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 as to secure perfect vision through them
is the most difficult branch of the art of the optician, and upon
his skill in practising it will depend more than upon anything
else his ultimate success and reputation. The shaping of a pair of
lenses in the way we have described is not beyond the power of any
person of ordinary mechanical ingenuity, possessing the necessary
delicacy of touch and appreciation of the problem he is attacking.
But to make a perfect objective of considerable size, which shall
satisfy all the wants of the astronomer, is an undertaking
requiring such accuracy of eyesight, and judgment in determining
where the error lies, and such skill in manipulating so as to
remove the defects, that the successful men in any one generation