Fig. 1.—a, double convex; b, piano-convex; c, concavo-convex or converging meniscus; d, double concave; e, piano-concave; f, divergent meniscus.

The first three, a, b, c, which are thicker at the centre than at the margins, are convergent or positive; and the second three, d, e, f, are divergent or negative. All lenses are formed by the union of prisms, and therefore have to a great extent the properties of prisms Fig. 2 will show the way in which the prisms


FIG. 2.

are united to form a biconvex lens, and the concentration of the rays of light by such prisms or lens.

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