How the Rainbow is an effect of the laws of refrangibility

Continuation of these discoveries; mutual action of bodies on light. — Very singular experiment. Consequences of these experiments. Mutual action of bodies on light. This whole theory of light is related to the theory of the universe. Matter has more properties than one thinks.

With the reflection of light, its inflection, its refraction, its refrangibility being known, the origin of colors being discovered, and the very thickness of bodies necessary to cause certain colors being determined, we still have to examine two properties of light, no less astonishing and no less new.

1. The power of light to act near the surfaces.

It is a mutual action of light on bodies, and of bodies on light.

2. A relationship between colors and musical tones, between the objects of sight and those of hearing.

But here we will only speak of the reciprocal action of bodies on light, because it relates to the great principle of nature by which all bodies act on each other.

Regarding the analogy between the seven primitive colors and the seven musical tones, it is a discovery that is not yet deep enough, which cannot yet lead to anything.

These two crystals, touching at one point, produce rings of different colors, red, blue, green, white, etc. Perform this same test in a dark room, where you have done the experiment with the prism exposed to the light that comes to it through a hole. You remember that, in this prism experiment, you saw the decomposition of light and the anatomy of its rays: you placed a sheet of white paper opposite this prism; this paper received the seven primitive colors, each in its order. Now expose your two glasses to any colored ray you please, reflected from this paper: you will always see colored rings form between these glasses; but all these rings are then of the color of the rays that come to you from the paper. Expose your glasses to the light of red rays, you will only have red rings between your glasses (figures 41 and 42); but what must surprise you is that between each of these red rings there is a completely black ring. To confirm this fact and the singularities attached to it even more, present your two glasses, no longer to the paper, but to the prism, so that one of the rays that escape from this prism, a red one for example, comes to fall on these glasses: only red rings are still formed between the black rings; place the sheet of white paper behind your glasses: each black ring produces a red ring on this sheet of paper, and each red ring, being reflected towards you, produces black on the paper.

It follows from this experiment that the air or water which is between your glasses reflects light in one place, and in another place lets it pass, transmits it. I confess that I cannot admire enough here this depth of research, this more than human sagacity, with which Newton pursued these so imperceptible truths; he recognized by measurements and by calculation these strange proportions.

At the point of contact of the two glasses, no light is reflected to our eyes: immediately after this contact, the first small layer of air or water that touches this black point reflects rays to you; the second layer is twice as thick as the first, and reflects nothing; the third layer is triple in thickness to the first, and reflects; the fourth layer is four times thicker, and does not reflect; the fifth is five times thicker, and reflects; and the sixth, six times thicker, transmits, and does not reflect.

So that the black rings go in this progression, 0, 2, 4, 6, 8; and the luminous and colored rings in this progression, 1, 3, 5, 7, 9[1]. What happens in this experiment happens in the same way in all bodies, which all reflect a part of the light, and receive another part into their substances. It is therefore a property again demonstrated to the mind and to the eyes, that the solid surfaces are not what reflect the rays. For, if the solid surfaces actually reflected: 1° the point where the two glasses touch would reflect and would not be dark; 2° each solid part that would give you a single kind of rays should also send you back all kinds of rays; 3° the solid parts would not transmit light in one place, and would not reflect it in another place, because, being all solid, they would all reflect; 4° if the solid parts reflected light, it would be impossible to see oneself in a mirror, as we have said, since the mirror, being furrowed and rough, could not send back the light in a regular manner. It is therefore certain that there is a power acting on bodies, without touching bodies, and that this power acts between bodies and light. Finally, far from light bouncing off the bodies themselves and returning to us, we must believe that the greater part of the rays that go to strike solid parts, remain there, are lost there, are extinguished there.

This power, which acts on the surfaces, acts from one surface to the other: it is mainly from the last ultimate surface of the transparent body that the rays bounce back; we have already proven it. It is, for example, from the points B B B (figure 43), more than from this point A, that the light is reflected.

We must therefore admit a power, which acts on the rays of light from one of these surfaces to the other, a power that alternately transmits and reflects the rays. This interplay of light and bodies was not only suspected before Newton; he counted several thousands of these alternative vibrations, of these transmitted and reflected jets. This action of bodies on light, and of light on bodies, still leaves many uncertainties in the way of explaining it.

The one who discovered this mystery could not, in the course of his long life, do enough experiments to assign the certain cause of these effects. But, if by his discoveries he had only taught us new properties of matter, would not that already be a great enough service rendered to philosophy[2]? He does not dwell on it in any way; he was content with the facts, without daring to determine anything about the causes.

We will therefore not push this introduction to light any further, perhaps we have said too much in simple elements; but most of these truths are new for many readers. Before moving on to the other part of philosophy, let us remember that the theory of light has something in common with the theory of the universe which we are about to enter. This theory is that there is a kind of marked attraction between bodies and light, just as we are going to observe one between all the globes of our universe: these attractions are manifested by different effects; but it is always a tendency of bodies towards each other, discovered with the help of experiment and geometry.

Among so many properties of matter, such as these alternations of transmission and reflection of light rays[3], this repulsion that light experiences in a vacuum, in the pores of bodies and on the surfaces of bodies; among these properties, I say, we must especially pay attention to this power by which rays are reflected and bent, to this force by which bodies act on light, and light on them, without even touching them. These discoveries must at least serve to make us extremely cautious in our decisions about the nature and essence of things. Let us remember that we know nothing at all except by experience. Without touch, we would have no idea of the extent of bodies; without eyes, we could not have guessed light; if we had never experienced movement, we would never have believed matter to be mobile; a very small number of senses that God has given us serve to discover to us a very small number of properties of matter. Reason supplies the senses we lack, and still teaches us that matter has other attributes, such as attraction, gravitation; it probably has many others that relate to its nature, and of which perhaps one day philosophy will give some ideas to men.

For my part, I confess that, the more I reflect on it, the more surprised I am that people are afraid to recognize a new principle, a new property in matter. It probably has an infinite number of them; nothing resembles anything else in nature. It is very probable that the Creator made water, fire, air, earth, plants, minerals, animals, etc., on entirely different principles and plans. It is strange that people rebel against new riches that are presented to us: for is it not enriching man to discover new qualities of the matter from which he is formed[4]?

↑ It is a question here of the average thickness of the layers of air. The diameters are proportional to the square roots of the thicknesses. (D.)

↑ In the 1738 editions, after the word philosophy, one read here: “He conjectured that light emanates from the sun and luminous bodies by alternations, by vibrations; that of these vibrations of the luminous body the first operates a reflection, the second a transmission, and so on to infinity. He had also prepared experiments which led to showing how this interplay of nature is related to the great principle of attraction; but he did not have time to finish his experiments. He had also conjectured that there is in nature a very elastic and very rare matter, which becomes less rare the further it is from opaque bodies; that the light rays excite vibrations in this elastic matter; and it must be admitted that this hypothesis would explain almost all the mysteries of light, and especially the attraction and gravitation of bodies; but a hypothesis, even if it could explain everything, should not be admitted. It is not enough for a system to be possible to deserve to be believed, it must be proven. If Descartes’ vortices could be sustained against all the difficulties with which they are overwhelmed, they would still have to be rejected, because they would only be possible: thus we will make no real foundation on Newton’s own conjectures. “If I speak of it, it is rather to make known the history of his thoughts than to draw the slightest induction from his ideas, which I regard as the dreams of a great man: he does not dwell on them in any way, he was content with the facts, without daring to determine anything about the causes. Let’s move on to the other discovery on the relationship that exists between the rays of light and the tones of music.” From the 1741 edition, almost all of this passage was suppressed. The author had only kept the last four lines, starting from the words: He does not dwell on it, etc. (B.) ↑ Voltaire only cites the name of this famous theory. Newton admits that the luminous molecules in which he makes the luminous matter consist have ends of different shapes, and that they acquire a rotational movement on themselves, in addition to the translational movement; depending on the end that presents itself, there is easy reflection or easy transmission. This theory, so ingenious, fell with the system. (D.) ↑ In the 1738 editions, and even in that of 1741, chapter xiii ended with the variant that was read, page 501. After which came a chapter xiv, which the author suppressed after 1741, and which is as follows: