CHAPTER XXII.

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.

[Verse: "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_.

[Verse: "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.

[Verse: "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_.

[Verse: "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_?