257. OPTICS is that branch of Physics which treats of the phenomena of light.

Definition of Light.

258. LIGHT is that physical agent which, acting upon the eye, produces the sensation of sight.

Two Theories of Light.

259. Two theories have been advanced to account for the phenomena of light: the Emission Theory, and the Undulatory, or Wave theory.

According to the emission theory, light consists of infinitely small particles of matter, shot forth from luminous. bodies with immense velocity, which, falling on the retina of the eye, produce the sensation of sight.

According to the undulatory theory, light, like heat, is caused by the vibrations of the molecules of bodies. It is transmitted by a highly elastic medium called ether.

(257.) What is Optics? (258.) What is Light? (259.) What two theories of light have been advanced? Explain the emission theory. Explain the wave theory.

This medium, which also transmits radiant heat, extends through space, penetrates all bodies, and exists in the intervals between their molecules. The molecular vibrations of a luminous body are imparted to the neighboring ether, and are propagated through it by a succession of spherical waves; these waves falling on the retina of the eye excite the sensation of sight.

Light and radiant heat are very closely related to each other; they are generated in the same manner and are propagated through the same medium, but they differ from each other in their wave length, and as a consequence in their mode of action on bodies.

In sound the particles of air vibrate to and fro in the direction of propagation; in light and radiant heat the particles of ether vibrate to and fro in a direction perpendicular to that of propagation. In sound the vibrations are longitudinal, or in the direction of the rays; in light and radiant heat they are transversal, or perpendicular to the rays.

The idea of transversal vibrations may be illustrated by a rope made fast at one end and held by the hand at the other. If the free end be moved rapidly to and fro, at right angles to the rope a succession of waves will run along the rope, whilst the particles of the rope simply vibrate back and forth in perpendiculars to the rope. If a stone be dropped into a pool of still water, a series of waves will be propagated outward, whilst the particles of water simply rise and fall, their motion being perpendicular to the direction of propagation.

Luminous Bodies.-Sources of Light.

200. Bodies that emit light are said to be luminous; those that are seen by light derived from others are said to be illuminated. Luminous bodies generate light; illuminated bodies reflect and diffuse it. The sun is a luminous

body; the moon is illuminated by it.

The principal sources of light are the sun, the stars, heat, chemical combination, phosphorescence, and electricity.

How is light imparted to the ether? How propagated? Relation between light and radiant heat. Difference. Illustrate the idea of transversal vibrations. (260.) Define a luminous body. An illuminated body. Illustrate.

The ultimate cause of the sun's light is unknown. The sun is surrounded by a gaseous envelope, called the photosphere, which appears to be in a state of intense ignition. The molecular vibrations of this envelope are undoubtedly the immediate sources of solar light and solar heat. The stars are similar to the sun, but on account of their enormous distances from us they send us but a small amount of light and heat.

If a body be heated its molecules are thrown into vibration, and when its temperature reaches 900° or 1000° F., it begins to be luminous in the dark. Beyond that its brightness increases as its temperature rises.

The light developed by chemical combinations is mostly due to the heat that accompanies t.cm. Combustion is an example; the affinity between the oxygen of the air and the carbon of the fuel causes them to rush together under favorable circumstances, thus generating heat and ultimately light itself.

Phosphorescence is the property that some bodies have of giving out light under certain conditions; it is often observed in decaying animal and vegetable matter and in some minerals.

Electricity is the source of a species of light that rivals in intensity that of the sun itself. It will be treated of hereafter.

Media. Opaque and Transparent Bodies.

261. A MEDIUM is anything that transmits light; thus, free space, air, water, and glass, are media.

Media owe their property of transmitting light to the ether which pervades them. This ether exists in the spaces between the particles of all bodies, but not always in such a state as to permit the transmission of light.

A TRANSPARENT BODY is one that permits light to pass through it freely, as glass, diamonds, rock-crystal, and water.

When bodies permit light to pass through them, but not in such quantity as to allow objects to be seen through them,

What is Phosphorescence? Illustrate. What is its cause? (261.) What is a Medium? Examples. What is a Transparent Body?

they are called translucent. Thus, scraped horn, ground glass, oiled paper, and thin porcelain are translucent.

An OPAQUE BODY is one that does not permit light to pass through it. Thus, iron, wood, and granite are opaque bodies.

Absorption of Light.

262. No body is perfectly transparent; all intercept or absorb more or less light, but some absorb much more than others. If light be transmitted through great thicknesses of media which in thin layers are transparent, a quantity of light is absorbed, and it often happens that the transmitted light is not of sufficient intensity to produce the sensation of sight.

The atmosphere seems perfectly transparent, but it is a known fact that much of the light of the sun is absorbed in reaching the earth, as is shown by the greater brilliancy of the stars in the higher regions, as on mountain tops. In the high regions of the atmosphere, objects are more clearly seen than nearer the earth; indeed so great is the clearness of vision in these regions, that it becomes exceedingly difficult to judge of distances. Opaque bodies absorb all of the light falling upon them which is not reflected.

The physical cause of absorption of light by bodies is some peculiarity of molecular constitution, which breaks up and neutralizes the waves of light that enter them.

Rays of Light.- Pencils. Beams.

263. A RAY of Light is a line along which light is propagated. It is normal to the advancing wave front. When the source is very distant the wave fronts are sensibly plane and the rays parallel.

When the ether is uniformly distributed throughout a medium,

A Translucent Body? An Opaque Body? (262.) Explain the phenomenon of absorption. Effect of atmospheric absorption Physical cause of absorption ? (263.) What is a ray of light?

the waves of light are concentric spheres, and the rays of light are straight lines, because a perpendicular to one wave front will be perpendicular to all of the successive stages of that front. Media, in which the ether is uniformly distributed, are, with respect to light, called homogeneous. All other media are called heterogeneous. When the waves of light are not concentric spheres, the rays of light are curved. Such, for example, are the rays of light transmitted through the atmosphere.

A PENCIL OF RAYS is a small group of rays meeting in a common point, such as the rays proceeding from a candle or a lamp.

When the rays proceed from a common point, they are are said to be divergent. When they proceed towards a common point, they are said to be convergent.

A BEAM OF RAYS is a small group of parallel rays, such as enter a small hole in a shutter, from a distant body, as the sun.

Velocity of Light.

264. It was shown by RŒMER, a Danish astronomer, in 1678, that light occupies nearly 81 minutes in coming from the sun to the earth, which gives a velocity of 186,000 miles per second.

He ascertained the velocity of light by a succession of observations on the eclipses of Jupiter's first satellite. In Fig. 155, S represents the sun, T, the earth, J, Jupiter, and e, Jupiter's first satellite. The darkened portion of the figure beyond Jupiter represents the shadow of that planet cast by the sun. It is known by computation, that Jupiter's first satellite revolves about that planet once in 42 hours, 28 minutes, and 36 seconds, and by entering the shadow of Jupiter, is eclipsed at each revolution.

What is the direction of a ray in a homogeneous medium? What is a homogeneous medium? A heterogeneous medium? Direction of a ray in such a medium? What is a Pencil of Rays? Example. Convergent? Divergent? What is a Beam of Rays? Example. (264.) What is the velocity of light? By whom determined?