A sphere is a round body like a ball or globe. It is round equally in all directions. Spherical curvature means that the curvature is identical in all directions. Spherical lenses mean that the surfaces of the lenses are spherical in shape. A spherical lens is really a piece of the outer portion of a sphere. A spherical plus curvature is ground on the surface of the lens with a minus tool. A minus curvature is ground with a plus tool.
When you get a prescription for a pair of glasses, the symbol of plus (+) or minus (-) is always designated, but seldom the sphere. It is assumed to be spherical. Thus a prescription for a 3/4 diopter plus lens will be +0.75, a minus 3/4 of a diopter will read -0.75; likewise +1.50 or -1.50, +3.00 and -3.00.
Spherical lenses can be correctly made in mass production. On a small comparative scale, they are now being made in mass production on precise automatic machines. They are being made to be sold ready-to-wear in frames, and also for the greater portion of spherical lenses, which are prescribed by oculists and optometrists.
Mass-produced spherical lenses can be more precise and less subject to mistakes and imperfections than the spherical lenses that are being ground in small shops. Mass-produced lenses can be, and are, produced cheaply.