re either plane, convex, or concave. The first reflect the rays of light in a direction exactly similar to that in which they fall upon them, and therefore represent bodies of their natural magnitude. The convex ones make the rays diverge much more than before reflection, and therefore greatly diminish the images of those objects which they show: while the concave ones, by collecting the rays into a focus, not only magnify the objects they show, but will burn very fiercely when exposed to the rays of the sun; and hence they are commonly known by the name of burning mirrors. See Burning Mirrors.

In ancient times the mirrors were made of some kind of metal; and from a passage of the Mosaic writings we learn that the mirrors used by the Jewish women were made of bras. The Jews certainly had been taught to use that kind of mirrors by the Egyptians; from whence it is probable that brazen mirrors were the first kind used in the world. Any kind of metal, indeed, when well polished, will reflect very powerfully; but of all others silver reflects the most, though it has been in all countries too expensive a material for common use. Gold also is very powerful; and metals, or even wood, gilded 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, became generally known, it hath been universally employed for those plane mirrors used as ornaments to houses; but in making reflecting teleopes, they have been found much inferior to metallic ones. It doth not appear that the same superiority belongs to the metallic burning mirrors, considered merely as burning glasses; since the mirror with which M. Macquer melted platinum, though only 22 inches diameter, and which was made of quicksilvered glass, produced much greater effects than M. Vilette's metallic speculum, which considerably exceeded it in size. It is very probable, however, that this mirror of M. Vilette's was by no means 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 Philosophical Transactions by Mr Mudge. See Glass-Grinding.