451. What is light?

Light, according to Newton, is the effect of luminous particles which dart from the surfaces of bodies in all directions. According to this theory, the solar light which we receive would depart from the sun and travel to the earth.

According to Huyghens, light is caused by an infinitely elastic ether, diffused through all space. This ether, existing everywhere, is excited into waves, or vibrations, by the luminous body.

The theory of light is so undetermined that neither the views of Newton, nor those of Huyghens, can be said to be exclusively adopted. Writers upon natural philosophy seize hold of either or both of those theories, as they present themselves more or less favourably in the explanation of natural phenomena. In "The Reason Why," as we have to speak of the effects of light rather than of its cause, we shall avoid, as far as possible, the doubtful points. But let no one be discouraged by the fact that the theory of light, as, indeed, of all the imponderable agents, is imperfectly understood. Rather let us rejoice that there are vast fields of discovery yet to be explored; and that light, the most glorious and inspiring element in nature, invites us from the sun, the moon, and the stars, and from the face of every green leaf and variegated flower, to search out the wonders of its nature, and further to exemplify the goodness and wisdom of God.

"And God said, Let there be light: and there was light."

452. What is the distance of the sun from the earth?

Ninety five millions of miles.453. At what rate of velocity does light travel?

At the rate of 192,000 miles in a second, through our atmosphere; and 192,500 miles in a second through a vacuum.454. How long does light take to travel from the sun to the earth?

Eight minutes and thirteen seconds.455. What is the constitution of the sun?

It is a spherical body, 1,384,472 times larger than the earth.456. From what does the luminosity of the sun arise?

From a luminous atmosphere, or, as M. Arago named it, photosphere, which completely surrounds the body of the sun, and which is probably burning with great intensity.457. What are the minor sources of light?

Light may be produced by chemical action, by electricity, and by phosphoresence, in the latter of which various agencies unite.458. What is a ray of light?

A ray of light is the smallest portion of light which we can recognise.459. What is a medium?

A medium is a body which affords a passage for the rays of light.460. What is a beam of light?

A beam of light is a group of parallel rays.461. What is a pencil of light?

A pencil of light is a body of rays which come from or move towards a point.

"And God saw the light, that it was good: and God divided the light from the darkness."—Gen. i.

462. What is the radiant point?

The radiant point is that from which diverging rays of light are emitted.463. What is the focus?

The focus is the point to which converging rays are directed.

Diverging, starting from a point, and separating. Converging, drawing together towards a point.464. What is the constitution of a ray of light?

A ray of white light, as we receive it from the sun, is composed of a number of elementary rays, which, with the aid of a triangular piece of glass, called a prism, may be separated, and will produce under refraction the following colours:—

1. An extreme red ray—a mixture of red and blue, the red predominating.

2. Red.

3. Orange—red passing into and combining with yellow.

4. Yellow—the most luminous of all the rays.

5. Green—yellow passing into and combining with the blue.

6. Blue.

7. Indigo—a dark and intense blue.

8. Violet—blue mingled with red.

9. Lavender grey—a neutral tint.

10. Rays called fluorescent, which are either of a pure silvery blue, or a delicate green.465. Why is a ray of light, which contains these elementary rays, white?

Because the colour of light is governed by the rapidity of the vibrations of the ether-waves. When a ray of light is refracted by, or transmitted through a body, its vibrations are frequently disturbed and altered, and thus a different impression is made upon the eye.

Light which gives 37,640 vibrations in an inch, or 458,000,000,000,000 in a second of time, produces that sensation upon the eye which makes the object that directs the vibrations appear red. Yellow light requires 44,000 vibrations in an inch, and 535,000,000,000,000 in a second of time. And the other colours enumerated (see 464) all require different velocities of vibration to produce the colours by which they are distinguished.

"The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light."—Matt. v.

Accepting the theory of vibrations, and applying it to the elucidation of the phenomena of light—it is unnecessary, we think, to believe that a ray of white light contains rays in a state of colour. It is said that if we divide a circular surface into parts, and paint the various colours in the order and proportions in which they occur in the refracted ray, and then spin the circle with great velocity, the colours will blend and appear white. But such is not the case; the result is in some degree an illusion, arising out of the sudden removal of the impression made upon the eye by the colours; and if a piece of white paper be held by the side of the coloured circle in motion, the latter will be found to be grey. When it is remembered that in colouring a white surface with thin colours, the white materially qualifies the colours, it must be admitted that the experiment fails to support the assertion that the colours of the spectrum produce white. But there can be no difficulty in understanding that a ray of light undergoing refraction, becomes divided into minor rays, which differing in their degrees of refrangibility, vary also in the velocity of their vibrations, and produce the several sensations of colour.466. Why is a substance white?

Because it reflects the light that falls upon it without altering its vibrations.467. Why is a substance black?

Because it absorbs the light and puts an end to the vibrations.468. Why is the rose red?

Because it imparts to the light that falls upon it that change in its vibratory condition, which produces on our eyes the sensation of redness.469. Why is the lily white?

Because it reflects the light without altering its vibrations.470. Why is the primrose yellow?

Because, though it receives white light, it alters its vibrations to 44,000 in an inch, and 535,000,000,000,000 in a second, and this is the velocity of vibration which produces upon the eye a sensation of yellow.

"But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness."—Matt. v.

471. Why are there so many varieties of colour and tint in the various objects in nature?

Because every surface has a peculiar constitution, or atomic condition, by which the light falling upon it is influenced. In tropical climates, where the brightness of the sun is the most intense, there the colours of natural objects are the richest; the foliage is of the darkest green; the flowers and fruits present the brightest hues; and the plumage of the birds is of the most gaudy description. In the temperate climates these features are more subdued, still bearing relation to the degree of light. And at a certain depth of the ocean, where light penetrates only in a slight degree, the objects that abound are nearly colourless.

It has been held by many philosophers (and the theory is so far conclusive that it cannot be dispensed with) that there is an analogy between the vibratory causes of sound, and the vibratory causes of colour. Any one who has seen an Æolian harp, and listened to the wild notes of its music, will be aware that the wires of the harp are swept by accidental currents of air; that when those currents have been strong, the notes of the harp have been raised to the highest pitch, and as the intensity of the currents has fallen, the musical sounds have deepened and softened, until, with melodious sighing, they have died away. No finger has touched the strings; no musical genius has presided at the harp to wake its inspiring sounds; but the vibration imparted to the air, as it swept the wires, has alone produced the chromatic sounds that have charmed the listener. If, then, the varied vibrations of the air are capable of imparting dissimilar sensations of sounds to the ear, is it not only possible, but probable, that the different vibrations of light may impart the various sensations of colours to the eye?