How the Rainbow is an effect of the laws of refrangibility

New discoveries on the cause of colors, which confirm the preceding doctrine.

Demonstration that colors are caused by the thickness of the parts that compose bodies, without the light being reflected from these parts.

More in-depth knowledge of the formation of colors. Great truths drawn from a common experiment.

Newton’s experiments. Colors depend on the thickness of the parts of bodies, without these parts themselves reflecting the light.

All bodies are transparent. Proof that colors depend on thickness, without the solid parts actually sending back the light.

All colors come to us from the mixture of the seven primordial colors that the rainbow and the prism show us distinctly[1].

The bodies most suitable for reflecting red rays, and whose parts absorb or let other rays pass through, will be red, and so on.

This does not mean that the parts of these bodies actually reflect the red rays; but that there is a power, a force hitherto unknown, which reflects these rays from near the surfaces and from within the pores of the bodies.

Colors are therefore in the sun’s rays, and bounce back to us from near the surfaces, and the pores, and the void.

What is the apparent power of bodies to reflect these colors to us consists of, which makes scarlet appear red, meadows green, and a clear sky blue: for to say that this comes from the difference of their parts is to say a vague thing that teaches nothing at all.

A child’s pastime, which seems to have nothing in itself but what is contemptible, gave Mr. Newton the first idea of these new truths that we are going to explain. Everything must be a subject of meditation for a philosopher, and nothing is small in his eyes.

He noticed that in these soap bubbles that children make, the colors change from moment to moment, counting from the top of the bubble as the thickness of this bubble decreases, until finally the weight of the water and soap which always falls to the bottom breaks the balance of this light sphere, and makes it disappear.

He presumed that colors could well depend on the thickness of the parts that make up the surfaces of bodies, and, to be sure of this, he performed the following experiments.

Let two crystals touch at one point: it doesn’t matter if they are both convex[2]; it is enough for the first to be so, and for it to be placed on the other in this way.

Let water be placed between these two glasses (figure 40) to make the experiment more perceptible, which also takes place in the air; when these glasses are pressed a little against each other, a small transparent black spot appears at the point of contact of the two glasses: from this point, surrounded by a little water, colored rings are formed in the same order and in the same way as in the soap bubble.

Finally, by measuring the diameter of these rings and the convexity of the glass, Newton determined the different thicknesses of the parts of water that gave these different colors; he calculated the thickness necessary for water to reflect white rays:

This thickness is about four parts of an inch divided into a million, that is to say four millionths of an inch; azure blue and colors leaning towards violet depend on a much smaller thickness. Thus the smallest vapors that rise from the earth, and which color the air without clouds, being of a very thin surface, produce this celestial blue that charms the eye.

Other equally subtle experiments have also supported this discovery, that colors are attached to the thickness of the surfaces.

The same body that was green when it was a little thick became blue when it was made thin enough to reflect only blue rays, and to let others pass through. These truths, of such a delicate research and which seemed to elude human sight, well deserve to be followed closely; this part of philosophy is a microscope with which our mind discovers infinitely small magnitudes.

All bodies are transparent, you just have to make them thin enough so that the rays, finding only one sheet, one leaf to pass through, pass through this sheet. Thus, when gold leaf is exposed to a hole in a dark room, it sends back yellow rays through its surface which cannot be transmitted through its substance, and it transmits green rays into the dark room, so that the gold then produces a green color: a new confirmation that colors depend on the different thicknesses.

An even stronger proof is that, in the experiment of this convex-plane glass, touching this convex glass at one point, water is not the only element which, in various thicknesses, gives various colors: air has the same effect; only the colored rings it produces between the two glasses have a larger diameter than those of water.

There is therefore a secret proportion established by nature between the strength of the constituent parts of all bodies and the primitive rays that color the bodies; the thinnest sheets will give the weakest colors; and to give black, exactly the same thickness, or rather the same thinness, the same fineness, as the small upper part of the soap bubble, in which a small black point was perceived, will be necessary, or else the same thinness as the point of contact of the convex glass and the flat glass, which contact also produces a black spot.

But, once again, one should not believe that bodies send back light by their solid parts, on the grounds that colors depend on the thickness of the parts.

There is a power attached to this thickness, a power that acts near the surface; but it is not at all the solid surface that repels, that reflects. This truth will be even more visibly demonstrated in the next chapter than it has been proven so far. It seems to me that the reader must have reached the point where nothing should surprise him anymore; but what he has just seen leads even further than one thinks, and so many singularities are, so to speak, only the frontiers of a new world.