Let us look at some of the instruments designed by a master for shaping glass discs into lenses. Some of the best telescopes in existence are from the hands of Mr. John A. Brashear, of Allegheny, Pennsylvania. The grinding tools he employs he has contoured in such wise as to produce desired curves free from error. The first polishers are of the ordinary form with square or circular facets equally distributed over the surface of the tool, as in Figs. H and 8. When the polish is brought to its best, the glass is allowed to cool slowly to a normal temperature, and is then carefully studied as to its defects. These are removed and the surfaces finished with iron tools, of the same diameter as the surface to be worked, each tool being laid off into six sections, as in Figs. 3, 4, 5, 6, 7. The tool being warmed, pitch is spread over its leaf-like spaces, which are given the proper curve by being pressed down on the previously wetted concave surface; the pitch and tool are next quickly cooled with water. In the shaping of these spaces rests success. The zone, a, a, in the first figure, needing the greatest amount of abrasion, meets the widest part of the leaflet, but in order that no zonal error may be introduced, as in b, c, c, b, of the second figure, it is gently tapered in each direction, the amount of taper being governed by the lateral stroke given to the polisher, as well as by the amount of departure of the zone from the normal curve. [Illustration: Tools for producing optical surfaces. John A. Brashear, Allegheny, Pa.] But after all the astronomers aided by lenses thus carefully shaped are few, while millions of people suffer from defects of sight which are overcome by suitably formed spectacles. Bi-focal Spectacles. In this field a recent minor improvement is worthy of mention. Benjamin Franklin many years ago made a pair of spectacles in which the upper half of each glass was ground for far seeing, the lower half for near seeing. To-day such bi-focal spectacles are not made in halves, with an unpleasant broken line across them. In each of the new eyeglasses toward the base a small lens of dense quality is enclosed; through this lens a wearer looks at objects nearby; through the upper part of the eyeglass he looks at distant objects. The joining of the three parts is effected so skilfully as not to be discernible. [Illustration: Bi-focal lens for spectacles.] Economy of Heat.