In grinding the balls, they are placed singly within the conical hole of the grinder, a small quantity of oil and emery is then put into the space between the larger side of the cone and the ball, and the disks being put in rapid revolution, the ball and grinder are slipped in between them, while they are in motion. The grinder is held horizontally by the handle, and pressed sideways against the ball to keep the conical grinding surface equally in contact with the ball, which is at the same time slowly but uniformly traversed by the grinder around the disks, within about one inch of their edges, as of course the further the ball is kept from the center of the disks, the more rapidly it will be rotated.

After a few revolutions of the ball around the disks, the latter become slightly indented with circular grooves, which serve as guides for the path in which the ball is traversed. Care is required to keep the disks pressing against the ball sufficiently tight to cause its rotation within the grinder, or otherwise from the surfaces of the disks becoming charged with emery they will act as laps and grind facets upon the ball.

It is quite necessary that the ball should be constantly traversed around the disks with uniform motion, as should it be permitted to linger for a longer time at1 one part of the circle than another, the ball would be more ground at that part, and become oval. The necessary supply of emery and oil is given without removing the ball from between the disks, by keeping up the circular motion with the one hand, while a little oil is dropped upon the ball as it revolves, and the emery may be sprinkled upon it in like manner.

The grinding is continued until the ball is made truly spherical, and so near to the required size that upon trial it will barely enter the ring gage, previously prepared of the exact diameter. The final adjustment for size is given in the polishing process, which is effected with dry crocus, sometimes applied on a conical tool of boxwood of exactly the same form as the brass grinder, the revolving disks being covered with leather or cloth to prevent the ball from being scratched. But where great accuracy of size is required, this method is almost too active for the final adjustment, and the method more completely under control, is to polish the balls by rubbing them in all directions with the fingers, within a conical brass tool supplied with dry crocus. This removes the circular marks given between the disks, produces a good lustre, and allows of the adjustment for size being effected with almost any required degree of exactness.

Spheres in glass, agate or other hard substances that do not admit of being turned with cutting tools, are prepared as nearly as admissible of the spherical form by grinding them by hand after the method of the lapidary. The balls are completed by grinding them between the revolving disks, with a brass or iron grinder just the same as the hardened steel balls, except that water is employed with the emery instead of oil, partly with the view of reducing the heat occasioned by the friction, which in the case of grinding glass spheres is liable to cause them to become cracked, and therefore in grinding glass balls the velocity of the disks should be only moderate, and water should be supplied in sufficient quantity to keep the balls tolerably cool.

The glass balls are lastly polished with putty powder, applied on a wooden polishing tool, the conical surface of which is covered with wash leather, by passing the latter through the hole, and securing it around the margins with a few tacks.

The method of grinding marbles for children is described in the catalogue of grinding processes page 1078.

The spherical surfaces of lenses, are produced by grinding them in counterpart tools, or disks of metal, prepared to the same curvatures as required in the lenses, and employed as the medium for the application of the grinding and polishing powders. The tools are made in pairs, concave and convex, and are first employed mutually to correct each other's errors; as the accuracy of the surfaces of the lenses is principally dependent on the tool upon which they are ground, being accurately formed to the counterpart figure.

For the formation of the grinding tools, a concave and a convex template are first made to the radius of the curvature of the required lens. The templates of large radius, are sometimes cut out of crown glass by cementing it upon a bench, and mounting a glazier's diamond upon the end of a light radius bar, sometimes only a rod of wood, with a brad awl stuck through the rod into the bench, the distance from the diamond to the awl being the radius of the curve. The glass having been cut with the diamond, is separated, the one cut forming the concave and convex edges, which are afterwards ground together with a little emery and water, for this purpose the templates are laid upon the bench and rubbed edge to edge; one of the pieces is occasionally turned end for end to verify the curves. See Tech. Repos. 1822 page 365.

More generally however templates of large and medium radii are made out of sheet brass, the templates of long radii are cut with a strong radius bar and cutter, and those of only a few inches radii are cut in the turning lathe. The brass concave and convex gages are cut at separate operations, as it is necessary to adjust the radius to compensate for the thickness of the cutter, and the brass templates are not usually corrected by grinding, as practically it is found more convenient to fit the tools themselves together.

The templates having been made of the required radius, are used for the preparation of the grinding and polishing tools, which for convex lenses consist of a concave rough grinding tool of cast iron, called a shell, shown in section in fig. 1127, the wooden pattern of which is turned to the curve of the template, and the shell is left from the casting; a similar shell, turned to a radius of about three-eighths of an inch larger than the template, serves as the foundation of the polisher, the preparation of which is described in page 1267. For common glasses, that are ground several together, a convex tool of cast iron, called a runner, of about half an inch less radius than the templates, is also required, as the basis upon which the lenses are cemented, as shown in fig. 1129.