Errors of Descartes on Light

Father Malebranche examined the errors of the senses. He was not himself free from the subtler errors of genius.

He adopted without proof Descartes’ 3 elements.

But he altered many things in that enchanted castle, and, making even fewer experiments than Descartes, he, too, built a system.

According to him, vibrations of the luminous body produce shocks on tiny soft vortices, capable of compression, all made of subtle matter.

How do these tiny soft vortices could transmit light to our eyes?

How the action of the sun could pass instantly through so many small compressed bodies—of which even a very small number would suffice to dampen that action?

How these vortices would not mix as they whirled against each other?

How they could be elastic?

Finally, why he supposed vortices at all—what would Father Malebranche have answered?

On what foundation did he build this imaginary edifice?

Must it be that men who spoke only of truth wrote nothing but fairy tales!

One experiment destroys all these so‑called luminous vortices.

Receive sunlight on a concave mirror; oppose to this concave mirror, as best you can, a lens so that the two tips of the two luminous cones meet in the air.

You create the most intense heat possible on Earth.

If the tips of these cones were vortices striving to escape in all directions, as is claimed, would they not make at the meeting point a prodigious combat?

Wouldn’t the effect be perceptible at some distance from the cone tips? Yet one inch away from this point you feel not the slightest warmth: imagine after that tiny vortices.

What, then, is the matter of light?

It is fire itself. It is fire which burns at a short distance when its parts are less tenuous, or more rapid, or more concentrated, and which softly illuminates our eyes when it acts from farther away, when its particles are finer and less rapid and less concentrated.

Thus a lit candle would burn the eye only a few lines from it, and illuminates the eye several inches away.

Thus the sun’s rays, scattered through the air, illuminate objects, and, concentrated by a burning glass, melt lead and gold.

What is fire?

It is an element I know only by its effects.

Man is not made to know the intimate nature of things.* He can only calculate, measure, weigh, and experiment.

Fire does not always give light, and light does not always shine.

But only the element of fire can illuminate and burn.

Fire not yet released—whether in an iron bar or in wood—cannot send out rays from the surface of that wood or iron; therefore it cannot be luminous; it only becomes so when that surface is ablaze.

The rays of the full moon produce no perceptible heat in the focus of a burning glass, although they produce considerable light. The reason is clear.

The degrees of heat are always proportional to the density of the rays.

The sun, at the same elevation, darts 90,000 times more rays than the full moon reflects to us on the horizon.

Thus, for moonlight rays at the focus of a burning glass to produce only as much heat as sunlight would on a surface of the same size as the glass, there would need to be 90,000 times more rays there than there are.

Those who have wished to make 2 beings of light and fire have therefore been mistaken.

They based themselves on the fact that not every fire gives light and not every light gives heat: it is as if one made two beings of everything that can serve two uses.

This fire is darted in all directions from the radiant point.

• That is why it is seen from every side.

It must therefore always be considered with the geometers as lines radiating from a center to the circumference.

Thus every bundle, every mass, every shaft of rays, coming from the sun or any fire, must be considered as a cone whose base is on our pupil and whose tip is in the fire that darts it.

This matter of fire shoots from the sun to us and to Saturn, etc., with a speed that overwhelms the imagination.

Calculation shows that the sun is 24,000 Earth radii away.

It follows that light travels from that star to us (in round numbers) a billion feet per second.

But a one‑pound cannonball, pushed by a half‑pound of powder, travels in one second only six hundred feet; therefore the speed of a ray of sunlight is, roughly, 1,666,600 times greater than that of a cannonball:

Therefore, if a single atom of light were only the 1/1,600,000 part of a pound, rays of light would necessarily have the effect of cannon.

Even if they were a thousand billion times smaller, a single moment of light emission would destroy everything living on the Earth’s surface.

How inconceivably small, then, must these rays be to enter our eyes without harming them?

The sun, which darts this luminous matter at us in 8 minutes.

The stars send it to us in several years.

They supply light endlessly without being exhausted—much as musk endlessly sends out odorous particles around itself without losing perceptible weight.

The sun hurls its rays is proportional to its mass.

• Its mass is around 1 million times greater than that of the Earth

The sun rotates in 25.5 days.

Some people imagined that I claimed this light was attracted by the Earth from the substance of the sun.

• But I have never said such thing.

Others claimed that the sun would soon lose all its substance and must send millions of pounds of light every minute.

But if one considers that light barely weighs, that perhaps the sun gives off barely an ounce per year, and that it receives from all the other suns, one would not make such hasty criticisms.

The speed of the sun’s substance going to us in a straight line disproves Descartes’ plenum.

1. How could a straight line reach us through millions of layers of matter moving in curved lines, and through so many different movements?*

2. How could such a subtle body in 8 minutes cross the space of 400,000 times 33,000,000 leagues from a star to us if it had to penetrate through resisting matter?

Each ray would have to displace in a moment 33,000,000 leagues of subtle matter 400,000 times over.

This supposed subtle matter would resist in an absolute plenum as much as the most compact matter.

1 pound of gold powder, pressed in a box, resists as much as 1 pound lump of gold.

Thus, a ray from a star would have to exert far more force than if it had to pierce a cone of gold whose axis was thirteen trillion two hundred billion leagues long.

Light, when moving from one element into another, from one medium to another, does not pass entirely through.

A large part is reflected.

The air reflects back more than it transmits.

Thus it would be impossible for any starlight to reach us—it would all be absorbed, all reflected, before even one ray could come halfway through our atmosphere.

What if this ray had many other atmospheres to cross? But in the chapters where we explain the principles of gravitation, we will see a host of arguments proving that this supposed plenum was a fable.

Let us pause here for a moment to see how slowly truth establishes itself among men. It has been nearly fifty years since Roemer demonstrated, through observations of the eclipses of Jupiter’s satellites, that light travels from the sun to the Earth in about seven and a half minutes; yet, not only is the contrary still maintained in several physics books, but here is how one speaks in a three‑volume collection drawn from the observations of all the academies of Europe, printed in 1730, page 35, volume I:

“Some have claimed that from a luminous body like the sun there is a continual outflow of an infinity of tiny insensible parts, which carry light to our eyes; but this opinion, which still smacks a little of the old philosophy, is not sustainable.”

This opinion is nevertheless demonstrated in more than one way, and far from smelling of the old philosophy, it is directly contrary to it: for what could be more contrary to empty words than so many measurements, calculations, and experiments?

Other opponents arose who attacked this truth of the emission and progression of light with the same weapons that men, more respected than enlightened, once dared to use so imperiously and so vainly to attack Galileo’s opinion on the motion of the Earth.

Those who fight reason with authority employ Holy Scripture, which is meant to teach us how to live well, to draw from it lessons for their philosophy; they have truly made Moses into a physicist. If this is simplicity, they deserve pity. If they think, by this artifice, to make those who disagree with them odious, they deserve even more pity; they should remember that those who condemned Galileo on a similar pretext covered their country with a shame that only the name of Galileo can erase.

They say we must believe that daylight does not come from the sun, because, according to Genesis, God created light before the sun.

But these gentlemen do not realize that, according to Genesis, God also separated the light from the darkness, and called the light day, and the darkness night, and made a day from evening and morning, etc.—and all that before creating the sun.

It would then follow, by the reasoning of these “physicists,” that the sun does not make the day, and that the absence of the sun does not make the night.

They also add that God separated the waters from the waters, and they understand by this separation the sea and the clouds. But, according to them, it would then follow that the vapors that form the clouds are not, as they are, raised by the sun. For according to Genesis, the sun was created only after this separation of the lower and upper waters; yet they admit in that very passage that it is the sun that raises these upper waters. They are therefore in contradiction with themselves.

Will they deny the motion of the Earth because Joshua commanded the sun to stop? Will they deny the germination of seeds in the Earth because it is said that the grain must rot before sprouting?

They must therefore acknowledge, with all people of good sense, that it is not truths of physics that must be sought in the Bible, and that we must learn there how to become better, not how to understand nature.