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 rendered 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.

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 crystal, and to convert the end of it into a spherule by melting it at the flame of a candle. Mr Nicholson observes, that window-glass affords excellent spherules. A thin piece from the edge of a pane of glass one tenth of an inch thick 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 a thin thread of glass about $\frac{1}{100}$th 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 to be bright, perfect, and round. Ground lenses are such as are rubbed into the shape required, and polished.