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we promised to say sometmng aDout lenses and their uses, and wo now proceed to fulfil our promise. A lens is a medium of any transparent substance so shaped that the rays of light passing through it are either collected or dispersed. The name, lens, is taken from the Latin word for a small flat bean, which the burning glasses of the ancients, to which the term was first applied, somewhat resembles. Each beam of light has what is callod its axis, and this is the middle ray, and a right line passing through the center of a lens, where the rays aro not at all affected, is its axis. The point at which the rays of the sun or other light are collected by passing through a lens, is oalled the focus of the lens, and when rays of light fall upon a lens, they are said to do so either directly or obliquely. No. 1 is a double convex lens, or two convex surfaces on the ono glass; No. 2 is a plano-convex lens, having one side flat and the other curved outwards; those magnify by increasing the angle of vision. No. 3 is a double concave lens, having two surfaces curved inwards; No. 4 is a plano-concave, having a plane surface and one curved inward; these concave glasses diminish objects soen through them by collecting instead of dispersing the rays and diminishing the anglo of vision. No. 5 is a meniscus lens, having one of its sides concave and the othor side convex, gradually ncaring each other, until they meet at tha edge. No. 6 is a concavo-convex, having surfaces like a meniscus, only that the curves recpilo from each other, instead of nearing, ns in the former. What tha focn3 of a lens is, the engraving will Illustrate. Suppose A B to be a plano-oonvexlens, of course the distance of the focus depends upon the convexity, but It may always be calculated, because if the circle of which the curve A B forms a part were continued, it would be at C, or exactly at the opposite side of the tirclo on the axis of the lens. In the double convox lens, the focus is nearly in the center of the circle, because they nre bent to double the angle which they would bo by a simple planoconvex. Many wonders are related of the burning glass, which is a double convex lens; thus it is said that Archimedes burnt the Roman fleet, in the harbor of Syracuse, by moans of glasses and mirrors, and there is little doubt that they were extensively used by the Egyptian and Hindoo priests, for lighting fires miraculously and performing "mirn-cles" generally. The largest burning glass is now at Pekin, where it was left by ivn English offieer; it is three feet in diameter, and is three and a half inohes thick through its center, and weighs 212 pounds ; ton grains of common slate were fused by it in two seconds, and ten grains of cast iron in three. Many great men have amused themselves with these I philosophical toys, among whom we may ,C mention Napier, the inventor of logarithms, |j and Newton, the demonstrator of gravity.
