, a speculum, looking-glass, or any polished body, whose use is to sorm the images of distinct objects by reflexion of the rays of light.

Mirrors are either plane, convex, or concave. The first sort reflects the rays of light in a direction exactly similar to that in which they fall upon it, and therefore represents bodies of their natural magnitude. But the convex ones make the rays diverge much more than before reflexion, and therefore greatly diminish the images of those objects which they exhibit: while the concave ones, by collecting the rays into a focus, not only magnify the objects they shew, but will also burn very fiercely when exposed to the rays of the sun; and hence they are commonly known by the name of burning Mirrors.

In ancient times the Mirrors were made of some kind of metal; and from a passage in the Mosaic writings we learn, that the Mirrors used by the Jewish women, were made of brass; a practice doubtless learned from the Egyptians.

Any kind of metal, when well polished, will reflect very powerfully; but of all others, silver reflects the most, though it has always been too expensive a material for common use. Gold is also very powerful; and all metals, or even wood, gilt and polished, will act very powerfully as burning Mirrors. Even polished ivory, or straw nicely plaited together, will form Mirrors capable of burning, if on a large scale.

Since the invention of glass, and the application of quicksilver to it, have become generally known, it has been universally employed for those plane Mirrors used as ornaments to houses; but in making reflecting telescopes they have been found much inferior to metallic ones. It does not appear however that the same superiority belongs to the metallic burning Mirrors, considered merely as burning speculums; since the Mirror with which Mr. Macquer melted platina, though only 22 inches diameter, and made of quicksilvered glass, produced much greater effects than M. Villette's metal speculum, which was of a much larger size. It is very probable, however, that M. Villette's Mirror was not so well polished as it ought to have been; as the art of preparing the metal for taking the finest polish, has but lately been discovered, and published in the Philos. Transactions, by Dr. Mudge of Plymouth, and, after him, by Mr. Edwards, Dr. Herschel, &c.

Some of the more remarkable laws and phenomena of plane Mirrors, are as follow:

1. A spectator will see his image of the same size, and erect, but reversed as to right and left, and as far beyond the speculum as he is before it. As he moves to or from the speculum, his image will, at the same time, move towards or from the speculum also on the other side. In like manner if, while the spectator is at rest, an object be in motion, its image behind the speculum will be seen to move at the same rate. Also when the spectator moves, the images of objects that are at rest will appear to approach or recede from him, after the same manner as when he moves towards real objects.

2. If several Mirrors, or several fragments or pieces of Mirrors, be all disposed in the same plane, they will only exhibit an object once.

3. If two plane Mirrors, or speculums, meet in any angle, the eye, placed within that angle, will see the image of an object placed within the same, as often repeated as there may be perpendiculars drawn determining the places of the images, and terminated without the angle. Hence, as the more perpendiculars, terminated without the angle, may be drawn as the angle is more acute; the acuter the angle, the more numerous the images. Thus, Z. Traber found, at an angle of one-3d of a circle, the image was represented twice, at 1/4th thrice, at 1/6th five times, and at (1/12)th eleven times.

Farther, if the Mirrors be placed upright, and so contracted; or if you retire from them, or approach to them, till the images reflected by them coalesce, or run into one, they will appear monstrously distorted. Thus, if they be at an angle somewhat greater than a right one, the image of one's face will appear with only one eye; if the angle be less than a right one, you will see 3 eyes, 2 noses, 2 mouths, &c. At an angle still less, the body will have two heads. At an angle somewhat greater than a right one, at the distance of 4 feet, the body will be headless, &c. Again, if the Mirrors be placed, the one parallel to the horizon, the other inclined to it, or declined from it, it is easy to perceive that the images will be still more romantic. Thus, one being declined from the horizon to an angle of 144 degrees, and the other inclined to it, a man sees himself standing with his head to another's feet.

Hence it appears how Mirrors may be managed in gardens, &c, so as to convert the images of those near them into monsters of various kinds; and since glass Mirrors will reslect the image of a lucid object twice or thrice, if a candle, &c, be placed in the angle between two Mirrors, it will be multiplied a great number of times.

Laws of Convex Mirrors.

1. In a spherical convex Mirror, the image is less than the object. And hence the use of such Mirrors in the art of painting, where objects are to be represented less than the life.

2. In a convex Mirror, the more remote the object, the less its image; also the smaller the Mirror, the less the image.

3. In a convex Mirror, the right hand is turned to the left, and the left to the right; and magnitudes perpendicular to the Mirror appear inverted.

4. The image of a right line, perpendicular to the Mirror, is a right line; but that of a right line oblique or parallel to the Mirror, is convex.

5. Rays reflected from a convex Mirror, diverge more than if reflected from a plane Mirror; and the smaller the sphere, the more the rays diverge.

Laws of Concave Mirrors.

The effects of concave Mirrors are, in general, the reverse of those of convex ones; rays being made to converge more, or diverge less than in plane Mirrors; the image is magnified, and the more so as the sphere is smaller; &c, &c.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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