Ground LENSES, are such as are ground or rubbed into the desired shape, and then polished. Different shapes have been proposed for lenses; but in the article OPTICS, n° 251 (Encycl.), it has been shewn that, after all, the spherical is the most practically useful. By many of the methods of grinding, however, the artificer, with his utmost care, can only produce an approximation to a truly spherical figure; and, indeed, gentlemen have, for the most part, nothing to depend on for the sphericity of the lenses of their telescopes, but the care and integrity of the workmen. In the 41st volume of the Transactions of the Royal Society of London, a machine is described by Mr Samuel Jenkins, which, as it is contrived 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. The following description of this machine will enable our readers fully to comprehend its construction, and the mode of using it: A is a globe covered with cement, in which are fixed the pieces of glass to be ground. This globe is fastened to the axis, and turns with the wheel B. C is the brass cup which polishes the glass: this is fastened to the axis, and turns with the wheel D. The motion of the cup C, therefore, is at right angles with the motion of the globe A; whence it follows demonstrably, that the pieces of glass ground by this double motion must be formed into the segments of spheres.