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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。











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