God: Reasons not appreciated by all minds. Reasons of the materialists.

Having now understood what light is, where it comes from, how and in what time it reaches us, let us look at its properties and its effects—effects unknown until our day. The first of these effects is that it seems to rebound from the solid surface of all objects, to bring their images into our eyes.

All men, all philosophers—even Descartes, Malebranche, and those who departed furthest from common ideas—have equally believed that it was indeed the solid surfaces of bodies that returned the rays to us. The smoother and more solid a surface, it was said, the more it reflects light; the more a body has large, straight pores, the more it lets rays pass through its substance. Thus the polished mirror, whose back is coated with a surface of quicksilver, reflects all the rays to us; thus that same mirror without quicksilver, having straight and wide pores in great number, lets a large part of the rays pass through. The wider and straighter a body’s pores, the more transparent it is: such, they said, is the diamond; such is water itself. These were the generally accepted ideas, and no one doubted them.

Yet all these ideas are entirely false—so often what seems most plausible is furthest from the truth. Philosophers fell into this error in the same way the vulgar do, when they think that the sun is no bigger than it appears to the eye. Here is the philosophers’ mistake.

There is no body whose surface we can truly make perfectly smooth. Yet many surfaces appear smooth to us, and perfectly polished. Why do we see smooth and even what is not? The most even surface is, compared to the tiny bodies that compose light, only a heap of mountains, cavities, and gaps—just as the tip of the finest needle is in fact bristling with ridges and roughness that the microscope reveals.

All the bundles of light rays that fall on these unevennesses would reflect according to how they struck them: thus, since they strike unevenly, they would never reflect regularly—and so we would never see ourselves in a mirror. Moreover, glass probably has a thousand times more pores than matter; yet every point of its surface sends rays back, therefore they are not returned by the glass.

The light that brings our image from a mirror thus certainly does not come from the solid parts of that mirror’s surface; it also does not come from the solid parts of the mercury and tin spread behind the glass. These parts are no more flat, no more even, than the glass itself. The solid parts of tin and mercury are incomparably larger and broader than the solid particles that make up light; so if tiny particles of light strike these large parts of mercury, they will scatter in every direction like lead pellets falling on rubble. What unknown power, then, makes light rebound toward us in an orderly way? It already appears that it is not the bodies that send it back to us. What seems most known, most indisputable to men, becomes a greater mystery than the weight of air once was. Let us examine this problem of nature—our astonishment will only grow. One can learn here only with surprise.

Take a piece—a cube of crystal, for example; here is what happens to the rays of the sun that fall on this solid and transparent body (figure 2):

1. A small part of the rays rebounds to your eyes from its first surface A, without even touching that surface, as will be more fully proven.

2. A very small part of the rays is received into the substance of this body at B; it plays there, is lost there, and is extinguished there—which is why few crystals are perfectly transparent, especially when they are thick.

3. A third part reaches the interior C of the glass, and from near the surface it returns into the air, and some rays come to your eyes.

4. A fourth part passes into the air.

5. A fifth part—the largest—comes back from beyond the far surface D into the crystal, passes through it again, and reflects to your eyes. Let us examine only these last rays, which, escaping from the far surface D, and encountering the air, rebound from this air toward the eye, re‑entering through the crystal.

Certainly they did not meet in this air solid parts on which they could have bounced: for if instead of air they encounter water at this surface B, few rays return then; they enter the water, they penetrate it in great number. Yet water is about 800 to 900 times denser, more solid, less rarefied than air. Still, these rays do not rebound from the water, and yet they rebound from the air in this glass: therefore it is not from the solid parts of bodies that light is reflected.

Here is an even more singular and decisive observation: Expose in a dark room this crystal A B (figure 3) to the rays of the sun, so that the beams of light reaching its surface B form an angle of more than 40 degrees with the perpendicular P.

Most of these rays then no longer enter the air: they all re‑enter this crystal the instant they leave it; they come back, as you see, in an imperceptible curve.

Certainly it is not the solid surface of the air that pushed them back into the glass; several of these rays entered the air before, when they fell less obliquely; why then, at an obliquity of 40 degrees 19 minutes, do most of these rays no longer pass through? Do they find at this angle more resistance, more matter in this air than they find in the crystal they had penetrated? Do they find more solid parts in the air at 40 degrees and one‑third than at 40? Air is about 2,400 times rarer, lighter, less solid than crystal: thus these rays should have passed into the air with 2,400 times more ease than they penetrated the thickness of the crystal. Yet, despite this overwhelming appearance of ease, they are repelled: they are therefore repelled by a force here 2,400 times more powerful than air; they are therefore not repelled by air; the rays, once again, are not reflected to our eyes by the solid parts of bodies.

Light rebounds so little from the solid parts of bodies that it sometimes rebounds, in fact, from the void itself: this fact deserves great attention.

You have just seen that light, falling at an angle of 40 degrees 19 minutes on crystal, rebounds almost entirely from the air it encounters at the far surface of this crystal; that if the light falls at an angle smaller by a single minute, even less passes from that surface into the air.

Newton asserted that if one found the way to remove the air from beneath this piece of crystal, then no rays would pass through, and all the light would reflect: I tested this; I had an excellent prism set in the middle of a copper plate; I fitted this plate to the top of an open receiver placed on the pneumatic machine; I brought the machine into my dark room. There, letting light in through a hole onto the prism, and making it fall at the required angle, I pumped out the air for a very long time; those present saw that as we pumped the air, less and less light passed into the receiver, and finally almost none passed through at all. It was a very pleasant sight to see this light reflect off the prism, entirely onto the floor.

The experiment thus demonstrates that light, in this case, rebounds from the void; but we know well that the void cannot act. What then can we conclude from this experiment? Two very palpable things: first, that the surface of solids does not return light; second, that there is in solid bodies an unknown power that acts on light—and it is this second property that we will examine later.

Here we need only prove that light is not reflected to us by solid parts.

Here is another proof of this truth.

Any opaque body, reduced to a thin sheet, lets rays of a certain kind pass through its substance, and reflects other rays; yet if light were sent back by bodies, all the rays that fall equally on these sheets would be reflected from them. Finally, we shall see that never has such an astonishing paradox been proven in so many ways. Let us then begin to familiarize ourselves with these truths.

1. This light, thought to be reflected by the solid surface of bodies, in fact rebounds without having touched that surface.

2. Light is not sent back from behind a mirror by the solid surface of quicksilver; it is sent back from within the pores of the mirror, and the pores of the quicksilver itself.

3. The pores of this quicksilver do not need to be very small to reflect light, as was thought until now; on the contrary, they must be wide.

This will be another cause of surprise, for those who have not studied this philosophy, to hear that the secret of making a body opaque is often to widen its pores, and that the way to make it transparent is to narrow them. The order of nature will appear completely reversed: what seemed to produce opacity will, in fact, produce transparency; and what seemed to produce transparency will be what makes bodies opaque. Yet nothing is more true, and even the simplest experiments demonstrate it.

Dry paper, whose pores are very large, is opaque: no ray of light passes through it; narrow its pores by soaking it with water or oil, and it becomes transparent; the same happens with cloth, with salt.

It is good to inform the public that a man who recently wrote against these truths, with much more arrogance and scorn than knowledge, had wanted to mock Newton over these discoveries. “If the secret,” he said, “of making a body transparent is to narrow its pores, then we must make windows smaller to have more light in our rooms,” etc.

I reply that it is quite improper to play the joker when one claims to speak as a philosopher, and that mocking Newton is too bold an enterprise; I reply above all that this joker should have considered that it is very true that large openings, whose light is blocked, will not give light; and that a thin body, pierced with an infinity of tiny straight holes exposed to the sun, illuminates us greatly. Oiled paper, wet cloth, for example, are thin bodies whose pores the oil or water has narrowed and straightened, and the light passes through these more regular pores; but it will not pass through the largest sieves if their holes cross and intercept the rays.

Before taking a mocking tone, one should be very sure one is right; and when one is certain at last of being right, one should not mock.

Let us return, and summarize that there are unknown principles that produce these marvels—causes that make light rebound before it touches a surface, that send it back from the pores of transparent bodies, that return it from the midst of the void itself. We are unavoidably compelled to admit these facts, whatever the cause may be.

Let us then study the other mysteries of light, and see whether from these surprising effects we may rise to some incontestable principle that must be admitted as much as the effects themselves.