His best
lenses are made by himself: those which he buys are not to be
depended upon. The best flint glass is obtained from Paris in
blocks, which he divides, grinds, and polishes to perfect form.
I was attracted by a newly made machine, placed on a table in the
sitting-room; and on inquiry found that its object was to grind
and polish lenses. Mr. Lancaster explained that the difficulty
to be overcome in a good machine, is to make the emery cut the
surface equally from centre to edge of the lens, so that the lens
will neither lengthen nor shorten the curve during its
production. To quote his words: "This really involves the
problem of the 'three bodies,' or disturbing forces so celebrated
in dynamical mathematics, and it is further complicated by
another quantity, the 'coefficient of attrition,' or work done by
the grinding material, as well as the mischief done by capillary
attractionand nodal points of superimposed curves in the path of
the tool. These complications tend to cause rings or waves of
unequal wear in the surface of the glass, and ruin the defining
power of the lens, which depends upon the uniformity of its
curve. As the outcome of much practical experiment, combined
with mathematical research, I settled upon the ratio of speed
between the sheave of the lens-tool guide and the turn-table;
between whose limits the practical equalization of wear (or cut
of the emery) might with the greater facility be adjusted, by
means of varying the stroke and eccentricity of the tool.
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