Lens in Optics, a piece of glass, or other transparent substance, having its two surfaces so formed that the rays of light have their direction changed by passing through it; so that they either converge, tending to a point beyond the lens; or diverge, as if they proceeded from a point before the lens; or become parallel, after converging or diverging. Some lenses are convex, that is, thicker in the middle than towards their circumferences; those that swell on both sides are called double convex lenses; some are concave, or thinner in the middle; some are plano-convex, or flat on one side and swelling on the other; some are plano-concave, or flat on one side and concave on the other; and lastly, some are concave on both sides. According to some opticiaus, the greatest diameter of a lens is half an inch, if it exceed that thickness they do not call it a lens, but a lenticular glass. Lenses are made either by blowing or grinding. Blown lenses are small globules of glass melted in the flame of a lamp: ground lenses are reduced by grinding and polishing.

A variety of simple apparatus is employed in the processes of grinding and polishing lenses, amongst which the following was recently introduced to the notice of the members of the London Mechanics' Institution, by Dr. Birkbeck, in one of his interesting lectures on improved mechanical inventions, a shows the edge of a circular lap or slab, used for grinding flat glasses upon; b a circular tool or block, upon the under surface of which the glasses to be ground are cemented; c is a reciprocating bar; d a box containing any weighty matter; e a long morticed aperture in the frame, through which the bar c freely works; f a crank; g a winch; h a double pulley wheel, the axis of which rests in the block i; j a single pulley wheel. Now on turning the crank by the winch g, the bar c gives to b an eccentric motion; the attrition of b on the surface of the lap a being increased or diminished at pleasure by increasing or diminishing the load in the box d. It should be noticed, that the cord which passes round the pulley h is crossed previous to its embracing the periphery of the pulley i, consequently a motion is given to the lap a the reverse of that given to b, which is considered to produce the best effect of grinding.

The apparatus described is devoted to the producing of plane surfaces to optical glasses; but the apparatus on the other side of the machine, is at the same time, by similar arrangements, employed in grinding concave or convex surfaces. For this purpose a variety of laps and other tools, similar to those delineated in the margin, are so made as to fit on the bed l, which bed is adjustable by four equidistant screws. The pulley o is driven by another band on the pulley h, and the required pressure given by another loaded box p; the use of the lower tool t was not apparent to us (when we took our sketches), but we conjecture it is intended to fix a diamond at the point, for cutting the glasses out of a true circular figure, by being screwed on at m. The several tools used are adapted for ready changing, that the operations may be performed with celerity. See Optical Instruments.