of labor he cannot perfectly clear his glass of the noxious veins,
and he has to break it up into smaller pieces. When he finally
succeeds, the disk has the form of a thin grindstone two feet or
upward in diameter, according to the size of the telescope to be
made, and from two to three inches in thickness. The glass is then
ready for the optician.

[Illustration with caption: THE OPTICIAN'S TOOL.]

The first process to be performed by the optician is to grind the
glass into the shape of a lens with perfectly spherical surfaces.
The convex surface must be ground in a saucer-shaped tool of
corresponding form. It is impossible to make a tool perfectly
spherical in the first place, but success may be secured on the
geometrical principle that two surfaces cannot fit each other in
all positions unless both are perfectly spherical. The tool of the
optician is a very simple affair, being nothing more than a plate
of iron somewhat larger, perhaps a fourth, than the lens to be
ground to the corresponding curvature. In order to insure its
changing to fit the glass, it is covered on the interior with a
coating of pitch from an eighth to a quarter of an inch thick.
This material is admirably adapted to the purpose because it gives
way certainly, though very slowly, to the pressure of the glass.
In order that it may have room to change its form, grooves are cut
through it in both directions, so as to leave it in the form of
squares, like those on a chess-board.

[Illustration with caption: THE OPTICIAN'S TOOL.]

It is then sprinkled over with rouge, moistened with water, and