GROUND LENSES are such as are rubbed into the shape required, and polished. Several shapes have been proposed, but the spherical has been found to be the most practically useful. Yet by various modes of grinding, the artificer can produce no more than an approximation to a figure exactly spherical, and men of letters or others must depend entirely on the care and integrity of workmen for the sphericity of the lenses of their telescopes. Mr Jenkins has described a machine, which being so contrived as to turn a sphere at one and the same time on two axes, cutting each other at right angles, will produce the segment of a true sphere, merely by turning round the wheels, and that without any care or skill in the workmen. See MACHANICS.