Errors of Descartes on Light

The Greeks have said:

“Light is an accident. This accident is the act of the transparent as transparent. Colors are what move transparent bodies. Luminous and colored bodies have qualities similar to those they stir in us, for the great reason that nothing can give what it does not have. Finally, light and colors are a mixture of hot, cold, dry, and moist; for the moist, the dry, the cold, and the hot being the principles of everything, colors must certainly be a compound of them.”

This absurd gibberish persisted in several schools.

Descartes said:

Light is a fine, subtle matter which strikes our eyes. Colors are the sensations that God awakens in us, according to the various movements by which this matter reaches our organs.

Descartes

So far, Descartes was right. He should have stopped there. But he wanted to establish a system.

He had laid down as the first foundation of his philosophy that nothing should be believed without evidence.

Yet, in contempt of his own rule, he imagined 3 elements formed of so‑called cubes, which he supposed to have been made by the Creator and to have broken apart by turning on themselves as they emerged from God’s hands.

These 3 imaginary elements were, as is well known:

The thickest part of those cubes — the coarse element, from which, according to him, were formed the solid bodies of the planets, the seas, and even the air;

The impalpable dust produced by the shattering of these dice, which infinitely fills the interstices of the infinite universe (in which he supposed there was no void);

The middles of those supposed broken dice, worn equally on all sides and finally rounded into spheres — which he fancied into being light itself, scattering them freely throughout the universe.

The more ingeniously this system was imagined, the more one feels that it was unworthy of a philosopher. And since none of this was proven, one might just as well have adopted the hot and the cold, the dry and the moist.

Error for error — what does it matter which one prevails?

According to Descartes, light does not travel from the sun to our eyes; rather, it is a globular matter spread everywhere, which the sun pushes, and which presses our eyes — like a stick pushed at one end presses instantly at the other.

He was so persuaded of this system that, in his seventeenth letter of the third volume, he states and repeats explicitly:

“I confess that I know nothing in philosophy, if the light of the sun is not transmitted to our eyes in an instant.”

Descartes was a great genius. Yet he knew little true philosophy.

He lacked the experience of the century that followed him. That century is as superior to Descartes as Descartes was to antiquity.

1. If light were a fluid always spread through the air, we would see clearly at night — since the sun, under the horizon, would always push this fluid of light in every direction, and the impression would reach our eyes. Light would circulate like sound.

We would see an object beyond a mountain; in fact, we would never have so fine a day as during a total solar eclipse — for the moon, passing between us and the sun, would (at least according to Descartes) press the globules of light and only increase their action.

2. The rays deflected by a prism and forced to take a new path demonstrate that light actually moves and is not merely a heap of globules simply pressed.

Light follows three different paths when entering a prism: its three routes — in the air, in the prism, and out of the prism — are different; more than that, it accelerates its movement in the prism’s body.

Is it not then strange to say that a body which visibly changes place three times and increases its motion does not move at all?

And yet, a book has just appeared that dares to say that the progression of light is an absurdity.

3. If light were a mass of globules — a fluid existing in the air and everywhere — a small hole made in a dark room should illuminate it entirely: because the light, then pushed in all directions through this tiny hole, would act everywhere, just as ivory balls arranged in a ring or square would all move if a single one were strongly pressed.

But the opposite happens: light received through a small opening, which lets through only a small cone of rays and travels twenty‑five feet, illuminates barely half a foot of the spot it strikes.

4. It is known that light, emanating from the sun to us, crosses in about eight minutes that immense distance — which a cannonball, maintaining its speed, could not do in twenty‑five years.

The author of The Spectacle of Nature, a very estimable work, fell into an error here that can mislead beginners for whom his book is written.

He says that light comes from the stars in seven minutes, according to Newton: he mistook the stars for the sun.

Light from the nearest stars reaches us in six months, according to a certain calculation based on very delicate — and very faulty — experiments.

It is not Newton, but Huygens and Hartsoeker, who made this supposition.

He adds, to prove that God created light before the sun, that light is spread through all nature and is felt when luminous stars push it; but it has been demonstrated that it arrives from fixed stars in a very long time.

If it must travel that distance, it was clearly not spread beforehand.

It is good to be on guard against such errors, repeated daily in many books that are echoes of each other.

Roemer’s Demonstration

Here, in a few words, is the substance of Roemer’s visible demonstration that light takes seven to eight minutes on its journey from the sun to the earth.

From the earth at C, one observes a satellite of Jupiter (figure 1), which is eclipsed regularly once every 42½ hours.

If the earth were immobile, the observer at C would see thirty emergences of this satellite in thirty times 42½ hours; but at the end of that time, the earth is now at D.

The observer no longer sees this emergence exactly after those thirty intervals: one must add the time light takes to travel from C to D, and that time is measurably significant.

But this space CD is still smaller than space GH on this circle.

This circle is the great orbit described by the earth; the sun is at its center; light coming from Jupiter’s satellite crosses CD in 10 minutes and GH in 15 or 16 minutes.

The sun is between G and H: therefore light comes from the sun in seven or eight minutes.

This beautiful observation was long contested; finally, one had to concede the experiment, and prejudice tried to evade the experiment itself.

“It proves at most,” people said, “that the matter of light existing in space, and continuous from the sun to our eyes, takes seven to eight minutes to transmit the sun’s impression to us.”

But should one not see that such an offhand answer contradicts all mechanical principles?

Descartes knew well — and said — that if the matter of light were like a long stick pressed by the sun at one end, the impression would be communicated instantly to the other end.

Thus, if a satellite of Jupiter pressed a supposed luminous matter — considered as a thread of globules, stiff, stretched to our eyes — we would not see the satellite’s emergence minutes later, but at the instant of the emergence.

If, as a last subterfuge, one retreats to saying that the matter of light must be regarded not as a rigid body but as a fluid, then one falls back into an error unworthy of any physicist, an error that betrays ignorance of the action of fluids: for such a fluid would act in all directions, and — as has been said — there would never be night or eclipse.

The movement would be far slower in this fluid, and it would take centuries instead of seven minutes for the sunlight to reach us.

The discovery of Roemer therefore proved incontestably the propagation and progression of light.

If ancient prejudice still struggles against such a truth, let it at least yield to the new discoveries of Mr. Bradley, which confirm it so admirably.

Bradley’s experiment is perhaps the finest feat ever accomplished in astronomy.

(🔽 The following paragraphs narrate Bradley’s experiment, the design of the parallactic telescope, and how he and earlier astronomers like Hooke, Flamsteed, Molyneux, and Graham determined the parallax of the Earth by measuring subtle shifts in the position of a star in Draco. They confirmed that light not only travels with enormous speed but that its travel time is uniform across stars at vastly different distances — a final, devastating blow to Descartes’s “luminous matter” system, and a triumph for empirical science.)

In the end, the passage concludes:

Newton, Roemer, Bradley, etc., conducted only experiments, and judged solely by facts — unlike Descartes, who “imagined,” who “did not examine this world; he created one.”