518. Why, when we move before a mirror, does the image draw near to the reflecting surface as we draw near to it, and retire when we retire?

Because the lines and angles of reflection are always equal to the lines and angles of incidence.519. What is the line of incidence?

If a person stands in a direct line before a mirror, the line through which the light travels from him to the mirror is the line of incidence.

Incidence—falling on.

"Blessed be the Lord, who daily loadeth us with benefits, even the God of our salvation."—Psalm lxviii.

520. What is the line of reflection?

The line of reflection is the line in which the rays of light are returned from the image formed in the glass to the eye of the observer.

Reflection—a turning back.521. What is the angle of incidence?

The angle of incidence is the angle which rays of light, falling on a reflecting surface, make with a line perpendicular to that surface.

522. What is the angle of reflection?

The angle of reflection is the angle which is formed by the returning rays of light, and a line perpendicular to the reflecting surface. It is always equivalent to the angle of incidence.

Take a marble and roll it across the floor, so that it shall strike the wainscot obliquely. Let A in the diagram represent the point from which the marble is sent. The marble will not return to the hand, nor will it travel to the line B, but will bound off, or be reflected, to C. Now B is an imaginary line, perpendicular to the reflecting surface; and it will be found that the path described by the marble in rolling to the surface and rebounding from it, form, with the line B, two angles that are equal. These represent the angles of incidence and of reflection, and explain why the reflection of a person standing at A before a mirror, would be seen by another person standing at C. This simple law in optics explains a great many interesting phenomena, and therefore it should be clearly impressed upon the memory.

"And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."—Gen. i.

523. Why do windows reflect the sun in the evening?

Because the eye of the observer is in the line of the reflection.524. Why do windows not reflect the sun at noon?

They do, but our eyes are not then in the line of the reflection.

It is obvious from the foregoing diagram that the evening rays of reflection fall upon the eyes of spectators, while the reflections at noon are so perpendicular that they are lost.525. Why do the sun and moon appear smaller when near the meridian, than when near the horizon?

Because, when near the horizon, they are brought into comparison with the sizes of terrestrial objects; but when near the meridian they occupy the centre of a vast field of sky, and as there are no objects of comparison surrounding them, they appear smaller.

This is one "Reason Why," assigned by some observers. But there is also another reason to be found in the fact that, when the sun or moon is near the horizon, we view it through a greater depth of atmosphere than we do when at the meridian. (See Fig. 13.) A straight line passed upward through the air, would not be so long as that which passes to S. Consequently, as the air is generally impregnated with moisture, at the time when these effects are observed, the rays of light are caused to diverge more, and the disc of the sun or moon appears magnified. Probably both of these reasons contribute to the effect. This latter reason also explains why the disc of the sun or moon may sometimes appear oval in shape, the lower stratum of air being more loaded with moisture than that through which we view the upper part of the disc.

"There is no darkness nor shadow of death, where the workers of iniquity may hide themselves."—Job xxxiv.

526. Why do our shadows lengthen as the sun goes down?

Because light travels only in straight lines, and as the sun descends, the direction of his rays becomes more oblique, thereby causing longer shadows.527. What is the cause of the optical illusions frequently observed in nature?

There are various kinds of natural optical illusions:—

The mirage, in which landscapes are seen reflected in burning sands.

The fata morgana, in which two or three reflections of objects occur at the same time.

The Ærial spectra, or Ærial reflections, &c.

The optical illusions above enumerated owe their origin to various atmospheric conditions, in which refractions and reflections are multiplied by the different densities of atmospheric layers. They chiefly occur in hot countries, where, from the varying effects of heat, the conditions of atmospheric refraction and reflection frequently prevail in their highest degree.

"In the morning ye say, it will be foul weather to-day, for the sky is red and lowering."—Matt. xvi.

528. Why do we have twilight mornings and evenings?

Because the coming and the departing rays of the sun are refracted and reflected by the upper portions of the atmosphere. (See Fig. 13.)529. How long before the sun appears above the horizon does the reflection of his light reach us?

The time varies with the refracting and reflecting power of the atmosphere, from twenty minutes to sixty minutes. But the sun's position is usually eighteen degrees below the horizon when twilight begins or ends.530. Why is the sky blue?

The white light of the sun falls upon the earth without change; it is then reflected back by the earth, and as it passes through the atmosphere portions of it are again returned to us, and this double reflection produces a polarised condition of light which imparts to vision the sensation of a delicate blue. (See 549.)531. Why do the clouds appear white?

Because they reflect back to us the solar beam unchanged.532. Why does the sky appear red at sunset?

Because the light vapours of the air, which are condensed as the sun sets, refract the rays of light, and produce red rays. The refraction which produces red requires only a moderate degree of density.533. Why do the clouds sometimes appear yellow?

Because there is a larger amount of vapour in the air, which produces a different degree of refraction, resulting in yellow.534. Why does a yellow sunset foretell wet weather?

Because it shows that the air is heavy with vapours. The refraction that produces yellow requires a greater degree of density.

"When it is evening ye say it will be fair weather, for the sky is red."—Matt. xvi.

535. Why does a red sunset foretell fine weather?

Because the redness shows that the vapours in the air towards the West, or wet quarter, are light, as is evidenced by the degree of refraction of the sun's rays.536. Why does a red sunrise foretell wet?

Because it shows that towards the East, or dry quarter, the air is charged with vapour, and therefore probably at other points the air has reached saturation.537. Why does a grey sunrise foretell a dry day?

Because it shows that the vapours in the air are not very dense.538. Why is "a rainbow in the morning the shepherd's warning?"

Because it shows that in the West, or wet quarter, the air is saturated to the rain point.539. Why is "a rainbow at night the shepherd's delight?"

Because it shows that the rain is falling in the East, and as that is a dry quarter, it will soon be over. Rainbows are always seen in opposition to the sun.