men of invention and industry-第70部分
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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。 As the
result of these considerations in the construction of the
machine; the surface of the glass 'comes up' regularly all over
the lens; and the polishing only takes a few minutes' workthus
keeping the truth of surface gained by using a rigid tool。〃
The machine in question consists of a revolving sheave or ring;
with a sliding strip across its diameter; the said strip having a
slot and clamping screw at one end; and a hole towards the other;
through which passes the axis of the tool used in forming the
lens;the slot in the strip allowing the tool to give any stroke
from 0 to 1。25 inch。 The lens is carried on a revolving
turn…table; with an arrangement to allow the axis of the lens to
coincide with the axis of the table。 The ratio of speed between
the sheave and turn…table is arranged by belt and properly sized
pulleys; and the whole can be driven either by hand or by power。
The sheave merely serves as a guide to the tool in its path; and
the lens may either be worked on the turn…table or upon a chuck
attached to the tool rod。 The work upon the lens is thus to a
great extent independent of the error of the machine through
shaking; or bad fitting; or wear; and the only part of the
machine which requires really first…class work is the axis of the
turn…table; which (in this machine) is a conical bearing at top;
with steel centre below;the bearing turned; hardened; and then
ground up true; and run in anti…friction metal。 Other details
might be given; but these are probably enough for present
purposes。 We hope; at some future time; for a special detail of
Mr。 Lancaster's interesting investigations; from his own mind and
pen。
'12' The translations are made by W。 Cadwalladr Davies; Esq。
'13' This evidence was given by Mr。 W。 Cadwalladr Davies on the
28th October; 1880。
End