A single ray of light has all possible colors

Descartes says that “the globules of his elements spin on themselves, in addition to their tendency to move in a straight line, and that these different spins produce the different colors.”

But do his elements, his globules, his spinning, even need the touchstone of experience to reveal their falsity?

A multitude of demonstrations annihilate these chimeras. Here are the simplest and most evident ones.

Arrange balls next to each other: suppose they are pushed in all directions, each spinning on itself in all directions; just from this description, it is impossible for these contiguous balls to move regularly in straight lines.

Moreover, how would you see this blue point and this green point (figure 28) on a wall?

There they are marked on the wall.

They must cross in the air at point A before reaching your eyes. They intersect. Their supposed spinning must change at the point of intersection.

The spins that created the blue and the green no longer remain the same and so, there would no longer be a blue or green point.

A Flemish Jesuit made this objection to Descartes.

Descartes replied that these balls don’t actually spin, but only have a tendency to spin.

That’s what Descartes said in his letters. Is the act of the transparent as transparent any more intelligible?

You’ll tell me that:

• this difficulty exists in all systems
• these rays, which come from the blue and green points, intersect
• this intersection of rays should always prevent vision
• it is always incomprehensible how intersecting rays reach our eyes in the right order.

But this scruple will soon be resolved if you consider that any part of matter has incomparably more pores than solid substance.

A ray of sunlight, over 30 million leagues long, likely doesn’t contain even a foot of solid matter end-to-end.

So it’s entirely possible that one ray could pass through another in this material without any disturbance (figure 29).

But not only do they pass this way, they also pass over each other like two sticks.

But, you’ll say, rays emanating from a center wouldn’t precisely, in strict mathematical terms, reach the same point on a circumference.

There will always be a tiny deviation.

Two men wouldn’t see the exact same point of the same object.

Of a billion people looking at a surface, no two will see exactly the same point.

In Descartes’ “plenum,” this intersection of rays is impossible.

But everything is equally impossible in a full universe. There is no motion at all that doesn’t imply and prove the existence of the void.

Malebranche comes next and says:

“Descartes was mistaken. His spinning globules are untenable. But it’s not light globules, it’s tiny spinning vortices of subtle matter, capable of compression, that cause colors. Colors, like sounds, consist in pressure vibrations.”

It seems impossible to discover the exact relationships of these (color) vibrations”.

But note he was speaking this way to the Academy of Sciences in 1699, while these relationships had already been discovered in 1675—not the vibration proportions of non-existent vortices, but the proportions of the refrangibility of rays that contain colors.

What he thought impossible had already been demonstrated visually and confirmed by the senses—which would have greatly displeased Father Malebranche.

Other philosophers sense the weakness of these assumptions and say:

“colors come from more or fewer rays reflected by colored bodies.

White reflects the most. Black reflects the least.

Brighter colors bring more rays to the eye.

Red strains the eyes a bit, so it has more rays than green, which is more restful.""

This hypothesis appears to be a crude error as soon as you compare a painting in dim light and in bright light.

You always see the same colors. White lit by a single candle remains white; and green lit by a thousand candles remains green.

Newton will tell you to believe only your eyes and mathematics.

Sit in a completely dark room where light enters only through a very small hole. The light beam will strike paper and reveal the color of whiteness.

Place a glass prism transversely in that beam (figure 30), then about sixteen or seventeen feet away, place a sheet of paper facing the prism.

You know that light bends (refracts) as it enters the prism from air, and bends again the other way as it exits into air. If it didn’t bend, it would fall on the floor of the room at point Z. But since it must deviate, it will hit the paper.

There you will see the entire secret of light and color.

This beam isn’t a single ray, as once thought; it’s a bundle of seven principal rays, each carrying a primordial color unique to itself.

All the colors of nature are born from combinations of these seven; and when they are all reflected together, they create whiteness.

Study this admirable mechanism. We’ve already hinted that not all light rays refract equally; what happens here is clear visual proof.

These seven rays escape from the body of that beam, splitting as it exits the prism, and arrange themselves on the paper, each occupying an oval area.

The ray with the least “rigidity” or matter bends the most. The strongest ray (figure 31), the densest, bends the least.

Do you see these seven rays breaking apart above one another?

Each paints on the paper the color it inherently contains.

1. Red bends the least.
2. then orange
3. the third, yellow
4. the fourth, green
5. the fifth, blue
6. the sixth, indigo
7. violet bends most, the last to rise above the others

So a single beam of white light is actually composed of 7 beams, each with its own color.

• Their combination is white.

Take a convex lens like those in glasses, which bring rays to a focus.

Expose it to the incoming beam: you’ll only see a white circle at the focus.

Now use the same lens to focus the seven rays from the prism: it recombines them into one beam (figure 32). The color is white. Therefore, all rays together produce whiteness.

Black, by contrast, is a body that reflects no rays.

If you isolate a ray from the prism and expose it to a mirror, a burning glass, or another prism, it never changes color, never separates into others.

To carry such a color is its essence—unalterable, like gold in a crucible.

Newton called this property of light—this unevenness in how its rays refract as refrangibility.

People denied when Mariotte had failed to replicate Newton’s experiments in France.

It was easier to say Newton had lied than to say Mariotte was unlucky with his prisms.

Even after the experiments succeeded, prejudice persisted.

As late as 1730, some journals and books denied these well-known experiments performed across Europe.

Similarly, after the discovery of blood circulation, some still argued against it and mocked those who supported it, calling them “circulators.”

When forced to accept the facts, critics still resisted the terminology.

They objected to “refrangibility” just as they had to “attraction” and “gravitation.” But what does a term matter, as long as it points to a truth? When Columbus discovered Hispaniola, he could name it what he liked. Shouldn’t inventors name what they discover?