gently warmed. The roughly ground lens is then placed upon it, and
moved from side to side. The direction of the motion is slightly
changed with every stroke, so that after a dozen or so of strokes
the lines of motion will lie in every direction on the tool. This
change of direction is most readily and easily effected by the
operator slowly walking around as he polishes, at the same time
the lens is to be slowly turned around either in the opposite
direction or more rapidly yet in the same direction, so that the
strokes of the polisher shall cross the lens in all directions.
This double motion insures every part of the lens coming into
contact with every part of the polisher, and moving over it in
every direction.

Then whatever parts either of the lens or of the polisher may be
too high to form a spherical surface will be gradually worn down,
thus securing the perfect sphericity of both.

[Illustration with caption: GRINDING A LARGE LENS.]

When the polishing is done by machinery, which is the custom in
Europe, with large lenses, the polisher is slid back and forth
over the lens by means of a crank attached to a revolving wheel.
The polisher is at the same time slowly revolving around a pivot
at its centre, which pivot the crank works into, and the glass
below it is slowly turned in an opposite direction. Thus the same
effect is produced as in the other system. Those who practice this
method claim that by thus using machinery the conditions of a
uniform polish for every part of the surface can be more perfectly
fulfilled than by a hand motion. The results, however, do not
support this view. No European optician will claim to do better