The structure of our eyes

To understand the human eye as a physicist — one who considers only vision — we must first know that the outer white covering, the rampart and ornament of the eye, transmits no ray of light.

The stronger and smoother this white of the eye is, the more it reflects light; and when some strong emotion drives new “spirits” to the face, which tighten and shake this membrane, sparks seem to flash from it.

In the center of this membrane rises slightly the cornea, thin, hard, and transparent, exactly like the glass of your watch placed on a sphere.

Beneath this cornea is the iris, another membrane which, colored by itself, spreads its color onto the transparent cornea that covers it: this iris makes eyes blue or black. It is pierced in its center, which therefore always appears black; and this center is the pupil of the eye.

It is through this opening that rays of light enter: it widens by an involuntary movement in dark places to admit more rays; it narrows again when great brightness offends it.

The rays admitted by this pupil have already undergone a strong refraction when passing through the cornea. Imagine this cornea like your watch glass; it is convex on the outside, concave on the inside: all the oblique rays are broken in the thickness of this glass; but then its concavity more or less restores what its convexity has broken. The same thing happens in our cornea.

The rays, thus bent and broken, then pass into a transparent fluid behind it. This liquid is called the aqueous humor. Anatomists still disagree about the exact shape of this little reservoir;

But whatever its shape, nature seems to have placed there this clear and limpid fluid to perform refractions, to transmit light purely, to let the lens (the crystalline body) behind it move forward without effort and change shape freely, and to maintain the necessary moisture, etc.

Finally, after passing through this water, the rays meet a kind of liquid diamond, shaped like a lens, and set in a fine, diaphanous membrane.

This “diamond” is the crystalline lens; it is the part that bends all the oblique rays: it is the principal organ of refraction and of vision, perfectly similar in this respect to a convex spectacle lens.

Consider this crystalline lens or convex glass lens (figure 7):

• The perpendicular ray A penetrates it without deviation.
• But the oblique rays B C bend in the thickness of the glass, drawing closer to the perpendiculars drawn to the points where they strike.

When they exit the glass to pass into the air, they bend again, moving away from the perpendicular: this second bending is precisely what makes them converge at D, the focus of the lens.

Now the retina — that delicate membrane, that extension of the optic nerve lining the back of our eye — is the focus of the crystalline lens; it is upon this retina that the rays end up. But before reaching it, they pass through yet another medium: the vitreous humor, less solid than the lens, less fluid than the aqueous humor.

It is in this vitreous humor that the rays have time to gather before finally coming together on points at the back of our eye. Imagine then, beneath this crystalline lens, the vitreous humor upon which the lens rests; this humor holds the lens in its hollow and is rounded toward the retina.

The rays, as they leave this last humor, complete their convergence. Each bundle of rays coming from a point of the object strikes a point of our retina.

A diagram, in which each part of the eye is labeled, will explain this mechanism better than lines and letters (figure 8).

Several philosophers of antiquity believed that, far from the rays of light reflected from objects painting an image at the back of our eyes, rays of light actually came out of our eyes themselves, went to seek the objects, and brought back some sort of “intentional species.” This idea was worthy of the rest of Greek physics — I do not say Roman physics, for the Romans hardly had any.

It was Giambattista Porta, an Italian, who in 1560 first revealed the true causes of vision and, through the simple experiment of a white cloth exposed to a ray of sunlight in a dark room,[2] suspected that the same thing must happen in the eye as in that room. He did not dare imagine that the rays penetrated all the way to the retina; he believed that objects painted themselves on the crystalline lens — and everyone believed him, until Kepler and Descartes finally explained the whole mechanism of vision, all the refractions that occur in our eyes, what makes vision short, and what can help it. Dr. Hooke, Newton’s precursor, later went so far as to demonstrate by experiment that an object, to be perceived, must trace at least on the retina an image the eight-thousandth part of an inch wide.

Having explained the structure of the eyes solely for the purposes of optics, we can easily understand why we so often need the help of lenses, and what use spectacles have.

Often an eye will be too flat, either because of the shape of its cornea or because its crystalline lens has been dried by age or disease; then the refractions will be weaker and fewer, and the rays will no longer gather on the retina. Consider this too-flat eye, which is called the eye of the farsighted person.

To simplify, look only at three bundles — three cones of rays — that fall from the object into this eye; they will meet at the points A A A, beyond the retina: it will see objects blurry (figure 9).

Nature has provided a remedy for this inconvenience, by giving the eye muscles the power to lengthen or flatten the eye, to move it toward or away from the retina. Thus, in this aged or diseased eye, the crystalline lens can move forward slightly and approach D D; then the space between the lens and the back of the retina becomes greater, the rays have time to gather on the retina instead of going beyond it. But when this power is lost, human ingenuity compensates: a convex lens is placed between the object and the weakened eye.

The effect of this lens is to bring the rays closer together; the eye receives them closer together and in greater number, and they end at a point on the retina as they should — then vision is clear and distinct.

Now look at another eye, which has the opposite problem (figure 10): it is too round. The rays meet too early, as you see at point B; they cross too quickly, separate again at B, and form a blur on the retina. This is what is called a nearsighted eye.

This problem diminishes as age brings others — dryness and weakness: they gradually flatten this too-round eye; and this is why it is said that “short-sightedness lasts longer.” It is not that it truly lasts longer than normal sight; but that, at a certain age, the dried-out eye flattens: then someone who had to bring his book to within three or four inches of his eye can sometimes read at a foot’s distance. But his vision soon becomes blurred and confused again, and he can no longer see distant objects. Such is our condition: a defect is almost never repaired except by another.

This eye is too round.

It needs a lens that prevents the rays from meeting too quickly: this lens will do the opposite of the first; instead of being convex on both sides, it will be slightly concave on both sides, and the rays will diverge in this one instead of converging in the other.

They will therefore meet farther away than they did before in the eye; and then this eye will enjoy perfect sight. The convexity and concavity of lenses are matched to the defects of our eyes: that is why the same spectacles that bring clear sight to one old man will be of no help to another, for there are no two illnesses, no two men, no two things in the world exactly alike — except for the first principles of homogeneous bodies.

It is said that antiquity did not know spectacles; yet it knew burning mirrors. The discovery of one truth is not always a reason for discovering the other truths connected to it. The attraction of the magnet was known, but its direction escaped notice. The demonstration of the circulation of the blood was in the very bloodletting practiced by every Greek doctor; and yet no one suspected that blood circulated.

But how did the Greeks and Romans manage, without magnifying glasses, to engrave those stones whose details we cannot admire today without one? On the other hand, if the art of making spectacles was known to the ancients, how was it lost? A secret can be lost, but every truly useful art is preserved.

It is believed that it was in the time of Roger Bacon, at the start of the 13th century, that spectacles — then called “besicles” — and magnifying glasses that give new eyes to the elderly were invented: for he is the first who spoke of them clearly, and people only began to mention them then. Spectacles were used for nearly four hundred years without anyone knowing exactly by what mechanics they aided our eyes — just as we still use the compass without knowing the cause that directs the magnetized needle.

You have just seen the effects that refraction produces in our eyes, whether the rays arrive without intermediary help or pass through lenses: you understand that without this refraction in our eyes, and without the reflection of rays from the surfaces of objects toward us, the organs of sight would be useless.

The means that nature employs to create this refraction, the laws it follows, are mysteries we shall now unfold. But we must first finish what we have to say about vision; we must answer those questions so natural:

Why do we see objects beyond a mirror, and not on the mirror itself?

Why does a concave mirror make the object larger? Why does a convex mirror make the object smaller? Why do telescopes bring things closer and enlarge them? By what artifice does nature make us aware of sizes, distances, positions? And finally, what is the true reason we see objects as they are, though in our eyes they are painted upside down?*

There is nothing in all this that does not merit the curiosity of every thinking being. But we would not dwell on these subjects, so often treated by illustrious writers, and we would simply refer you to them — if we did not have to present some rather new truths, interesting for a small number of readers.