. . .A convex glass is thickest in the middle, and thinner towards the edges. A concave glass is thin in the middle, and thicker towards the extremities. Of these there are various forms, which are represented in fig. 5. A, is a plano-convex lens, which has one side plane, and the other spherical or convex. B, is a plano-concave, which is plane on the one side and concave on the other. C, is a double-convex, or one which is spherical on both sides. D, a double-concave, or concave on both sides. E, is called a meniscus, which is convex on one side and concave on the other. F, is a concavo-convex, the convex side of which is of a smaller sphere than the concave. In regard to the degree of convexity or concavity in lenses, it is evident that there may be almost an infinite variety. For every convex surface is to be considered as the segment of a circle, the diameter and radius of which may vary to almost any extent. Hence, lenses have been formed by opticians, varying from . . . Read More