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"L ANGUAGE, which is older than science, abounds in expressions whose precise meaning does not always conform with reality. People spoke before observing, and the word sanctioned by usage has sometimes been found untrue in the light of subsequent scientific research. For example, we speak of darting a glance at a person or object, to signify a quick look at that person or object.
"As a matter of fact, when we look at anything do we send forth visual rays from our eyes to procure for us information concerning the thing looked at? Does that which makes us see go out from our eyeballs? Certain terms in common use might imply as much; but to take these terms literally would be a serious mistake.
"In the act of looking nothing goes out from our eyes; on the contrary, all that we see comes to them from the thing seen. Our eyeballs do not send out anything; they receive. We do not dart our glances or looks, but direct them toward the objects we wish to examine; that is, we open our eyes and turn them toward these objects in order to receive that which shows them to us.
"And what do we thus receive? We receive light which, coming from the thing seen, gives us knowledge of it by penetrating to the interior of the eye. The gate by which this light-message enters is the orifice to be seen in the front of the eyeball in the shape of a round black spot. It is called the pupil.
"Light tells us about distant objects somewhat as heat announces the presence of a fire and as sound calls attention to the tinkling bell or the booming cannon. The heat comes from the fire and not from us; the sound comes from the bell or the cannon, not from us. Light, which enables us to see, comes from what is seen and not from us.
"Again, we speak of 'palpable darkness,' meaning profound obscurity. The adjective palpable is applied to anything perceptible to the sense of touch. Strictly speaking, air, although a very tenuous substance, is in reality palpable because one needs only to wave the hand in order to feel its contact, gentle as a light breeze. Mist and fog, which are great masses of vapor, might still more fittingly be called palpable. But can this be said of darkness? Can it be likened to a sort of fog that hides things from us by becoming thick and reveals them again by becoming thin? Is it a material substance, a substance that can be felt, and one that shuts us in like a black veil?
"No, darkness is never palpable; it does not thicken like fog, for it is nothing, absolutely nothing but the absence of light. It has no existence of its own; when darkness descends, nothing really falls from the sky, but rather light has been taken away. Darkness is to light what silence is to sound.
"Sound is a very real thing with its aërial waves that travel from the sonorous body to our ears. Light also is a real thing. With incomparable speed it travels from the luminous object to our eyes. Let the sonorous body cease to vibrate, the luminous object cease to shine, and in the one case we have silence, in the other darkness; in short, two negative quantities.
"Any object, to be visible, must give out light. If it does not, it is by that very fact invisible, however perfect the eye may be. In the same way, a bell that does not ring cannot be heard, no matter how keen the sense of hearing. No light, no vision possible; no sound-waves, no hearing possible.
"It is said, however, that certain animals—the cat in particular—are able to see in complete darkness. If that is your opinion, undeceive yourselves; no animal can see when light is entirely wanting.
"The cat, it is true, has the advantage of us: its large eyes, the pupils of which can contract and almost close when exposed to light of dazzling brilliancy, or enlarge to receive more abundantly the dim light of dark places—its large eyes, I say, enable it to find its way where to duller vision all is utter darkness.
"But in reality there is only partial darkness where the cat finds the little light it needs. If light were entirely wanting the animal would open its big eyes in vain; it would see nothing at all, absolutely nothing; and to find the way about it would have to depend on the long hairs of its mustache, just as the blind man depends on his stick.
"Would you like to see how the cat contrives to regulate the admission of light into its eyes? Watch it in the sun. You will see the pupil reduced to a narrow slit like a black line. In order not to be dazzled by the blinding glare, the animal has nearly closed the passage to the light; it has nearly closed the pupil while leaving the eyelids wide open. Take the cat into the shade. The slit of both eyes enlarges and becomes an oval. Put it in semi-darkness and the oval dilates until it becomes almost a circle; and the dilation increases as the light gets dimmer.
"Thanks to its pupils, which open wide and can thus still receive a little light where to others the darkness is complete, the cat can find its way in the dark and hunt by night still better than by day, as it is then invisible to the mice while it sees them well enough.
"Iron heated white-hot to be hammered on the anvil lights up the blacksmith's dark shop. The flame of the lamp lights the interior of our dwellings at night. Any substance, if heated sufficiently, becomes, like the white-hot iron and the lamp-flame, a source of light. The sun is the world's torch, the radiant furnace that gives light, heat, and animation to everything that has life.
"It is a ball of fire about a million and a half times as large as the earth. Its enormous distance from us—more than thirty millions of leagues—reduces it in our eyes to a disk only a couple of spans across; for the more distant an object is the smaller it appears. Small as its remoteness makes it appear, nevertheless it remains for us the undisputed king of the heavens. Whoever should try to look it in the face would immediately be dazzled and compelled to lower his eyelids.
"The stars, countless in number, are so many suns, comparable with ours in brightness and size; but their distance from us is so prodigious that these colossal stars look like mere points of light. Indeed, the greater number of them are not even visible without the aid of a good telescope. These distant suns light other worlds and take hardly any part in the earth's illumination.
"Our sun is the only dispenser of light to our world. To its rays we owe our day; their withdrawal causes our night.
"Illumined by the sun, objects send back or reflect in all directions the light that reaches them, somewhat as a wall or rock reflects the aërial waves that constitute sound. Reaching our eyes, this reflected light causes us to see what surrounds us; it is a sort of luminous echo comparable with the sonorous echo. The illuminated object thus becomes itself a source of light, but of a borrowed light having its true origin elsewhere.
"We have in the heavens above us a splendid example of this borrowed light. The moon has no light of its own. It is a dark body which becomes bright by reflecting the light from the sun. Its half that faces the sun is bright, the other dark. According to the relative positions of earth, sun, and moon, the last named turns toward us at certain times the whole of its bright half, and then the moon is full; later only a part of this half, which shows us the moon in the shape of a crescent; finally the half that does not get the rays of the sun, and for the time being the moon is invisible although still present in our sky. If it shone by its own light the moon would not have these changing aspects, but would always appear to us as does the sun, in an invariably round and luminous form.
"Every object that arrests light casts a shadow. Hold your hand before the lamp when it is lighted in the evening. On the opposite wall you will see a dark shape, the shadow of your hand. In like manner, when the sun shines, the shadow of a tree, wall, or house can be seen on the ground. What, then, is shadow? It is the space left untouched by light, the latter being intercepted by some obstruction.
"Under the cover of a tree, at the foot of a wall, in the shelter of a rock, the direct rays of the sun are cut off; but the light reflected by neighboring objects, themselves lighted directly, always penetrates there. Hence we have an intermediate state between broad daylight and total darkness, a semi-obscurity which would be total darkness were it not for the light reflected by neighboring objects on which the sun shines. It is partial darkness. Where there is no light, either direct or reflected, we have black night."