onstration, or proof. By some, it has been described as consisting of very minute particles, which are thrown off from what are called luminous bodies, in all directions, and with immense velocity; while others consider it as the effect of an undulation, or vibration, produced by luminous bodies in the thin and elastic medium which is interposed between them and the seat of our vision; this vibration producing an effect upon our organs, which we recognize as light, analogous to the impression of sound upon the ear, caused by the atmosphere. This theory is called the undulatory theory of light; and the former one, in which light is supposed to consist of material particles, is called the theory of emission. Whatever may be the cause, or absolute nature, of light, we know it is a remarkable property of luminous bodies; that it enables us to see the luminous objects themselves, as well as others; and that its absence produces darkness.

All visible bodies may be divided into two classes self-luminous and non-luminous. Under the first head are comprised all those bodies which possess in themselves the property of exciting the sensation of light, or vision; such as the heavenly luminaries, terrestrial flames of all kinds, phosphorescent bodies, and those substances which shine by being heated, or by friction. Under the second class, we recognize such bodies as have not, of themselves, the power of throwing off particles or undulations of light, but which possess the power of reflecting the light which is cast upon them by self-luminous bodies. A non-luminous body may thus, by reflection, receive light from another nonluminous body, and communicate it to a third, and so

on; all reflected light, however, is inferior, in point of brilliancy, to that which comes direct from a self-luminous body. The transmission of light was formerly supposed to be instantaneous; but recent observations have shown that, like sound, it requires a certain time to pass from one place to another, though the velocity of its motion is truly astonishing, as has been manifested in various ways. Astronomers have proved, by observing the eclipses of Jupiter's satellites when that planet is nearest, and when it is farthest, from the earth, that light moves from the sun to the earth, a distance of 95 millions of miles, in seven and a half minutes, or about 200,000 miles during a single vibration of a pendulum! So prodigiously great is this velocity, that, as far as any of the common affairs of life may be concerned, light may be said to be instantaneous in its universal action.

Light proceeds in a straight direction from the luminous body which produces it. The direct shining of the sun, or any other luminous body, is in the form of rays, or thin, ethereal lines, each acting independently of the other. No such separation of parts, however, is observable in common circumstances, in consequence of the diffusive properties of our atmosphere. Seeing is simply the reception of the direct or reflected ray from an object, by our eye. Until the rays of the sun reach the spot on which we are placed, we are neither conscious of light, nor of the presence of the sun as an object. In the same manner, a candle, being lighted, and exposed in the open country in a dark night, all who are able to see it are within the influence of its rays; but beyond a given distance,

these rays are too weak to produce vision; and all who are in this remote situation cannot see the smallest appearance of the light. Yet the number of rays which proceed even from a common candle is so vast as to be beyond the power of imagination to conceive; for if such a light is visible within a sphere of 4 miles, it follows that, if the whole of that space were surrounded with eyes, each eye would receive the impression of a ray of light. In proportion as light advances from its seat of production, it diminishes in intensity. The ratio of diminution is agreeable to that which governs physical forces; that is, the intensity of the light will diminish as the square of the distance increases, or at the rate of 1, 4, 16, &c. But, in proportion as we lose in intensity, we gain in volume; the light is the weaker the farther it is from the candle, but it fills a wider space.

In discussing the properties of light, it is important to consider the medium through which it passes, as air, water, glass, &c. Any parcel of rays passing from a point, is called a pencil of rays; the point at which converging rays meet, is called a focus. Rays may be parallel, convergent, or divergent, which terms will not require an explanation. The point towards which they tend, but which they are prevented from reaching by some obstacle, is called the imaginary focus.

REFRACTION is the bending of rays of light from the course they first pursued. If the rays, after passing through a medium, enter another of different density, perpendicular to its surface, they are not refracted, but proceed through this medium, in their original direction. For instance, if the rays of the sun were to

strike upon the surface of a river at right angles, or perpendicularly to its surface, they would go straight to the bottom, and the line which they pursued in the air would be continued in the water. But if they enter obliquely to the surface of a medium either denser or more rare than what they moved in before, they are made to change their direction in passing through that medium; in other words, they are refracted. The mode of refraction depends on the comparative density or rarity of the respective media. If the medium which the rays enter be denser, they move through it in a direction nearer to the perpendicular drawn to its surface. On the contrary, when light passes out of a denser into a rarer medium, it moves in a direction farther from the perpendicular. This refraction is

greater or less; that is, the rays are more or less bent, or turned aside, from their course, as the second medium, through which they pass, is more or less dense than the first. To prove this in a satisfactory way, take an upright empty vessel into a darkened room, which admits but a single beam of light obliquely through a hole in the window-shutter. Let the empty vessel stand on the floor a few feet in advance of the window which admits the light, and let it be so arranged that, as the beam of light descends towards the floor, it just passes over the top of the side of the vessel next the window, and strikes the bottom on the side farthest from the window. Let the spot where it falls be marked. Now, on filling the vessel with water, the ray, instead of striking the original spot, will fall considerably nearer the side towards the window. And