Improvements in Spectacle Lenses and in the Method of Grinding same.
Year: 1890

page_text: Date of Application, 3rd Dec., 1890
Complete Specification Left 28th Mar 1891-Accepted 2nd May. 1891
PROVISIONAL SPECIFICATION.
Improvements in Spectacle Lenses and in the Method of Grinding
same.
GEORGE PRESCOTT 9 Merrion Row Dublin Ophthalmic Optician and Electrical
Engineer do hereby declare the nature of this invention to be as follows:-
The lenses generally used for the correction of astigmatic vision have hitherto
been ground with cylindrical curvature on one side at least not because of such
cylindric lens being the most perfect corrective but because it was the most
approximate one offered by the arts. A more perfect corrective would be a lens of
elliptic curvature. Elliptic lenses whose radii are shorter in the shorter diameter
have been used but such lenses are correctives of astigmatism in the horozontal or
vertical meridians: and as the corrections for intermediate positions are of equal if
not greater importance from their frequency of occurrence, have devised a simple
and effective apparatus for grinding elliptic lenses so as to have their longer or
shorter rádius of curvature in any other meridian than their longer or shorter
diameter.
For this purpose I fit on the grinding lathe a disc shaped chuck and on the
periphery of this chuck I cement the pieces of glass to be ground. A cylindric
grinding tool with the usual media is now applied to the rotating lenses with the
result that lenses have on one face two radii of curvature, to wit, that of the
cylindric tool, and that of the chuck, and it is only necessary to cement the glasses
on the chuck with the desired meridian parallel to the plane of the chuck, to have
that meridian in any other than the longer or shorter diameter of the oval
spectacle lens.
Dated thin and dow of Decembar 1800
COMPLETE SPECIFICATION.
Improvements in Spectacle Lenses and in the Method of Grinding
same.
GEORGE PRESCOTT 9 Merrion Row Dublin Optician do hereby declare the
nature of this invention and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement :-
Astigmatic vision has hitherto been reduced by the use of lenses with a cylindric
surface. Such method is open to the objection that only the principal meridians are
treated correctly, the intermediate surfaces not being calculably dealt with, and
although lenses with shortest radius of curvature in the shortest diameter have been
ground, such lenses offer a correction for a maximum error in one meridian only,
so far as the cylindric effect is concerned, and that meridian one of least frequency.
I have devised a simple apparatus for grinding lenses having the same character of
surface as that which occurs on an astigmatic cornea. Such a lens would be generally
described by an optician as an elliptic lens, because a section through the lens at
My lens presents a
right angles to its axis would shew a peripheral elliptic curve.
surface of revolution, to wit, an eccentric circle of revolution, and the method of
grinding is as follows. On the grinding lathe is fixed a disc chuck of a radius
which is that intended for the curvature of the surfaces of the lenses in one
meridian, and of such thickness as will permit the desired number of pieces of glass
to be cemented side by side around the edge of the chuck. For instance if a
number of lenses are to be ground on the chuck, and the lenses are to be each

page_text: .
one inch in diameter and are to be cemented in two rows around the chuck, then the
chuck may be two inches thick. A cylindric grinding tool such as is generally
used in lens grinding is now applied, with the usual media, to the pieces of glass
cemented on the chuck. The chuck is rotated. The glasses are (by their more or
less rapid rotation, the application of the grinding tool with cylindric surface and
the emery and other usual media) ground and polished, and when finished the
lenses have a shorter radius of curvature in one meridian, viz., that of the cylindrical
surfaced tool, or that of the chuck, whichever is shorter, and a longer radius of
curvature in the other principal meridian, together with a graduation of surface in
the intermediate spaces similar to that which occurs on the human astigmatic
cornea.
From the above description it appears that it is within the power of the operator
to so cement the pieces of glass on the chuck relatively to their longer or shorter
diameter, (assuming the glasses to be of oval form) and the direction of the rotation
of the chuck to secure that the shorter or longer radius of curvature shall be in
any meridian relatively to the longer or shorter diameter, and by a simple and
inexpensive process. Glasses intended to have a minus refraction may be cemented
on the inside of a "bell" chuck, and then treated as above described.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is,
Lenses for ophthalmic purposes, which present surfaces of revolution having
on one side different radii of curvature, the shortest radius of curvature not in
the shortest diameter, (assuming the lenses to be oval) and the method of grinding
same, as described.
Dated this 26th day of March 1891.
GEORGE PRESCOTT.