ger of the lock. The condensed air being thus allowed to escape, impels the bullet in the barrel with a very considerable mo, mentum. Twelve pennyweights of air being thus injected into a ball 3 inches in diameter, has been found to drive out fifteen balls successively with great velocity. Sometimes the air-gun has two barrels, the space between which is the receptacle for the condensed air. In this case, the condenser is fixed to the barrel, and comes through the butt-end of the gun.

The magazine air-gun is furnished with a winding or serpentine barrel, which holds about a dozen balls. These are introduced one after another into the straight barrel by means of a lever, and may be discharged with such rapidity as to answer the purpose of several guns.

When the concave ball is filling with condensed air by means of the syringe, a flash of light is often perceived, capable of kindling a piece of match. This curious fact must have some connexion with the rise of the mercury in the thermometer when placed in condensed air, as mentioned in a preceding note.E. ED.

227

LECTURE VII.

Of Optics.

LIGHT consists of an inconceivably great number of particles flowing from a luminous" body in all manner of directions; and these particles are so small, as to surpass all human comprehension,

That the number of particles of light is inconceivably great, appears from the light of a candle; which, if there be no obstacle in the way to obstruct the passage of its rays, will fill all the space within two miles of the candle, every way, with luminous particles, before it has lost the least sensible part of its substance.

-

A ray of light is a continued stream of these particles, flowing from any visible body in a straight line: and that the particles themselves are incomprehensibly small, is manifest from the following experiment. Make a small pinhole in a piece of black paper, and hold the paper upright on a table facing a row of candles standing by one another; then place a sheet of pasteboard at a little distance behind the paper, and some of the rays which flow from all the candles through the hole in the paper will form as many specks of light on the pasteboard as there are candles on the table before the plate, each speck being as distinct

The amaz- and clear as if there were only one speck from ing small. ness of the one single candle: which shows, that the parparticles ticles of light are exceedingly small, otherwise of light. they could not pass through the hole from so

many different candles without confusion.Dr. Niewentyt has computed that there flows more than 6,000,000,000,000 times as many particles of light from a candle in one second of time, as there would be grains of sand in the whole earth, supposing each cubic inch of it to contain 1,000,000.*

• These particles by falling directly upon our eyes, excite in our minds the idea of light.When they fall upon bodies, and are thereby reflected to our eyes, they excite in us the ideas of these bodies. And as every point of a visible body reflects the rays of light in all manner of directions, every point will be visible in every part to which the light is reflected from it. PLATE Thus, the object ACB is visible to an eye in any part where the rays Aa, Ab, Ac, Ad, Ae, Ba, Bb, Bc, Bd, Be, and Ca, Cb, Cc, Cd, Ce, come. Here we have shown the rays as if they were only reflected from the ends A and B, and from the middle point C of the object;

XV.

Fig. 1.

* These remarkable facts have been used as arguments against the materiality of light, and as proofs of the theory of Huygens and Euler, which has lately been revived by Dr. Young of the Royal Institution. All objections, however, of this kind vanish when we attend to the continuation of the impression of light upon the retina, and to the small number of luminous particles which are on that account necessary for producing constant vision. It appears from the accurate experiments of M. D'Arcy, (Mem. Acad. Par. 1765. p. 450, Priestley on Vision, vol. 2, p. 634,) that the impression of light upon the re ina continues two minutes and forty seconds, and as a particle of light will move thirty-two millions of miles, during that time, constant vision would be maintained by a succession of luminous particles, thirty-two millions of miles distant from rach other.-E. Ep.

XV.

every other point being supposed to reflect PLATE rays in the same manner. So that wherever Reflected a spectator is placed with regard to the body, light. every point of that part of the surface which is toward him, will be visible, when no intervening object stops the passage of the light.

As no object can be seen through the bore of a bended pipe, it is evident that the rays of light move in straight lines, while there is nothing to refract or turn them out of their rectilineal course.

While the rays of light continue in any* medium of uniform density, they are straight; but when they pass obliquely out of one medium into another, which is either more dense or more rare, they are refracted toward the denser medium: and this refraction is more or less, as the rays fall more or less obliquely on the refracting surface which divides the mediums.

To prove this by experiment, set the empty Fig. 2. vessel ABCD into any place where the sun shines obliquely, and observe the part where the shadow of the edge BC falls on the bottom of the vessel at E; then fill the vessel with water, and the shadow will reach no farther than e; which shows, that the ray aBE, which came straight in the open air, just over the edge of the vessel at B to its bottom at E, is refracted by falling obliquely on the surface of Refracted the water at B; and instead of going on in the light. rectilineal direction aBE, it is bent downward

Any thing through which the rays of light can pass, is called a medium; as air, water, glass, diamond, or even a vacuum.

in the water from B to e; the whole bend be ing at the surface of the water: and so of all the other rays abc.

If a stick be laid over the vessel, and the sun's rays be reflected from a glass perpendicularly into the vessel, the shadow of the stick will fall upon the same part of the bottom, whether the vessel be empty or full, which shows, that the rays of light are not refracted when they fall perpendicularly on the surface of any medium.

The rays of light are as much refracted by passing out of water into air, as by passing out of air into water. Thus, if a ray of light flows from the point e, under water, in the direction eB, when it comes to the surface of the water at B, it will not go on thence in the rectilineal course Bd, but will be refracted into the line Ba: Therefore,

To an eye at e looking through a plane glass in the bottom of the empty vessel, the point a cannot be seen, because the side Bc of the vessel interposes; and the point d will just be seen over the edge of the vessel at B. But if the vessel be filled with water, the point a will be seen from e; and will appear as at d, elevated in the direction of the ray eB.*

* Hence a piece of money lying at e, in the bottom of an empty vessel, cannot be seen by an eye at a, because the edge of the vessel intervenes; but let the vessel be filled with water, and the ray e a being then refracted at B, will strike the eye at a, and so render the money visible, which will appear as if it were raised up to ƒ in the line a Bf. A.

If the piece of money mentioned in the preceding note be a six. pence or a shilling, and if it be placed at the bottom of a wineglass, a curious optical deception will take place. The real piece of money will be seen near the surface of the water, while another piece similar to a half-crown or crown, will appear at the bottom of the glass. The cause of this is obvious.-E. ED.