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 that the virtual image is greatly diminished. Concave lenses, as the student has no doubt already guessed, do not give real images.

The effects produced by the action of concave mirrors may be produced with just as much facility by convex lenses. If a body is placed in a focus of a lens which receives the direct rays of the sun, the heat as well as the light will be concentrated at one point; and if the object is combustible, it will take fire sooner or later, according to the size of the lens. All the experiments mentioned by Buffon as being produced by a concave mirror are equally obtainable with a concave lens. When of sufficient diameter, the most refractory metals, such as platinum or iridium, may be melted and dissipated into vapour. Before lucifer matches and vesuvians were as common as they are now, it was not at all unusual to find smokers carrying a small burning-glass and a piece of tinder, for the purpose of lighting their pipes or cigars; and there hardly exists a boy who has not lighted a bonfire in the fields or playground by means of an old spectacle lens or telescope glass.

Amongst other applications of this property of lenses may be mentioned that of causing guns to fire at a certain time, by arranging a small burning-glass above the touch-hole. In the Gardens of the Palais Royal, at Paris, there is such a gun, so arranged that on sunny days it fires exactly at noon, or, in other words, at the moment the sun comes to the meridian. Every fine day towards twelve o'clock, crowds of Parisians who have nothing to do may be seen bending their steps towards the Palais Royal to set their watches by the gun, which they believe to be superior as a time-keeper to the finest chronometer in the world. There they stand, most of them old fellows with a scar or two about their faces, showing that they have nobly won the rest they appear to enjoy so innocently and calmly with