Year: 1898

page_text: I, WILLIAM ELLIOTT DEBENHAM, of 46, Haverstock Hill, London, Photographer,
do hereby declare the nature of this invention to be as follows:-
On a portion of the glass used for a spectacle lens, I grind or cast or mould a
flat or curved surface, giving to that portion a different focal power from that
of the remaining portion, so as to produce with a single piece, a glass of bifocal
power. As an example I take a periscopic or positive meniscus lens, having the
radii of its curvatures as one to two. On the concave side I grind and polish a
flat place, extending from the edge to about the centre. As little as practicable
is ground away from the edge but towards the centre the depression may be
deeper, and by these means not only is the magnifying power doubled, making the
flattened portion suitable for reading, but the axis of this portion is near its centre
instead of being at its upper edge... The axes of the two portions may be so
disposed as to be each within its own portion instead of being at the line of
junction or outside. By this means better definition is obtained over large areas.
|Biconvex, plano-convex, bi-concave, plano-concave and negative meniscus lenses
are treated in a similar way on either the upper or lower portion and on either side
to suit particular requirements.
As an alternative method I build up instead of taking away, and this method is.
more particularly applied to glasses which are moulded, and not ground on the side
on which the projection occurs, or if ground on that side at all, only on the pro-
jecting portion and not on the depressed portion of the surface. The surface
introduced may also be partly above and partly below the general curve or plane
on which it stands. With moulded lenses, or with those ground on one side and
ground as to one of the powers on the other side, a desirable figure is obtained by
moulding the inner side of a meniscus upon a spherical mould from which a
segment has been cut, leaving a plane or slightly curved surface which may extend
from near the centre, to the margin of the lens. This flattened portion may be
ground and polished without interfering with the rest of the surface. In this
example there is no raised edge at the junction of the two powers, but where there
is such an edge this edge is not polished but left rough, and blackened or covered
by a thin metal or other opaque band, to prevent the eye from being dazzled by
reflections or refractions. While partial grinding is being executed, the rest of
the surface is protected by a suitable varnish or coating.
The glass may be divided as to its powers, in a horizontal line, but I prefer
that the portion used for reading shall be semi-circular or approximately so along
its upper edge, so that the margins of the lens at both sides may be available for
distant vision.
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1
1000

page_text: I, WILLIAM ELLIOTT DEBENHAM, of 46, Haverstock Hill, London, Photographer,
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, reference being made to the accompanying drawings:-
On a portion B of the glass used for a spectacle lens, I grind or cast or mould
a flat or slightly curved surface, giving to the portion so treated a shorter focus
than that of the remaining portion marked A. As an example I take a periscopic
or positive meniscus lens having the radii of its curvatures as one to two. On
the concave surface I grind and polish a flat place B in the form of a segment
of a circle extending from the edge to about the centre of the lens. By this
means the magnifying power is doubled, and the axis of the reading portion of the
glass is brought within its surface, instead of being at the edge or junction of
the two powers.
Better definition is thus secured over large areas. For myopic
vision, biconcave, plano-concave, and negative meniscus lenses are treated in
a similar manner.
As an alternative method I build up instead of grinding away, and this method
is more particularly applied to lenses which are moulded, and not ground on that
side on which the building up occurs, or if ground on that side at all, only on the
built up part and not on that part which serves for distant vision. For such
lenses a desirable figure is obtained by shaping the inner side of a meniscus upon
a spherical mould, from which a segment has been cut leaving a plane or slightly
curved surface which extends from near the centre to the margin of the lens.
This flattened portion of the lens may be ground and polished without interfering
with the rest of the surface. A similar effect is produced by moulding a convex
circle, or segment of a circle, upon the plane or nearly plane surface of a lens.
In the drawing annexed hereto Figure 1 represents a spectacle lens of two
powers. Fig. 2 represents a section of the lens where the higher power B is pro-
duced by grinding below the concave surface. Figure 3 represents a section of a
Jens in which the flat or nearly flat portion B¹ starts from the concave surface with-
out a dépression or ridge.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is:-
1. The production of a bifocal lens by grinding a flat or slightly curved space
on the concave surface of a meniscus lens.
2. The production of a lens having similar properties by moulding or by mould-
ing and grinding combined.
3. The production by moulding, or by moulding and grinding combined, of a
bifocal lens of such form as cannot be produced by grinding alone; that is to say.
where the portion which is thickened for near vision as shown at B Figure 3
would be ground away if the general surface of that side of the lens were pro-
duced by grinding.
Dated 1001
عمل
J
1000