Chapter III: Preface (3)

SALV.: If, in spite of every sort of investigation, I am able to find no such differences, I believe I have thereby discovered that such difference does not exist. So in my opinion it is useless to pursue this further: rather let us proceed. Motion is only so far motion and acts as such, if it stands in relation to things which lack motion. In relation to things that are all in the same degree affected by it, it is as much without effect as if it did not take place. The wares with which a ship is loaded move, when they depart from Venice and arrive at Aleppo, passing Korfu, Candia, Cyprus etc; since Venice, Korfu and Candia remain fixed and do not move with the ship. But in respect to the bales, chests, and other pieces of baggage which are on the ship as cargo or ballast, the movement of the ship itself from Venice to Syria is as good as non-existent, since their position in relation to one another does not change; and this is due to the fact that the movement is a common one in which they all take part. If of the wares on the ship one bale moves only an inch away from the chest, this is for it a greater movement in relation to the chest, than the whole journey of 2,000 miles which they undergo in common.

Therefore, since plainly the motion which many movable bodies undergo in common is without effect and, with regard to their mutual position toward one another, it is as if it did not exist, for there is no change among them; and since it only affects the relative position of such bodies as do not share in the movement, for in this case the mutual relation is changed; since we have divided the universe into two parts, of which one must be movable and the other immovable; then for all purposes this movement will be of the same effect whether it is ascribed to the earth alone or to all the rest of the universe. For the working of such a motion is on nothing but the relative position in which the earth and the heavenly bodies stand to one another, and aside from this relative position nothing changes. If now it is indifferent for accomplishing this result whether the earth alone moves and the whole universe rests, or the earth rests and the whole universe is subject to one common movement, who can believe that Nature--who by common agreement does not employ great means when she can obtain the same result by smaller ones--would have undertaken to set in motion an immeasurable number of mighty bodies, and that with incredible velocity, to accomplish what could be obtained by the moderate motion of one single body around the center?

SIMPL.: I do not agree that that mighty movement would be as if it did not happen in regard to the sun, the moon, the innumerable host of fixed stars. Do you call it nothing that the sun goes from one meridian to another, rises from one horizon, sinks under another, brings now day, now night; that the moon goes through similar changes and likewise the other planets, as well as the fixed stars?

SALV.: All the changes mentioned by you are such only with respect to the earth. To demonstrate this, only imagine yourself away from the earth; there is then no rising or setting of the sun, no horizons, no meridians, no day, no night; in a word, by the movement mentioned no change in the relation of the moon to the sun or to any other star is evoked. All these changes have reference to the earth; they are supposed only because the sun is first visible in China, then Egypt, Greece, France, Spain, America, and so on, and so also for the moon and the other heavenly bodies. The same process would occur in the same way, if, without disturbing so vast a part of the universe, the earth alone should be revolved.

The difficulty is however doubled since a second very important one is added. That is, if one attributes to the firmament this mighty motion, one must regard it as necessarily opposed to the particular movements of all the planets, all of which indisputably have their own movements from west to east, and in comparison very moderate movements at that. One is then forced to the conclusion that they depart from that rapid daily motion, namely from east to west, to go in the opposite direction. But, if we suppose that the earth moves, the opposition of motions disappears and the single movement from west to east fits in with all the facts and explains them most satisfactorily.

SIMPL.: As far as this opposition of motions is concerned that has little importance, since Aristotle proves that the circular motions are not opposed to one another and that the apparent opposition cannot actually be called so.

SALV.: Does Aristotle prove that or merely suppose it, because it aids him for a certain purpose? If, according to his own declaration, those things are opposed which mutually destroy one another I do not see how two moving bodies which meet one another in a circular motion should do one another less harm than if they meet on a straight line.

SAGR.: Wait a moment, I pray. Tell me, Signore Simplicio, if two knights run into one another with leveled lances on the open field, if two squadrons or two streams on their way to the sea break through and unite with one another, would you call such collisions opposed movements?

SIMPL.: Of course we would call them opposed.

SAGR.: How then is there no opposition in circular motions? For the movements mentioned take place upon the surface of the earth or water, both of which are recognized to be circular in form and so the motions must be circular. Do you understand, Signore Simplicio, what circular motions are not opposed to one another? Two circles which touch each other on the outside and of which the revolution of one is in a reverse direction from that of the other. If, however, one circle is within the other, then motions in different directions must be opposed to one another.

SALV.: Whether opposed or not opposed is merely a strife of words. I know that in fact it is simpler and more natural to accomplish everything with one motion than to call in two. If you do not wish to call them opposite, then call them reverse. Moreover, I mention this introduction of a double movement not as something impossible, and in no way propose to deduce from it a strong proof for the motion of the earth, but merely a high degree of probability for it.

The improbability of the movement of the universe about the earth is tripled, however, by the complete upsetting of that arrangement which governs all the heavenly bodies whose circular motion is accepted not doubtfully but with full assurance. That is, that in such cases the larger the orbit the longer the time required for its completion, and the smaller, the shorter. Saturn, whose course surpasses all the planets in extent, completes it in thirty years. Jupiter revolves in a smaller circle in twelve years. Mars in two, the moon in a month. We see clearly in the case of the Medicean stars [the moons of Jupiter] that the one nearest Jupiter goes through its orbit in a very short time, namely, forty-two hours, the next nearest in three and a half days, the third in seven days, and the farthest removed in sixteen days. This thoroughly constant rule remains unchanged if we ascribe the twenty-four hour movement to the revolution of the earth, but if we suppose the earth to remain unmoved, we must proceed from the short period of the moon to increasingly greater periods, to the two year period of Mars, the twelve year period of Jupiter, the thirty year period of Saturn, and then abruptly to a disproportionately larger orbit, to which must also be ascribed the revolution in twenty-four hours. And these suppositions entail the smallest part of the disturbance of the otherwise constant law. For when one passes from the orbit of Saturn to those of the fixed stars and attributes to them even greater orbits, which correspond to the period of revolution of many thousands of years, one must pass from this by a much more disproportionate transition to that other movement and ascribe to them a period of revolution about the earth of twenty-four hours. But if the movement of the earth is supposed, the regularity of the period is accounted for in the best possible way; from the slow period of Saturn we arrive at the immovable fixed star.

A fourth difficulty also is encountered which must be added if we suppose the motion of the smaller sphere. I mean the great dissimilarity in movements of these stars, some of which must revolve at a tremendous rate in immense circles, others slowly in smaller circles, according as they are placed at greater or smaller distances from the pole. And not only the size of the different circles and so the velocity of movement varies greatly in different fixed stars, but also the same stars change their courses and their velocity; herein is the fifth difficulty. That is, those stars which 2,000 years ago stood on the equator of the stellar sphere and thereafter moved in the greatest circles, must now, since to-day they have moved several degrees from it, move more slowly and in smaller circles. Within a conceivable time it will happen that one of those which have been continually moving will eventually reach the pole and cease to revolve, then later, after a period of rest, begin to move again. The other stars, however, which undoubtedly move, all have, as has been said, as orbit an immense circle and move in it without change.

The improbability is increased (and this may be called a sixth difficulty) for him who investigates basic principles, by the fact that one cannot imagine the firmness which that immense sphere must possess, in whose depths so many stars are so solidly fixed that in spite of such varieties of motions they are held together in the revolution without in any way changing their relative positions. But if according to the most probable view the heavens are fluid, so that each star may describe its own orbit, by what law and according to what principles are their orbits governed, so that seen from the earth they appear as if held in one sphere? To accomplish this it seems to me it would be easier and more convenient to make them stationary instead of movable, just as the paving stones in the market place are kept in order more easily than the troops of children who race over them.

Finally the seventh objection; if we ascribe the daily revolution to the highest heavens we must suppose this to be of such power and force that it bears along the innumerable crowd of fixed stars, every one a body of immense mass and much larger than the earth, further, all the planets, although these by their nature move in an opposite direction. Moreover, we must suppose that the element of fire and the greater portion of the air is also borne along; therefore, singly and alone the little earth ball withstands stubbornly and independently this mighty force: a supposition that seems to me to have much against it. I cannot explain how the earth, a body freely suspended and balanced on its axis, inclined by nature as much toward motion as the rest, surrounded by a fluid medium, is not seized on by this general revolution. We do not encounter this difficulty, however, if we suppose the earth to move, a body so small, so inconsiderable in comparison with the whole universe that it could have no effect at all upon this.

FOOTNOTES:

[Footnote 6: Translated from the _Dialogo dei due Massima Systemi del Mondo_ (1632).]

V

WILLIAM HARVEY

1578-1657

_In 1615 William Harvey stated his theory of the circulation of the blood, which he derived from patient observations, in his lectures on anatomy. The theory was epoch-making in the history of physiology because it initiated the study of the chemical constituency of the blood and of its function in nutrition._

_Harvey, born April 1, 1578, in the south of England, attended the University of Cambridge, and took his degree in 1597. The following four years he studied at Padua under Fabricius. In 1602, when he returned to England, he began the practice of medicine, and in 1609 became connected with St. Bartholomew’s Hospital. He published his “Excercitatio” in 1628, served for several years as physician to Charles I, and retired in 1646 to private life. He died June 3, 1657._

_He described the process of his discovery as follows: “I frequently and seriously bethought me, and long revolved in my mind, what might be the quantity of blood which was transmitted, in how short a time its passage might be effected, and the like; and not finding it possible that this could be supplied by the juices of the ingested aliment without the veins on the one hand being drained, and the arteries on the other hand becoming ruptured through the excessive charge of blood, unless the blood should somehow find its way from the arteries into the veins, and so return to the right side of the heart; I began to think whether there might not be a motion, as it were, in a circle. Now this I afterwards found to be true; and I finally saw that the blood, forced by the action of the left ventricle into the arteries, was distributed to the body at large, and its several parts, in the same manner as it is sent through the lungs, impelled by the right ventricle into the pulmonary artery, and that it then passed through the veins and along the vena cava, and so round to the left ventricle in the manner already indicated,--which motion we may be allowed to call circular._”

THE CIRCULATION OF BLOOD IN ANIMALS[7]

Thus far I have spoken of the passages of the blood from the veins into the arteries, and of the manner in which it is transmitted and distributed by the action of the heart; points to which some, moved either by the authority of Galen or Columbus, or the reasonings of others, will give in their adhesion. But what remains to be said upon the quantity and source of the blood which thus passes, is of so novel and unheard-of character, that I not only fear injury to myself from the envy of the few, but I tremble lest I have mankind at large for my enemies, so much doth wont and custom, that become as another nature, and doctrine once sown and that hath struck deep root, and respect for antiquity influence all men: Still the die is cast, and my trust is in my love of truth, and the candour that inheres in cultivated minds. And sooth to say, when I surveyed my mass of evidence, whether derived from vivisections, and my various reflections on them, or from the ventricles of the heart and the vessels that enter into and issue from them, the symmetry and size of these conduits,--for nature doing nothing in vain, would never have given them so large a relative size without a purpose,--or from the arrangement and intimate structure of the valves in particular, and of the other parts of the heart in general, with many other things besides, I frequently and seriously bethought me, and long revolved in my mind, what might be the quantity of blood that was transmitted, in how short a time its passage might be effected, and the like; and not finding it possible that this could be supplied by the juices of the ingested aliment without the veins on the one hand becoming drained, and the arteries on the other getting ruptured, through the excessive charge of blood, unless the blood should somehow find its way from the arteries into the veins, and so return to the right side of the heart; I began to think whether there might not be _A Motion, As It Were, In A Circle_. Now this I afterward found to be true; and I finally saw that the blood, forced by the action of the left ventricle into the arteries, was distributed to the body at large, and its several parts, in the same manner as it is sent through the lungs, impelled by the right ventricle into the pulmonary artery, and that it then passes through the veins and along the vena cava, and so round to the left ventricle in the manner already indicated. Which motions we may be allowed to call circular, in the same way as Aristotle says that the air and rain emulate the circular motion of the superior bodies; for the moist earth, warmed by the sun, evaporates; the vapours drawn upwards are condensed, and descending in the form of rain, moisten the earth again; and by this arrangement are generations of living things produced; and in like manner too are tempests and meteors engendered by the circular motion, and by the approach and recession of the sun.

And so, in all likelihood, does it come to pass in the body, through the motion of the blood; the various parts are nourished, cherished, quickened by the warmer, more perfect, vaporous, spiritous, and, as I may say, alimentive blood; which, on the contrary, in contact with these parts becomes cooled, coagulated, and, so to speak, effete; whence it returns to its sovereign the heart, as if to its source, or to the inmost home of the body, there to recover its state of excellence, or perfection.

Here it resumes its due fluidity and receives an infusion of natural heat--powerful, fervid, a kind of treasury of life, and is impregnated with spirits, and it might be said with balsam; and thence it is again dispersed; and all this depends on the motion and action of the heart.

The heart, consequently, is the beginning of life; the sun of the microcosm, even as the sun in his turn might well be designated the heart of the world; for it is the heart by whose virtue and pulse the blood is moved, perfected, made apt to nourish, and is preserved from corruption and coagulation; it is the household divinity which, discharging its function, nourishes, cherishes, quickens the whole body, and is indeed the foundation of life, the source of all action.

FOOTNOTES:

[Footnote 7: From _An Anatomical Disquisition on the Motion of the Heart-Blood in Animals_.]

VI

ROBERT BOYLE

1627-1691

_Robert Boyle, fourteenth child of the Earl of Cork, was born January 25, 1627, in Munster, Ireland. He went to Eton, studied under the rector of Stalbridge, and later traveled on the Continent under private tutors. On the death of his father in 1644, he inherited the manor at Stalbridge. At the age of eighteen he became associated with the English scientific investigators at Oxford who later founded the Royal Society, and engaged actively in physical experiments and researches. The greatest of his achievements was his discovery of the law of the compressibility of gases. He died December 30, 1691._

THE DISCOVERY OF THE LAW OF THE COMPRESSIBILITY OF GASES[8]

We took a long glass tube, which, by a dexterous hand and the help of a lamp, was in such a manner crooked at the bottom, that the part turned up was almost parallel to the rest of the tube, and the orifice of this shorter leg of the syphon (if I may so call the whole instrument) being hermetically sealed, the length of it was divided into inches (each of which was subdivided into eight parts) by a straight list of paper, which, containing those divisions, was carefully pasted all along it. Then putting in as much quicksilver as served to fill the arch or bended part of the syphon, that the mercury standing in a level might reach in one leg to the bottom of the divided paper, and just to the same height or horizontal line in the other, we took care, by frequently inclining the tube, so that the air might freely pass from one leg into the other by the sides of the mercury (we took, I say, care), that the air at last included in the shorter cylinder should be the same laxity with the rest of the air about it. This done, we began to pour quicksilver into the longer leg of the syphon, which, by its weight pressing up that in the shorter leg, did by degrees straighten the included air; and continuing this pouring in of quicksilver till the air in the shorter leg was by condensation reduced to take up but half the space it possessed (I say possessed, not filled) before, we cast our eyes upon the longer leg of the glass, upon which we likewise pasted a slip of paper carefully divided into inches and parts, and we observed, not without delight and satisfaction, that the quicksilver in that longer part of the tube was 29 inches higher than the other. Now that this observation does both very well agree with and confirm our hypothesis, will be easily discerned by him that takes notice what we teach: and Monsieur Pascal and our English friend’s [Mr. Townley’s] experiments prove, that the greater the weight is that leans upon the air, the more forcible is its endeavor of dilation, and consequently its power of resistance (as other springs are stronger when bent by greater weights). For this being considered, it will appear to agree rarely well with the hypothesis, that as according to it the air in that degree of density, and correspondent measure of resistance, to which the weight of the incumbent atmosphere had brought it, was unable to counterbalance and resist the pressure of a mercurial cylinder of about 29 inches, as we are taught by the Torricellian experiment; so here the same air being brought to a degree of density about twice as great as that it had before, obtains a spring twice as strong as formerly. As may appear by its being able to sustain or resist a cylinder of 29 inches in the longer tube, together with the weight of the atmospherical cylinder that leaned upon those 29 inches of mercury; and, as we just now inferred from the Torricellian experiment, was equivalent to them.

(_The tube broke at this point and, unable to proceed after several similar efforts, Boyle tried the converse experiment--to determine the spring of rarefied air. A tube, about 6 feet in length, and sealed at one end, was nearly filled with mercury, and into it was placed_)--

A slender glass pipe of about the bigness of a swan’s quill, and open at both ends; all along of which was pasted a narrow list of paper, divided into inches and half-quarters. This slender pipe being thrust down into the greater tube almost filled with quicksilver, the glass helped to make it swell to the top of the tube; and the quicksilver getting in at the lower orifice of the pipe filled it up till the mercury included in that was near about a level with the surface of the surrounding mercury in the tube. There being, as near as we could guess, little more than an inch of the slender pipe left above the surface of the restagnant mercury, and consequently unfilled therewith, the prominent orifice was carefully closed with sealing-wax melted; after which the pipe was let alone for a while that the air, dilated a little by the heat of the wax, might, upon refrigeration, be reduced to its wonted density. And then we observed, by the help of the above-mentioned list of paper, whether we had not included somewhat more or somewhat less than an inch of air; and in either case we were fain to rectify the error by a small hole made (with a heated pin) in the wax, and afterward closed up again. Having thus included a just inch of air, we lifted up the slender pipe by degrees, till the air was dilated to an inch, an inch and a half, two inches, etc., and observed in inches and eighths the length of the mercurial cylinder, which, at each degree of the air’s expansion, was impelled above the surface of the restagnant mercury in the tube. The observations being ended, we presently made the Torricellian experiment with the above mentioned great tube of 6 feet long, that we might know the height of the mercurial cylinder for that particular day and hour, which height we found to be 29-3/4 inches.

FOOTNOTES:

[Footnote 8: From Thorpe, _Essays on Historical Chemistry_.]

VII

CHRISTIAN HUYGHENS

1629-1695

_Christian Huyghens was born at The Hague, April 14, 1629. He studied law in Breda, but becoming attracted to the study of mathematics he neglected his legal practice for it. In 1655 he improved the method of grinding telescopic lenses, and, assisted by his brother, discovered the sixth satellite of Saturn and the fact that it was belted with rings. In 1657 he presented to the States-General the first pendulum clock. In 1678 he evolved his wave theory of light, and published it at Leyden in 1690. He died at The Hague, June 8, 1695._

THE WAVE THEORY OF LIGHT[9]

Proofs in optics, as in every science in which mathematics is applied to matter, are founded upon facts from experience--as for example, that light moves in straight lines, that the angles of incidence and reflection are equal, and that light rays are refracted in accordance with the law of sines [i. e., that the ratio between the sines of the incident and refracted ray is constant for the same substance.] For this last law is now as generally and surely known as either of the others.

Most writers in optics have been content to assume these facts, but others more curious have attempted to discover the source and reason of these phenomena, looking upon them as being in themselves interesting data. Yet although they have propounded some ingenious theories, intelligent readers still require a fuller explanation before being entirely satisfied. Therefore I herein offer some considerations on the matter with the hope of making clearer this branch of physics which has not improperly gained the reputation of being very obscure.

I feel myself particularly indebted to those that first began to study these profound subjects, and to lead us to hope them capable of orderly explanation. Yet I have been surprised to find these very investigators accepting arguments far from clear as if proof conclusive. No one has yet offered even a probable explanation of the first two remarkable phenomena of light,--why it moves in straight lines, and why rays from any and all directions can cross one another without interference.

I shall attempt in this treatise to submit clearer and more probable reasons, along the lines of modern philosophy, first for the transmission of light, and, second, for its reflection when it meets certain bodies.

Further, I shall explain the fact of rays said to undergo refraction in passing through various transparent bodies. Here I shall consider also, the refractions due to the differing densities of the atmosphere. Later I shall investigate the remarkable refraction occurring in Icelandic crystals. Finally, I shall study the different shapes necessary in transparent and reflecting bodies in order to bring together rays upon a single point or to deflect them in different ways. Here we shall see how easy it is by our new theory to determine not alone the ellipses, hyperbolas, and other curves which M. Descartes has so shrewdly constructed for this end, but as well the curve that one surface of a lens must have when the other surface is known, as spherical, plane, or any other figure.

We cannot but believe that light is the motion of a certain material. Thus when we reflect on its production, we discover that here on the earth it is usually emitted from fire and flame, and that these unquestionably contain bodies in rapid motion, since they can soften and melt many other more solid substances. If we note its effects, we see that when light is brought to a point, as, for example, by concave mirrors, it can cause combustion the same as fire: that is, it can force bodies apart, a power that certainly argues motion, at least in that true science where one believes all natural phenomena to result from mechanical causes. Moreover, in my mind we must either admit this or give up all hope of ever understanding anything in natural science.

Since, according to this philosophy, it is believed certain that the sensation of sight is produced only by the impulse of some form of matter against the nerves at the base of the eye, we have yet another reason for believing light to be a motion in the substance lying between us and the body producing the light.

As soon as we consider, moreover, the enormous speed with which light travels in every direction, and the fact that when rays come from different directions, even from those exactly opposite, they cross without interference, it must be plain that we do not see luminous objects by means of particles transmitted from the objects to us, as a shot or an arrow moves through the air. For surely this would not allow for the two qualities of light just mentioned, particularly the latter (that of speed). Light, then, is transmitted in some other way, a comprehension of which we may get from our knowledge of how sound moves through the air.

We know that sound is sent out in all directions through the medium of the air, a substance invisible and impalpable, by means of a motion that is communicated successively from one part of the air to the next; and as this movement has the same speed in all directions, it must form spherical surfaces that keep enlarging until at last they strike the ear. Now there can be no doubt that light likewise reaches us from a luminous substance through some motion caused in the matter lying in the intervening space,--for we have seen above that this cannot take place through transmission of matter from one place to another.

If, moreover, light requires time for its passage--a matter we shall discuss in a moment--it will then follow that this movement is caused in the substance gradually, and therefore is transmitted, like sound, by surfaces and spherical waves. I call these _waves_ because of their likeness to those formed when one throws a pebble into water, which are examples of gradual propagation in circles, although from a different cause and on a plane surface.

In regard to the question of light requiring time for its transmission, let us consider whether there is any experimental evidence against it.

What experiments we can make here on the earth with sources of light placed at great distances (although indicating that it does not take a sensible time for light to pass over these distances) are subject to the objection that these distances are yet too small, and that we can only argue that the movement of light is enormously fast. M. Descartes thought it to be instantaneous and based his opinion upon much better reasons taken from the eclipse of the moon. Yet as I shall make clear, even this evidence is not decisive. I shall state the matter in a somewhat different way from his in order more easily to exhibit all the consequences.

Suppose S to be the position of the sun, E A part of the orbit of the earth, S E M a straight line intersecting in M, the orbit of the moon, represented by the circle A M.

Now if light requires time--say an hour--to move the distance between the earth and the moon, then [at the time of an eclipse] it follows that when the earth has come to E its shadow, or the stoppage of the light of the sun, will not yet have reached M [the moon], and will not for an hour. Counting from the instant the earth reaches E, it will be an hour before it will reach M if it is to be obscured there. This eclipse will not be seen from the earth for yet another hour. Suppose that during these two hours the earth has moved to X, the moon appearing eclipsed at M, the sun still being seen at S. For I assume as does Copernicus that the sun is fixed and since light moves in straight lines, is always seen in its true position.

But as a matter of fact, we are assured that the eclipsed moon always appears directly opposite the sun; while on the above supposition [that light takes an hour in passing between the moon and the earth], its position ought to be back of the straight line by the angle Y X M, the supplement of the angle S X M. But this is not the case, for this angle Y X M would be very easily noticed, it being about 33 degrees. For by our analysis (found in the essay on the causes of the phenomena of Saturn), the distance from the sun to the earth, S E, is about 12,000 times the diameter of the earth, and hence 400 times the distance of the moon, which is 30 diameters. The angle X M E then will be nearly 400 times as great as E S X, which is 5 minutes, i. e., the angular distance travelled by the earth in two hours [the earth traversing almost a degree in a day]. Thus the angle E M X is almost 33 degrees, and likewise the angle M X Y, being 5 minutes greater [than E M X].

Now it must be remembered that in this computation it is assumed that the speed of light is such as to consume an hour in passing from here to the moon. But if we assume it to take only a minute of time, then the angle Y X M would amount to only 33 minutes, and if it only takes ten seconds, this angle will be less than six minutes. Now so small an angle is not observable in a lunar eclipse and hence it is not permissible to argue that light is absolutely instantaneous.

It is rather unusual, we admit, to take for granted a speed 100,000 times as great as that of sound, which (following my experiments) travels about 180 toises [about 1150 feet] in a second, or during a pulse-beat. Yet this supposition is not at all impossible, for it is not necessary to carry a body at such speed but only for motion to traverse successively from one point to another.

Hence I do not hesitate in this matter to assume that the passage of light takes time, for on this assumption all phenomena can be explained, while on the contrary supposition none of them can be explained. In fact, it seems to me and to many others as well, that M. Descartes, whose purpose has been to discuss all physical matters clearly, and who has certainly succeeded in this better than any one before him, has written nothing on light and its qualities that is not either hard to understand or even incomprehensible.

Moreover, this idea that I have propounded as an hypothesis has lately been made a well nigh established fact by that keen calculation of Roemer, whose method I will here take occasion to describe, on the expectation that he will himself in the future fully confirm this theory.

His method, the same as the one we have just discussed, is astronomical. He shows not only that light takes time for its passage, but calculates also its speed and that this must be at least six times as much as the rate I have just given as an estimate.

In his demonstration he uses the eclipses of the small satellites that revolve around Jupiter, and very frequently pass into his shadow. Roemer’s reasoning is this:

Let S be the sun, B C D E the yearly orbit of the earth, J Jupiter and G H the orbit of his nearest satellite, for this one because of its short period is better suited to this investigation than any one of the other three. Suppose G to be the point where the satellite enters, and H where it leaves, Jupiter’s shadow.

Suppose that when the earth is at B, the satellite is seen to emerge [at G], at some time before the last quarter. Were the earth to remain stationary there, 42-1/2 hours would elapse before the next emergence would take place, for this much time is taken by the satellite in making one revolution in its orbit and returning to opposition to the sun. For example, if the earth remained at B during 30 revolutions, then after 30 times 42-1/2 hours, the satellite would again be seen to emerge. If in the meantime the earth has moved to C, farther from Jupiter, it is clear that if light requires time for its passage, the emergence of the satellite will be seen later when the earth is at C than when at B. For we must add to the 30 times 42-1/2 hours, the time occupied by light in passing over the difference between the distances [of the earth from Jupiter] G B and G C, i. e., M C. So in the other quarter, when the earth travels from D to E, approaching Jupiter, the eclipses will occur earlier when the earth is at E than when at D.

Now by many observations of these eclipses throughout ten years, it is shown that these inequalities are actually of some moment, amounting to as much as ten minutes or more: whence it is argued that in traversing the whole diameter of the earth’s orbit, K L, double the distance from the earth to the sun, light takes about 22 minutes.

The motion of Jupiter in its orbit while the earth passes from B to C or from D to E has been taken into consideration in Roemer’s calculation, where it is also proved that these inequalities cannot be caused by any irregularity or eccentricity in the movement of the satellite.

Now if we consider the enormous size of this diameter K L [the earth’s orbit] which I have estimated to be about 24,000 times that of the earth, we get some comprehension of the extraordinary speed of light.

Even if K L were only 22,000 diameters of the earth, a speed traversing this distance in 22 minutes would be equal to the rate of a thousand diameters a minute, i. e., 16 2-3 diameters a second (or a pulse-beat) which makes more than 1,100 times 100,000 toises, since one diameter of the earth equals 2,865 leagues, of which there are 25 to the degree, and since in accordance with the very precise calculation made by M. Picard in 1609 under orders from the king, each league contains 2,282 toises.

As I stated before sound moves only 180 toises per second. Hence the speed of light is over 600,000 times as great as that of sound, which, however, is very different from being instantaneous,--it is the difference between any finite number and infinity. The theory that light movements are propagated from point to point in time being thus demonstrated, it follows that light moves in spherical waves, as does sound.

But if they are alike in this regard, they are unlike in others, as in the original cause of the motion that transmits them, the medium through which they move, and the manner in which they are transmitted in it.

We know that sound is caused by the rapid vibration of some body (either as a whole or in part), this vibration setting in motion the adjoining air. But light movements must arise at every point of the luminous body, otherwise all the various parts of the body would not be visible. This fact will be clearer from what follows.

In my judgment, this movement of light-giving bodies cannot be more satisfactorily explained than by supposing that those that are fluid, e. g., a flame, and probably the sun and stars, consist of particles that float about in a much rarer medium, that sets them in violent motion, causing them to strike against the still more minute particles of the surrounding ether. In the case of light-giving solids such as red-hot metal or carbon we may suppose this movement to be caused by the rapid motions of the metal or wood, the particles on the surface exciting the ether. Hence the vibration producing light must be much shorter and faster than that causing sound, since we do not find that sound disturbances give rise to light any more than the wave of the hand through the air causes sound.

The next question is in regard to the nature of the medium through which the vibration produced by light-giving bodies moves. I have named it _ether_, but it plainly differs from the medium through which sound moves. The latter is simply the air we feel and breathe, and when it is removed from any space, the medium which carries light still remains. This is shown by surrounding the sounding body in a glass vessel, and exhausting the air by means of the air-pump that Mr. Boyle has devised, and with which he has performed so many striking experiments. In trying this experiment, however, it is best to set the sounder on cotton or feathers so that it cannot communicate vibrations to the glass receiver or the air-pump, a point hitherto neglected. Then, when all the air has been exhausted, one catches no sound from the metal when it is struck.

Hence we conclude not only that our atmosphere which cannot penetrate glass is the medium through which sound acts, but that the medium carrying light-vibrations is something different: for after the vessel is exhausted of air, light passes through it as easily as before.

The last point is proven even more conclusively by the famous experiment of Torricelli. [Fill a long closed glass tube with mercury, then invert it.] The top of the glass tube not filled by the mercury contains a high vacuum, but transmits light as well as when filled with air. This demonstrates that there is within the tube some form of matter different from air, and which penetrates either glass or mercury, or both, though both are impenetrable to air. And if a like experiment is tried with a little water on top of the mercury, it becomes equally clear that the substance in question traverses either glass or water or both.

In regard to the different methods of transmission of sound and light, in the case of sound it is easy to see what happens when one remembers that air can be compressed and reduced to a much smaller volume than usual, and that it tends with the same force to expand to its original volume. This quality, considered along with its penetrability retained in spite of such condensation seems to show that it consists of small particles that float about in rapid vibration in an ether consisting of still more minute particles. Sound, then, is caused by the struggle of these particles to escape when at any point in the course of a wave they are more crowded together than at some other point.

Now the wonderful speed of light considered with its other qualities, does not permit us to believe it to be transmitted in the same manner. Therefore I shall try to explain the way in which I think it must take place. I must first, however, describe that quality of hard substances through which they transmit motion one to another. If one take a number of balls of the same size of any hard substance, and place them touching one another in one line, he will find that on letting a ball of the same size strike against one end of the line, the motion is transmitted in an instant to the other end of the line. The last ball is driven from the line while the others are apparently undisturbed, the ball that struck the line coming to a dead stop. This is an illustration of a transmission of motion at great speed, varying directly as the hardness of the balls. Yet it is certain that this transmission is not instantaneous, but requires time. For if the movement, or if you wish, the tendency to move, did not pass from one ball to another in succession, they would all be set in motion at the same instant and would all move forward at the same time. Now this is so far from the case that only the last one leaves the row, and it has the speed of the ball that first struck the line.

There are other experiments, also demonstrating that all bodies, even those thought hardest, such as steel, glass and agate, are really elastic, and bend a little, no matter whether they are in rods, balls, or bodies of any other shape,--that is, they give slightly at the point where struck, and at once regain their former shape. Thus I have discovered that in letting a glass or agate ball strike on a large, thick, flat piece of the same substance the surface of which has been roughened by the breath, the place where it strikes is shown by a circular indentation that varies in size directly as the force of the blow. This indicates that the materials give when struck and then fly back,--an event that necessarily takes time.

Now to apply such a motion to the explanation of light, there is nothing in the way of our imagining the particles of ether to have an almost complete hardness, and an elasticity as perfect as we need wish. We need not here discuss the cause of either this hardness or elasticity, as this would lead us too far from the question at issue. I will remark, however, by the way, that these particles of ether, in spite of their minuteness, are also composed of parts and that their elasticity depends on a very rapid motion of a subtle substance traversing them in all directions and making them take a structure that offers a ready passage to this fluid. This agrees with the idea of M. Descartes, except that I would not, like him, give the pores the shape of round, hollow canals. This is so far from being at all absurd or incomprehensible that it is easily credible that nature uses an infinite series of different-sized molecules in order to produce her marvelous effects.

Moreover, although we do not know the cause of elasticity, we cannot have failed to notice that most bodies possess this characteristic; hence it is not unreasonable to suppose that it is a quality of the minute, invisible particles of the ether. And it is a fact that if one looks for some other method of accounting for the gradual transmission of light, he will have a hard time finding any supposition better suited than elasticity to explain the fact of uniform speed. This [uniform speed] seems to be a necessary assumption, for if the motion slowed down when distributed over a great mass of matter at a far distance from its source, then this great speed would at last be lost. On the other hand, we suppose ether to have the property of elasticity so that its particles regain their shape with equal activity whether struck a hard or gentle blow. Thus the rate at which light would move would remain constant.

FOOTNOTES:

[Footnote 9: Translated from _Traité de la Lumière_.]

VIII

ANTHONY VAN LEEUWENHOECK

1632-1723

_Born in Delft, Holland, October 24, 1632, Anthony Van Leeuwenhoeck, a lens-maker for microscopes, made several important biological discoveries. In 1673 he noticed the red globules in the blood; in 1675 he discovered animalculæ in water; in 1677 he described the spermatozoa; in 1690 he traced the passage of blood from the arteries into the veins. Among his other achievements were his investigations of the tubules of teeth, the solidity of hair, the structure of the epidermis, and his descriptions of insect anatomies. He announced most of his findings to the Royal Society of London. Against the generally accepted idea of spontaneous generation, he held that all things generated their kind. He died at Delft, August 26, 1723._

OBSERVATIONS ON ANIMALCULÆ[10]

In the year 1675, I discovered very small living creatures in rain water, which had stood but few days in a new earthen pot glazed blue within. This invited me to view this water with great attention, especially those little animals appearing to me ten thousand times less than those represented by M. Swammerdam, and by him called water-fleas, or water-lice, which may be perceived in the water with the naked eye.

The first sort I several times observed to consist of 5, 6, 7, or 8 clear globules without being able to discern any film that held them together, or contained them. When these animalcula or living atoms moved, they put forth two little horns, continually moving. The space between these two horns was flat, though the rest of the body was roundish, sharpening a little towards the end, where they had a tail, near four times the length of the whole body, of the thickness, by my microscope, of a spider’s web; at the end of which appeared a globule of the size of one of those which made up the body. These little creatures, if they chanced to light on the least filament or string, or other particle, were entangled therein, extending their body in a long round, and endeavoring to disentangle their tail. Their motion of extension and contraction continued a while; and I have seen several thousands of these poor little creatures, within the space of a grain of gross sand, lie fast clustered together in a few filaments.

I also discovered a second sort, of an oval figure; and I imagined their head to stand on a sharp end. These were a little longer than the former. The inferior part of their body is flat, furnished with several extremely thin feet, which moved very nimbly. The upper part of the body was round, and had within 8, 10, or 12 globules, where they were very clear. These little animals sometimes changed their figure into a perfect round, especially when they came to lie on a dry place. Their body was also very flexible; for as soon as they struck against the smallest fibre or string, their body was bent in, which bending presently jerked out again. When I put any of them on a dry place, I observed that, changing themselves into a round, their body was raised pyramidal-wise, with an extant point in the middle; and having laid thus a little while, with a motion of their feet, they burst asunder, and the globules were presently diffused and dissipated, so that I could not discern the least thing of any film, in which the globules had doubtless been enclosed; and at this time of their bursting asunder, I was able to discover more globules than when they were alive.

I observed a third sort of little animals, that were twice as long as broad, and to my eye eight times smaller than the first. Yet I thought I discerned little feet, whereby they moved very briskly, both in round and straight line.

There was a fourth sort, which were so small that I was not able to give them any figure at all. These were a thousand times smaller than the eye of a large louse. These exceeded all the former in celerity. I have often observed them to stand still as it were on a point, and then turn themselves about with that swiftness, as we see a top turn round, the circumference they made being no larger than that of a grain of small sand, and then extending themselves straight forward, and by and by lying in a bending posture. I discovered also several other sorts of animals; these were generally made up of such soft parts, as the former, that they burst asunder as soon as they came to want water.

May 26, it rained hard; the rain growing less, I caused some of that rain-water running down from the house top, to be gathered in a clean glass, after it had been washed two or three times with water. And in this I observed some few very small living creatures, and seeing them, I thought they might have been produced in the leaded gutters in some water that had remained there before.

I perceived in pure water, after some days, more of those animals, as also some that were somewhat larger. And I imagine, that many thousands of these little creatures do not equal an ordinary grain of sand in bulk; and comparing them with a cheese-mite, which may be seen to move with the naked eye, I make the proportion of one of these small water-creatures to a cheese-mite to be like that of a bee to a horse; for, the circumference of one of these little animals in water is not so large as the thickness of a hair in a cheese-mite.