502. What is the difference between the refraction and the reflection of light?

Refraction is the deviation of rays of light from their course through the interference of a different medium; reflection is the return of rays of light which, having fallen upon a surface, are repelled by it.503. What is the radiation of light?

The radiation of light is its emission in rays from the surface of a luminous body.504. Do all bodies radiate light?

All bodies radiate light; but those that are not in themselves primary sources of light, are said to reflect it.505. Do black bodies reflect any light?

Black bodies absorb the light that falls upon them. But they reflect a very small degree of light.506. Why is glass transparent?

Because its atoms are so arranged that they allow the vibrations of light to continue through their substance.

"As in water face answereth to face, so the heart of man to man."—Proverbs xxvii.

507. Does glass obstruct the passage of any portion of light?

Glass reflects (sends back) a very small portion of light. This may be observed by holding a piece of paper, or a hand, a few inches from a window, when a faint reflection of it will be visible. Probably the small amount of light reflected by transparent glass, which gives a passage to the greater part of the rays, may serve to illustrate the small amount of light reflected from black surfaces, which absorbs the greater portion of light.

Instead of a piece of white paper, hold a piece of black cloth two or three inches from the window-pane, and you will have two reflections so weak that the image of the cloth will be almost lost. The first reflection is that of the very small amount of light from the black surface on to the glass, and the second reflection is that of the inconceivably small amount returned by the glass, and by which the faint image of the black cloth is produced. But put the black cloth outside of the window-pane, and then hold an object before them, and you will find that the two weak reflectors, acting together, produce an improved image, or reflection.508. Why, if a book is held between a candle-light and the wall, does a shadow fall upon the wall?

Because the rays of light are intercepted by the book.509. Why do the rays pass over the edges of the book in a direct line with the flame of the candle?

Because light always travels in straight lines.510. Why is there some amount of light even where shadows fall?

Because, as all objects reflect light, some of them throw their light into the field of the shadow.511. Why are some substances opaque to light?

Because the arrangement of their particles will not admit of the vibrations of the luminous ether passing through them.

Opaque—impervious to rays of light.512. Why do we see our faces reflected in mirrors?

Because the rays of light from our faces are reflected by the surface of the quicksilver at the back of the glass.

"The day is thine, the night also is thine: thou hast prepared the light and the sun."—Psalm lxxiv.

513. Why does the quicksilver reflect the rays of light?

Because, being densely opaque to light, and presenting also a bright surface, it is a good reflector, and it throws back the whole of the rays.514. What has the glass to do with the reflection?

The glass has nothing to do with the reflection, except that it affords a field upon which the reflecting surface of the quicksilver is spread; and it keeps the air and dirt from dulling the quicksilver.

The parts of a mirror from which the quicksilver is rubbed away give no reflection that could assist the reflecting power of the quicksilver. That the surface of the glass does not reflect the image, is shown by the fact, that if you put the point of any object against the glass, the thickness between the point and the place where the reflection of it begins, will show the exact thickness of the glass.515. Why does a compound mirror (a multiplying mirror) exhibit a large number of images of one object.

Because all objects reflect rays of light in every direction, and therefore the different mirrors, being at various angles, receive each a reflection of the same object.516. Why does a window-pane appear to be a better reflector by candle-light than by day-light?

The reflecting power of glass is precisely the same by night as by day, and is always very feeble. But it appears to be greater by night, because the surrounding darkness increases the apparent strength of the reflection.517. How do we know that objects reflect light in every direction?

Because if we prick a hole in a card with a pin, and then look through that small hole upon a landscape, we can see some miles of country, and some thousands of objects; every part of every object throughout the whole scene, must have sent rays of light the small hole pricked in the card.

"Such knowledge is too wonderful for me; it is high, I cannot attain unto it."—Psalm cxxxix.

At one extremity of the landscape, viewed through the hole in the card, there may be a forest of trees; in the distance there may be hills bathed in golden light, and overhung with glittering clouds; in the mid-distance there may be a river winding its course along, as though it loved the earth through which it ran, and wished, by wandering to and fro, to refresh the thirsty soil; in the foreground may be a church, covered by a million ivy leaves; and grouping towards the sacred edifice may be hundreds of intending worshippers, old and young, rich and poor; flowers may adorn the path-ways, and butterflies spangle the air with their beauties; yet every one of those objects—the forest, the hills, the clouds, the river, the church, the ivy, the people, the flowers, the butterflies—must have sent rays of light, which found their way through the little hole in the card, and entered to paint the picture upon the curtain of the eye.

This is one of the most striking instances that can be afforded of the wonderful properties of light, and of the infinitude of those luminous rays that attend the majestic rising of the sun. Not only does light fly from the grand "ruler of the day" with a velocity which is a million and a half times greater than the speed of a cannon-ball, but it darts from every reflecting surface with a like velocity, and reaches the tender structure of the eye so gently that, as it falls upon the little curtain of nerves which is there spread to receive it, it imparts the most pleasing sensations, and tells its story of the outer world with a minuteness of detail, and a holiness of truth. Philosophers once sought to weigh the sunbeam; they constructed a most delicate balance, and suddenly let in upon it a beam of light; the lever of the balance was so delicately hung that the fluttering of a fly would have disturbed it. Everything prepared, the grave men took their places, and with keen eyes watched the result. The sunbeam that was to decide the experiment had left the sun eight minutes prior to pass the ordeal. It had flown through ninety-five millions of miles of space in that short measure of time, and it shot upon the balance with unabated velocity: but the lever moved not, and the philosophers were mute.