a piece of glass, or any other transparent substance, the surfaces of which are so formed, that the rays of light, by passing through it, are made to change their direction, either tending to meet in a point beyond the lens, or made to become parallel after converging or diverging; or lastly, proceeding as if they had issued from a point before they fell upon the lens. Some lenses are convex, or thicker in the middle; some concave, or thinner in the middle; some plano-convex, or plano-concave; that is with one side flat, and the other convex or concave; and some are called meniscuses, or convex on one side and concave on the other. See DIOPTRICS.

Lenses are of two kinds, either blown or ground.

Blown LENSES, are only made use of in the single microscope, and the common method of making them has been to draw out a fine thread of the soft white glass called crytal, and to convert the end of it into a sphere by melting it at the flame of a candle. Mr Nicholson observes that window glass affords excellent spheres. A thin piece from the edge of a pane of glass one-tenth of an inch broad was held perpendicularly, and the flame of a candle was directed against it by means of the blow-pipe, when it became soft, and the lower end descended by its own weight to the distance of about two feet, where it remained suspended by by a thin thread of glass about \( \frac{1}{60} \) of an inch in diameter. A part of this thread was applied endwise to the lower blue part of the flame of the candle without the blow-pipe, when the end became instantly white-hot, and formed a globule, which was gradually thrust towards the flame till it became sufficiently large. A number of these were made and examined, by viewing their focal images with a deep magnifier, when they appeared bright, perfect, and round.

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 MECHANICS.