefore they shine; and falling upon matter which is already half dissipated, such as is tinder, they thoroughly dissipate the parts thereof, and make them turn round. From whence, as I have newly shown, light and heat, that is to say fire, is generated.

6. The shining of glow-worms, some kinds of rotten wood, and of a kind of stone made at Bologna, may have one common cause, namely, the exposing of them to the hot sun. We find by experience that the Bologna stone shines not, unless it be so exposed; and after it has been exposed it shines but for a little time, namely, as long as it retains a certain degree of heat. And the cause may be that the parts, of which it is made, may together with heat have simple motion imprinted in them by the sun. Which if it be so, it is necessary that it shine in the dark, as long as there is sufficient heat in it; but this ceasing, it will shine no longer. Also we find by experience that in the glow-worm there is a certain thick humour, like the crystalline humour of the eye; which if it be taken out and held long enough in one's fingers, and then be carried into the dark, it will shine by reason of the warmth it received from the fingers; but as soon as it is cold it will cease shining. From whence, therefore, can these creatures have their light, but from lying all day in the sunshine in the hottest time of summer? In the same manner, rotten wood, except it grow rotten in the sunshine, or be afterwards long enough exposed to the sun, will not shine. That this doth not happen in every worm, nor in all kinds of rotten wood, nor in all calcined stones, the cause may be that the parts, of which the bodies are made, are different both in motion and figure from the parts of bodies of other kinds.

7. Also the sea water shineth when it is either dashed with the strokes of oars, or when a ship in its course breaks strongly through it; but more or less, according as the wind blows from different points. The cause whereof may be this, that the particles of salt, though they never shine in the salt-pits, where they are but slowly drawn up by the sun, being here beaten up into the air in greater quantities and with more force, are withal made to turn round, and consequently to shine, though weakly. I have, therefore, given a possible cause of this phenomenon.

8. If such matter as is compounded of hard little bodies be set on fire, it must needs be, that, as they fly out in greater or less quantities, the flame which is made by them will be greater or less. And if the ethereal or fluid part of that matter fly out together with them, their motion will be the swifter, as it is in wood and other things which flame with a manifest mixture of wind. When, therefore, these hard particles by their flying out move the eye strongly, they shine bright; and a great quantity of them flying out together, they make a great shining body. For flame being nothing but an aggregate of shining particles, the greater the aggregate is, the greater and more manifest will be the flame. I have, therefore, shown a possible cause of flame. And from hence the cause appears evidently, why glass is so easily and quickly melted by the small flame of a candle blown, which will not be melted without blowing but by a very strong fire.

Now, if from the same matter there be a part broken off, namely, such a part as consisteth of many of the small particles, of this is made a spark. For from the breaking off it hath a violent turning round, and from hence it shines. But though from this matter there fly neither flame nor sparks, yet some of the smallest parts of it may be carried out as far as to the superficies, and remain there as ashes; the parts whereof are so extremely small, that it cannot any longer be doubted how far nature may proceed in dividing.

Lastly, though by the application of fire to this matter there fly little or nothing from it, yet there will be in the parts an endeavour to simple motion; by which the whole body will either be melted, or, which is a degree of melting, softened. For all motion has some effect upon all matter whatsoever, as has been shown at art. 3, chap. XV. Now if it be softened to such a degree, as that the stubbornness of the parts be exceeded by their gravity, then we say it is melted; otherwise, softened and made pliant and ductile.

Again, the matter having in it some particles hard, others ethereal or watery; if, by the application of fire, these latter be called out, the former will thereby come to a more full contact with one another; and, consequently, will not be so easily separated; that is to say, the whole body will be made harder. And this may be the cause why the same fire makes some things soft, others hard.

9. It is known by experience that if hay be laid wet together in a heap, it will after a time begin to smoke, and then burn as it were of itself. The cause whereof seems to be this, that in the air, which is enclosed within the hay, there are those little bodies, which, as I have supposed, are moved freely with simple motion. But this motion being by degrees hindered more and more by the descending moisture, which at the last fills and stops all the passages, the thinner parts of the air ascend by penetrating the water; and those hard little bodies, being so thrust together that they touch and press one another, acquire stronger motion; till at last by the increased strength of this motion the watery parts are first driven outwards, from whence appears vapour; and by the continued increase of this motion, the smallest particles of the dried hay are forced out, and recovering their natural simple motion, they grow hot and shine, that is to say, they are set on fire.

The same also may be the cause of lightning, which happens in the hottest time of the year, when the water is raised up in greatest quantity and carried highest. For after the first clouds are raised, others after others follow them; and being congealed above, they happen, whilst some of them ascend and others descend, to fall one upon another in such manner, as that in some places all their parts are joined together, in others they leave hollow spaces between them; and into these spaces, the ethereal parts being forced out by the compressure of the clouds, many of the harder little bodies are so pent together, as they have not the liberty of such motion as is natural to the air. Wherefore their endeavour grows more vehement, till at last they force their way through the clouds, sometimes in one place, sometimes in another; and, breaking through with great noise, they move the air violently, and striking our eyes, generate light, that is to say, they shine. And this shining is that we call lightning.

10. The most common phenomenon proceeding from fire, and yet the most admirable of all others, is the force of gunpowder fired; which being compounded of nitre, brimstone and coals, beaten small, hath from the coals its first taking fire; from the brimstone its nourishment and flame, that is to say, light and motion, and from the nitre the vehemence of both. Now if a piece of nitre, before it is beaten, be laid upon a burning coal, first it melts, and, like water, quencheth that part of the coal it toucheth. Then vapour or air, flying out where the coal and nitre join, bloweth the coal with great swiftness and vehemence on all sides. And from hence it comes to pass, that by two contrary motions, the one, of the particles which go out of the burning coal, the other, of those of the ethereal and watery substance of the nitre, is generated that vehement motion and inflammation. And, lastly, when there is no more action from the nitre, that is to say, when the volatile parts of the nitre are flown out, there is found about the sides a certain white substance, which being thrown again into the fire, will grow red-hot again, but will not be dissipated, at least unless the fire be augmented. If now a possible cause of this be found out, the same will also be a possible cause why a grain of gunpowder set on fire doth expand itself with such vehement motion, and shine. And it may be caused in this manner.

Let the particles, of which nitre consisteth, be supposed to be some of them hard, others watery, and the rest ethereal. Also let the hard particles be supposed to be spherically hollow, like small bubbles, so that many of them growing together may constitute a body, whose little caverns are filled with a substance which is either watery, or ethereal, or both. As soon, therefore, as the hard particles are dissipated, the watery and ethereal particles will necessarily fly out; and as they fly, of necessity blow strongly the burning coals and brimstone which are mingled together; whereupon there will follow a great expansion of light, with vehement flame, and a violent dissipation of the particles of the nitre, the brimstone and the coals. Wherefore I have given a possible cause of the force of fired gunpowder.

It is manifest from hence, that for the rendering of the cause why a bullet of lead or iron, shot from a piece of ordnance, flies with so great velocity, there is no necessity to introduce such rarefaction, as, by the common definition of it, makes the same matter to have sometimes more, sometimes less quantity; which is inconceivable. For every thing is said to be greater or less, as it hath more or less quantity. The violence with which a bullet is thrust out of a gun, proceeds from the swiftness of the small particles of the fired powder; at least it may proceed from that cause without the supposition of any empty space.

11. Besides, by the attrition or rubbing of one body against another, as of wood against wood, we find that not only a certain degree of heat, but fire itself is sometimes generated. For such motion is the reciprocation of pressure, sometimes one way, sometimes the other; and by this reciprocation whatsoever is fluid in both the pieces of wood is forced hither and thither; and consequently, to an endeavour of getting out; and at last by breaking out makes fire.

12. Now light is distinguished into, first, second, third, and so on infinitely. And we call that first light, which is in the first lucid body; as the sun, fire, &c.: second, that which is in such bodies, as being not transparent are illuminated by the sun; as the moon, a wall, &c.: and third, that which is in bodies not transparent, but illuminated by second light, &c.

13. Colour is light, but troubled light, namely, such as is generated by perturbed motion; as shall be made manifest by the red, yellow, blue and purple, which are generated by the interposition of a diaphanous prisma, whose opposite bases are triangular, between the light and that which is enlightened.

For let there be a prisma of glass, or of any other transparent matter which is of greater density than air; and let the triangle A B C be the base of this prisma. Also let the strait line D E be the diameter of the sun's body, having oblique position to the strait line A B; and let the sunbeams pass in the lines D A and E B C. And lastly, let the strait lines D A and E C be produced indefinitely to F and G. Seeing therefore the strait line D A, by reason of the density of the glass, is refracted towards the perpendicular; let the line refracted at the point A be the strait line A H. And again, seeing the medium below A C is thinner than that above it, the other refraction, which will be made there, will diverge from the perpendicular. Let therefore this second refracted line be A I. Also let the same be done at the point C, by making the first refracted line to be C K, and the second C L. Seeing therefore the cause of the refraction in the point A of the strait line of A B is the excess of the resistance of the medium in A B above the resistance of the air, there must of necessity be reaction from the point A towards the point B; and consequently the medium at A within the triangle A B C will have its motion troubled, that is to say, the strait motion in A F and A H will be mixed with the transverse motion between the same A F and A H, represented by the short transverse lines in the triangle A F H. Again, seeing at the point A of the strait line A C there is a second refraction from A H in A I, the motion of the medium will again be perturbed by reason of the transverse reaction from A towards C, represented likewise by the short transverse lines in the triangle A H I. And in the same manner there is a double perturbation represented by the transverse lines in the triangles C G K and C K L. But as for the light between A I and C G, it will not be perturbed; because, if there were in all the points of the strait lines A B and A C the same action which is in the points A and C, then the plane of the triangle C G K would be everywhere coincident with the plane of the triangle A F H; by which means all would appear alike between A and C. Besides, it is to be observed, that all the reaction at A tends towards the illuminated parts which are between A and C, and consequently perturbeth the first light. And on the contrary, that all the reaction at C tends towards the parts without the triangle or without the prisma A B C, where there is none but second light; and that the triangle A F H shows that perturbation of light which is made in the glass itself; as the triangle A H I shows that perturbation of light which is made below the glass. In like manner, that C G K shows the perturbation of light within the glass; and C K L that which is below the glass. From whence there are four divers motions, or four different illuminations or colours, whose differences appear most manifestly to the sense in a prisma, whose base is an equilateral triangle, when the sunbeams that pass through it are received upon a white paper. For the triangle A F H appears red to the sense; the triangle A H I yellow; the triangle C G K green, and approaching to blue; and lastly, the triangle C K L appears purple. It is therefore evident that when weak but first light passeth through a more resisting diaphanous body, as glass, the beams, which fall upon it transversely, make redness; and when the same first light is stronger, as it is in the thinner medium below the strait line A C, the transverse beams make yellowness. Also when second light is strong, as it is in the triangle C G K, which is nearest to the first light, the transverse beams make greenness; and when the same second light is weaker, as in the triangle C K L, they make a purple colour.

14. From hence may be deduced a cause, why the moon and stars appear bigger and redder near the horizon than in the mid-heaven. For between the eye and the apparent horizon there is more impure air, such as is mingled with watery and earthy little bodies, than is between the same eye and the more elevated part of heaven. But vision is made by beams which constitute a cone, whose base, if we look upon the moon, is the moon's face, and whose vertex is in the eye; and therefore, many beams from the moon must needs fall upon little bodies that are without the visual cone, and be by them reflected to the eye. But these reflected beams tend all in lines which are transverse to the visual cone, and make at the eye an angle which is greater than the angle of the cone. Wherefore, the moon appears greater in the horizon, than when she is more elevated. And because those reflected beams go transversely, there will be generated, by the last article, redness. A possible cause therefore is shown, why the moon as also the stars appear greater and redder in the horizon, than in the midst of heaven. The same also may be the cause, why the sun appears in the horizon greater and of a colour more degenerating to yellow, than when he is higher elevated. For the reflection from the little bodies between, and the transverse motion of the medium, are still the same. But the light of the sun is much stronger than that of the moon; and therefore, by the last article, his splendour must needs by this perturbation degenerate into yellowness.

But for the generation of these four colours, it is not necessary that the figure of the glass be a prisma; for if it were spherical it would do the same. For in a sphere the sunbeams are twice refracted and twice reflected. And this being observed by Des Cartes, and withal that a rainbow never appears but when it rains; as also, that the drops of rain have their figures almost spherical; he hath shown from thence the cause of the colours in the rainbow; which therefore need not be repeated.

15. Whiteness is light, but light perturbed by the reflections of many beams of light coming to the eye together within a little space. For if glass or any other diaphanous body be reduced to very small parts by contusion or concussion, every one of those parts, if the beams of a lucid body be from any one point of the same reflected to the eye, will represent to the beholder an idea or image of the whole lucid body, that is to say, a phantasm of white. For the strongest light is the most white; and therefore many such parts will make many such images. Wherefore, if those parts lie thick and close together, those many images will appear confusedly, and will by reason of the confused light represent a white colour. So that from hence may be deduced a possible cause, why glass beaten, that is, reduced to powder, looks white. Also why water and snow are white; they being nothing but a heap of very small diaphanous bodies, namely, of little bubbles, from whose several convex superficies there are by reflection made several confused phantasms of the whole lucid body, that is to say, whiteness. For the same reason, salt and nitre are white, as consisting of small bubbles which contain within them water and air; as is manifest in nitre, from this, that being thrown into the fire it violently blows the same; which salt also doth, but with less violence. But if a white body be exposed, not to the light of the day, but to that of the fire or of a candle, it will not at the first sight be easily judged whether it be white or yellow; the cause whereof may be this, that the light of those things, which burn and flame, is almost of a middle colour between whiteness and yellowness.

16. As whiteness is light, so blackness is the privation of light, or darkness. And, from hence it is, first, that all holes, from which no light can be reflected to the eye, appear black. Secondly, that when a body hath little eminent particles erected straight up from the superficies, so that the beams of light which fall upon them are reflected not to the eye but to the body itself, that superficies appears black; in the same manner as the sea appears blackblack when ruffled by the wind. Thirdly, that any combustible matter is by the fire made to look black before it shines. For the endeavour of the fire being to dissipate the smallest parts of such bodies as are thrown into it, it must first raise and erect those parts before it can work their dissipation. If, therefore, the fire be put out before the parts are totally dissipated, the coal will appear black; for the parts being only erected, the beams of light falling upon them will not be reflected to the eye, but to the coal itself. Fourthly, that burning glasses do more easily burn black things than white. For in a white superficies the eminent parts are convex, like little bubbles; and therefore the beams of light, which fall upon them, are reflected every way from the reflecting body. But in a black superficies, where the eminent particles are more erected, the beams of light falling upon them are all necessarily reflected towards the body itself; and, therefore, bodies that are black are more easily set on fire by the sun beams, than those that are white. Fifthly, that all colours that are made of the mixture of white and black proceed from the different position of the particles that rise above the superficies, and their different forms of asperity. For, according to these differences, more or fewer beams of light are reflected from several bodies to the eye. But in regard those differences are innumerable, and the bodies themselves so small that we cannot perceive them; the explication and precise determination of the causes of all colours is a thing of so great difficulty, that I dare not undertake it.

Vol. I. Lat. & Eng.
C. XXVII.
Fig. 1-2]