Definitions.

Definitions.

1. By a ray of light is meant the least particle of light that can be either intercepted or separated from the rest. In the diagrams by which the science is illustrated, rays of light are represented by right lines.

2. Any small parcel of rays proceeding from or to a point, considered apart from the rest, is called a pencil of rays.

3. A beam of light is used to denote any considerable aggregate of light, or parcel of rays.

4. Parallel rays are such as move always at the same distance from each other, as represented by fig. 3, pl. I.

5. Converging rays are such as approach nearer and nearer to each other, and tend to unite in a point; they are represented by fig. 4, forming a cone, the base of which is at the place where the rays began to converge.

6. Diverging rays are those which continue to recede further and further from each other through their whole progress, and if proceeding from a point, form, as shown by fig. 5, a cone, the base of which is the termination of their course.

7. The point at which converging rays meet, is called the focus.

8. When converging rays are prevented from meeting by some obstacle, the point towards which they tend, and where they would have united, if not intercepted, is called the virtual or imaginary focus.

9. Void space, and whatever substance the rays of light pass through, is called by opticians, a medium.

10. Media are either dense or rare: one medium is said to be more dense than another, when it is heavier, or contains more matter under the same bulk; and vice versa, it is called more rare than another, when it is lighter, or contains less matter under the same bulk. Glass is more dense than water; water is more dense than air.

11. By a lens is meant a transparent body of a different density from the surrounding medium, and terminated by two surfaces, either both curved, or one plane and the other curved. As lenses are commonly made of glass, it is usual to call them glasses, with the addition of some word or words, designating their use, or the nature of their curve, or both; as a magnifying glass, an object or eye-glass of a telescope, a concave spectacle-glass, a convex spectacle-glass.

12. An incident ray is that which comes from any body to the reflecting surface; the reflected ray is that which is sent back from the reflecting surface. Children sometimes amuse

Definitions.-Opinions of the ancients respecting light.

themselves with presenting a piece of looking-glass to the sun, and casting a vivid spot of light on any object they please. In this case, the rays received directly from the sun are called incident, and the lucid spot is made by the reflected rays.

13. The angle of incidence is the angle which is formed by the incident ray with a perpendicular to the reflecting surface at the point of incidence; and the angle of reflection is the angle formed by the same perpendicular and the reflected ray. Thus, in fig. 6, a ray of light, a, falls in the direction ad, upon the surface ef, and is reflected in the direction db; cd is perpendicular to ef, therefore a dc is the angle of incidence, and c d b is the angle of reflection.

14. A mirror, or speculum, is an opaque body, the surface of which is very smooth and finely polished, so that it will reflect the rays of light which fall upon it, and by this means represent the images of objects opposed to it.

15. Plane mirrors are those reflecting bodies, the surfaces of which are perfectly plane, such as our common looking glasses.

16. Concave and convex mirrors are those the surfaces of which are curved. If a watch-glass were silvered on the round side, it would be a concave mirror; if it were silvered on the hollow side, it would be a convex mirror.

Of Light.

The nature of light has been a subject of speculation from the first dawnings of philosophy. Several of the earliest philosophers thought that objects became visible by means of something proceeding from the eye; while some maintained, that vision was occasioned by particles continually flying off from the surfaces of bodies, which met with others proceeding from the eye; but Pythagoras is said to have ascribed the effect solely to the particles proceeding from external objects, and entering the pupil of the eye. The ancients possessed much greater ingenuity to invent theories, than inclination (or perhaps opportunity) to determine the truth of them by experiment; and therefore, if Pythagoras actually promulgated this opinion, it seems to have been supported by no facts which placed it on an immutable basis. Hence it gained for many ages no greater credit than other opinions, the most incongruous and vague. It was not till about a thousand years after the time of Pythagoras, that J. Baptista Porta fully satisfied himself and others of vision being performed entirely by the intromission of light into the eye.

Several eminent philosophers have imagined that the sensation which we receive from light is to be attributed entirely to

Opinions respecting light.

the vibrations of a subtile medium or fluid, diffused throughout the universe, and put in action by the impulse of the sun. According to this hypothesis, light may be considered as analogous to sound, which is known to depend entirely on the pulsations of the air striking upon the car. But this theory is encumbered with many difficulties. If light depended altogether upon the vibrations of a fluid, a quick motion of the hand, or of a machine contrived for the purpose, would produce light at any time; or, rather, as no solid reason can be assigned why the fluid should cease to vibrate in the night, since the sun must always affect some part of it, we ought to have perpetual day. Again, the texture of certain bodies is actually changed by exposure to light, even though they be inclosed in glass; but if covered with the thinnest plate of metal, which excludes the light, no alteration takes place. Many other objections which militate with equal force against this theory, might be adduced, but these have never received answers which can be deemed satisfactory.

A late writer is decidedly of opinion that light is diluted fire, that is, fire weakened and diffused, as ardent spirit when mingled with water. It appears at first view favourable to this opinion, that light may be collected and condensed by the burning glass, so as to burn like the fiercest flame; on the contrary, flame itself may be attenuated, even by artificial means, to such a degree as to be perfectly innoxious. "The flame," says Dr. Goldsmith, "which hangs over burning spirit of wine, we all know to scorch with great power, yet these flames may be made to shine as bright as ever, yet be perfectly harmless. This is done by placing them over a gentle fire, and leaving them thus to evaporate in a close room without a chimney; if a person should soon after enter with a candle, he will find the whole room filled with innoxious flames. The parts have been two minutely separated, and the fluid, perhaps, has not force enough to send forth its burning rays with sufficient effect." But when we consider the remarkable discovery of Dr. Herschel, that light may be separated from the caloric* which accompanies it, the identity of the two substances, if not fully disproved, becomes very doubtful. The Doctor, when employed in making observations on the sun by means of telescopes, found that the coloured glasses used to prevent the inconvenience arising from the heat, very soon

Caloric is the term by which modern philosophers have agreed to distinguish that peculiar substance which produces heat.

Separation of light and heat.-Velocity of light.

cracked and broke in pieces when their colour was deep enough to intercept the light. On prosecuting the inquiry, in a course of experiments devised for the purpose, he found that the solar beam contained rays which gave no light, and yet produced a greater degree of heat than even those rays which produced the strongest light. The two species of rays may be separated from each other by a very easy experiment. If a glass mirror be held before the fire, it strongly reflects the rays of light, but the rays of caloric which it sends forth are very few; a metallic mirror, on the other hand, made of planished tin-plate, for example, reflects the rays of light indifferently, but it supplies calorific rays in great abundance. The glass mirror becomes hot; the metallic mirror does not alter its temperature. If a plate of glass be suddenly interposed between a glowing fire and the face, it intercepts completely the warming power of the fire, without causing any sensible diminution of its brilliancy; consequently it intercepts the rays of caloric, but allows the rays of light to pass. If the glass be allowed to remain in its station till it becomes as hot as its distance from the fire will allow, it ceases to intercept the rays of caloric, which pass through as freely as the rays of light. These data lead us inevitably to the conclusion, that light is not diluted fire or caloric.

We shall now proceed to notice the most important properties of light in an optical point of view; a task much more delightful than the investigation of its nature and essence; and eminently calculated to entrance the mind with astonishment.

In the very short space of one second, a ray of light traverses the prodigious extent of nearly two hundred thousand miles. The manner in which the velocity of light is calcu lated, is not less ingenious than the discovery is surprising. It was by observing the eclipses of Jupiter's satellites, which appeared to be eclipsed sooner or later than the times given by the tables of them, and the observation was always before or after the computed time according as the earth was nearer to or further from the planet Jupiter than the mean distance. To understand this fact more fully, suppose the earth in going its annual circuit round the sun, is at C, (fig. 1, pl. I,) and that an eclipse is there observed of a satellite of Jupiter, which regularly suffers an eclipse in forty-two hours and a half. If the earth never left C, but continued there immoveable, and Jupiter remained at the same distance, the eclipse of the satellite would always be observable at the expected interval of forty-two hours and a half; and consequently, in thirty times forty-two hours and a half, the spectator would see thirty eclipses. But the earth travelling onward to D, the spectator

Velocity and minuteness of light.

does not see thirty eclipses till some time after the stated period, for the further off the earth removes, the longer the time required for the light to reach the spectator. In traversing from C to D, light takes up about sixteen minutes and a half, therefore, as the sun is half way between C and D, it must perform its journey from him in half that time, that is, in eight minutes and a quarter; or according to the most exact calculation, in eight minutes and seven seconds. No difference in the velocity of light has ever been discovered, whether it is original, as from the stars, or reflected only, as from the planets.

Such being the rapidity with which the rays of light dart themselves forward, it becomes a matter of easy calculation, and may a little assist our conception to observe, that a journey which they perform in eight minutes, could not be performed by a cannon ball at its ordinary speed in less than thirty-two years. That the motion of light is inexpressibly rapid, we may easily convince ourselves, if we notice the firing of a cannon or a musket at a considerable distance, and observe the time which elapses between seeing the flash and hearing the report. Sound, it has been calculated, travels at the rate of 1140 feet, or 380 yards in a second; yet the time which intervenes between seeing the flash and hearing the report of the fire arm, is a satisfactory proof of the prodigious disparity between the velocity of light and sound.

If the velocity of light, then, be so very great, it may be inquired why it does not strike against objects with a proportionate force. If the finest sand were thrown against our bodies with a hundredth part of this velocity, each grain would be as fatal as the stab of a dagger; yet our eyes, the most exquisitely sensible of all our organs of perception, receive its impressions without the smallest pain. But we have sufficient evidence to convince us that the minuteness of the particles of light is still more extraordinary than their velocity, and that their minuteness, therefore, is the cause of their being harmless. A lighted candle will fill a sphere of four miles in diameter, and may be extinguished without having lost any sensible portion of its weight; yet it must have diffused several hundreds of millions more particles of light than there could be grains in the whole earth, if it were entirely composed of the finest sand.

It is a principle in mechanics, that the momentum of moving bodies, or the force with which they strike, is proportionate to the quantity of matter they contain multiplied by their velocity; consequently, if the particles of light were not infinitely smaller than we can conceive, nothing could