rection 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 than the American firm of Alvan Clark & Sons in
producing uniformly good object-glasses, and this firm always does the
work by hand, moving the glass over the polisher, and not the polisher
over the glass.

Having brought both flint and crown glasses into proper figure by this
process, they are joined together, and tested by observations either
upon a star in the heavens, or some illuminated point at a little
distance on the ground. The reflection of the sun from a drop of
quicksilver, a thermometer bulb, or even a piece of broken bottle,
makes an excellent artificial star. The very best optician will always
find that on a first trial his glass is not perfect. He will find that
he has not given exactly the proper curves to secure achromatism. He
must then change the figure of one or both the glasses by polishing it
upon a tool of slightly different curvature. He may also find that
there is some spherical aberration outstanding. He must then alter his
curve so as to correct this. The correction of these little
imperfections in the figures of the lenses so