When rays of light fall obliquely upon the surface of any transparent medium, they are slightly diverted from their course. This alteration of the course of the rays is called refraction, and the degree of refraction is influenced by the difference between the densities of the mediums through which light is transmitted.

In that case the ray pursues its course—there is no refraction.

A ray of light falling slantingly upon a window, in passing through it is slightly brought to the perpendicular; and if it then falls upon the surface of water, it is still further brought to the perpendicular in passing through the water.

It is; but the direction of the refraction is just the opposite to the instance just given; a ray of light passing through water into air, does not take a more perpendicular course, but becomes more oblique.

Because the rays of light that are reflected from it all pass through the same medium, the air.

Because the rays of light that pass through the water are reflected in a different degree to those that pass through the air.

Because, in passing through clouds of different densities the rays are bent out of their course.

Because the light reflected from the objects at the bottom is refracted as it leaves the water.

About one-third. A person bathing, and being unable to swim, should calculate before jumping into the water, that if it looks two feet deep, it is quite three feet.

Because we do not allow for the different refractive powers of water and of air.

Because of the refractive effects of the atmosphere. Rays of light, passing obliquely from the sun through the air to the earth, are refracted three or four times by the varying density of the medium. Each refraction bends the rays towards the perpendicular; and hence we see the sun before it rises and after it sets.

Because the ever varying density of the air which is flying away in hot currents, and succeeded by cold, constantly changes the refractive power of the medium through which the figures are viewed.

Because their light reaches us through variously heated and moving currents of air. In this case the earth is the kiln, and the stars the object that is viewed through the refractive medium.

Because it denotes that there are various ærial currents of different temperatures and densities, producing atmospheric disturbance.

The refraction of the sun's rays by the falling rain.

The colours belong to the elementary rays of light; and these rays having different degrees of refrangibility, some of them are bent more than others; they are therefore separated into distinct rays of different colours.

Because the rays of refracted light, reflected upon other drops of rain, are again refracted, and then reflected again, forming a secondary bow.

Because the secondary bow is a reflection of the primary bow, and, like all reflections, is reversed.

Because those rays which first reach the reflecting surface are the first returned. If you hold your open hand towards the looking-glass, the light passing from the point of your finger will reach the reflector and be returned before the rays that pass from the back parts of the hand. Hence the image of the hand will present the reflection of the finger point towards the point of the finger.

Because they are derived from the refraction and reflection of rays which have already been refracted and reflected, and thereby their intensity has been diminished.

A lunar rainbow is caused by the light of the moon, in the same manner as the solar rainbow is caused by the light of the sun.

Because the light of the moon is the reflected light of the sun, and is therefore less intense.

A halo is a luminous ring, which forms between the eye of the observer and a luminous body.

Haloes may appear around the disc of the sun, moon, or stars. But in this country the lunar haloes are the most remarkable and frequent.

The refraction of light as it passes through an intervening cloud, or a stratum of moist and cold air.

Because they are sometimes formed very high in the atmosphere, at other times very low. Being high, and farther removed from the spectator, and nearer the source of light, they appear smaller; while the nearer they are, the larger they appear.

Because they show that there is a great amount of atmospheric moisture, which will probably form rain.

Because they refract the rays of light in the same manner as the rain drops.

Because, like a large rain drop, it refracts the rays of light, and shows the elementary rays.

The refraction of the light that falls upon the surface of the pearl.

The refraction of the rays of light by the various facets of the diamond.

The refraction of light, and the production of prismatic colours, surrounds us with most interesting phenomena. The laundress, whose active labours raise over the wash-tub a soapy froth, performs inadvertently one of the most delicate operations of chemistry—the chemistry of the imponderable agents—and the result of her manipulations manifests itself in the delicate colours that dance like a fairy light over the glassy films that follow the motion of her arms. The laughing child, throwing a bubble from the bowl of a tobacco pipe into the air, performs the same experiment, and produces a result such as that which filled the philosophic Newton with unbounded joy. The foam of the seashore, the plumage of birds, the various films that float upon the surface of waters, the delicate tints of flowers, and the rich hues of luscious fruits, all combine to remind us, that every ray of light comes like an angelic artist sent from heaven, bearing upon his palette the most celestial tints, with which to beautify the earth, and show the illimitable glory of God.

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.

The radiation of light is its emission in rays from the surface of a luminous body.

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

Black bodies absorb the light that falls upon them. But they reflect a very small degree 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.

Because the rays of light are intercepted by the book.

Because light always travels in straight lines.

Because, as all objects reflect light, some of them throw their light into the field of the shadow.

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.

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.

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.

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.

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.

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.

Because the lines and angles of reflection are always equal to the lines and angles 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.

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.

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

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.

Because the eye of the observer is in the line of the reflection.

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.

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.

Because light travels only in straight lines, and as the sun descends, the direction of his rays becomes more oblique, thereby causing longer shadows.

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 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.

Because they reflect back to us the solar beam unchanged.

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.

Because there is a larger amount of vapour in the air, which produces a different degree of refraction, resulting in yellow.

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.

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.

Because it shows that the vapours in the air are not very dense.

Because it shows that in the West, or wet quarter, the air is saturated to the rain point.

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.

The most obvious distinction is, that light acts upon vision, and heat upon sensation, or feeling.

Another distinction is, that heat expands all bodies, and alters their atomic condition; while light, though usually attended by heat, does not display the same expansive force, but produces various effects which are peculiar to itself.

Yes. A ray of light, as well as containing elementary rays that produce colours under refraction, contains also chemical rays, and heat rays.

Because we have heat rays, as from dark hot iron, from various chemical actions, and from friction, which are unattended by the development of light. And we have light, or luminosity, such as that of phosophoresence, which is unaccompanied by any appreciable degree of heat.

But, besides this confirmation, further proof is afforded by the fact, that in passing rays of solar light through media that are transparent to heat, but not to light, the heat rays may be separated from the luminous rays, and vice versa.

Black glass, and black mica, which are nearly opaque to light, are transparent to heat to the extent of ninety degrees out of a hundred. While pale green glass, coloured by oxide of copper, and covered with a coating of water, or a thin coating of alum, will be perfectly transparent to light, but will be almost quite opaque to heat. These remarks apply, in a greater or less degree, to various other substances.

Both heat and light have been referred to minute vibratory motions which occur, under exciting causes, in a very subtile elastic medium.

They are both united in the sun's rays.

They are both subject to laws of absorption, radiation, reflection, and refraction.

They are both essential to life, whether animal or vegetable.

Both may be developed in their greatest intensity by electricity.

They are both imponderable.

"When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained:"

Heat frequently exists without light.

Light is usually attended with heat.

The point of heat at which the eye begins to discover luminosity has been estimated at 1,000 deg.

The light of a fire, or of a candle, or gas, travels with the same velocity as the light of the sun,—a velocity which would convey light eight times round the world while a person could count "one."

At the same velocity as all other light. And yet there are stars so distant that, although the light of the sun reaches the earth in eight minutes and a half, it requires hundreds of years to bring their light to us.

The intensity of a reflection depends upon the power of the reflecting surface. But, taking the sun and moon as the great examples of primary and reflected light, the intensity of the sun's light is 801,072 times greater than that of the moon.

Polarized light is light which has been subjected to compound refraction, and which, after polarization, exhibits a new series of phenomena, differing materially from those that pertain to the primary conditions of light.

The polarization of light appears to confirm in a high degree the vibratory theory of light; and to show that the vibrations of light have two planes or directions of motion. The mast of a ship, for instance, has two motions: it progresses vertically as the ship is impelled forward, and it rolls laterally through the motion of the billows.

Something like this occurs in the vibrations of light, only the vertical vibration is the condition of one ray, and the lateral vibration is the condition of another ray, and the vibrations of these two rays intersect each other in the solar ray. When these vibrations occur together, the ray has certain properties and powers. But by polarization the rays may be separated, and the result is two distinct rays, having different vibrations.

It then appears that various bodies are transparent to these polarized rays only in certain directions. And this fact is supposed to show that bodies are made up of their atoms arranged in certain planes, through or between which the lateral or the vertical waves of light, together or singly, can or cannot pass; and that the transparency or the opacity of a body is determined by the relation of its atomic planes to the planes of the vibrations of light.

Because rays of light flow from their bodies to each other's eyes, and convey an impression of their respective conditions.

In some popular works that have come under our notice, we find that the student is told that "we cannot absolutely see each other—we only see the rays of light reflected from each other." The statement is erroneous as expressed. We do not see the rays of light, for if we did so, the effect of vision would be destroyed, and all bodies would appear to be in a state of incandesence, or of phosphoresence. Rays of light, which are in themselves invisible, radiate from the objects we look upon, enter the pupil of the eye, and impress the seat of vision in a manner which conveys to the mind a knowledge of the form, colour, and relative size and position of the figure we look upon. If this is not seeing the object—what is? It would be just as reasonable to say, that we cannot hear a person speak—that we only hear the vibrations of the air. But as the vibrations are imparted to the air by the organs of voice of the speaker, as he sets the air in motion, and makes the air his messenger to us, we certainly hear him, and can dispense with any logical myths that confound the understanding, and contribute to no good result.

Actinism is the chemical property of light.

Actinism—ray power.

Because of the actinic, or chemical power of the rays of the sun.

Because of the chemical power of the sun's rays.

Because of the actinic powers that accompany the solar light.

Simply the darkening of preparations of silver, by the actinic rays.

Because the heat rays are in excess. The clouds shut off the scorching light; but, like the blue glass of the photographer's studio, they transmit actinism.

It quickens the germination of seeds; and assists in the formation of the colouring matter of leaves. Seeds and cuttings, which are required to germinate quickly, will do so under the effect of blue glass (which is equivalent to saying, the effect of an increased proportion of actinism), in half the time they would otherwise require.

In the spring, when the germination of plants demands its vitalising aid. In summer, when the maturing process advances, light and heat increase, and actinism relatively declines. In the autumn, when the ripening period arrives, light and actinism give way to a greater ratio of heat.

"But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him."—Corinth. Book i., ii.

Electricity is a property of force which resides in all matter, and which constantly seeks to establish an equilibrium.

Because it first revealed itself to human observation through a substance called, in the Greek language, electrum. This substance is known to us as amber.

Thales, a Greek philosopher, observed that, by briskly rubbing electrum, it acquired the property of attracting light particles of matter, which moved towards the amber, and attached themselves to its surface, evidently under the influence of a force excited in the amber.

It is a resinous substance, hard, bitter, tasteless, and glossy. It has been variously supposed to be a vegetable gum, a fossil, and an animal product. It is probably formed by a species of ant that inhabit pine forests. The bodies of ants are frequently found in its substance.

Because it excites in the sealing wax that force which was first observed in the amber. Sealing-wax, therefore, is called an electric (amber-like) body.

Simply because the term fluid is the most convenient that can be found to express our ideas when speaking of the phenomena of electric force. But of the nature of electricity, except through its observed effects, nothing is known.

All substances in nature, from the metals to the gases. But they differ very widely in their electrical qualities.

Electricity, when it exists, or is excited, in any body, to an amount which is in excess of the amount natural to that body, is called positive (called also vitreous).

Electricity, when it exists, or is excited, in any body, in an amount which is less than is the amount natural to that body, is called negative (called also resinous).

Because some philosophers believe that there is but one electricity, but that it is liable to variations of quantity or state, which they distinguish by positive and negative; while other philosophers believe that there are two electricities, which they name vitreous and resinous, because they may be induced respectively from vitreous and resinous substances, and they display forces of attraction and repulsion.

By changes in the condition of matter. As electricity resides in all substances, and is, perhaps, an essential ingredient in their condition, so every change in the state of matter—whether from heat to cold, or from cold to heat; from a state of rest to that of motion; from the solid to the liquid, or the æriform condition, or vice versa; or whether substances combine chemically and produce new compounds—in every change the electrical equilibrium is disturbed; and, in proportion to the degree of disturbance, is the force exerted by electricity to resume its balance in the scale of nature.

By passing through substances that are favourable to its diffusion; therefore they are called conducting or non-conducting bodies, according as they favour or oppose the transmission of the electrical current.

Metals, charcoal, animal fluids, water, vegetable bodies, animal bodies, flame, smoke, vapour, &c.

Rust, oils, phosphorous, lime, chalk, caoutchouc, gutta percha, camphor, marble, porcelain, dry gases and air, feathers, hair, wool, silk, glass, transparent stones, vitrefactions, wax, amber, &c. These bodies are also called insulators. Some of these substances, as chalk, feathers, hair, wool, silk, &c., though non-conductors when dry, become conductors when wetted.

Insulating—preventing from escaping.

The result is a violent action in which, intense heat and light are developed, and in the evolution of which the electric force becomes expended.

The electric force, passing through a conducting body to find its equilibrium, is checked in its course by an insulator, and emits a spark.

Currents of electricity pass towards each other along wires at the ends of which two charcoal points are placed. As long as the charcoal points remain in contact, the electric communication is complete, and no light is emitted, but, when they are drawn apart, intense heat and light are evolved.