Improvements in Bifocal Lenses.
Year: 1899

page_text: Date of Application, 23rd May, 1890-Accepted, 24th June, 1899
COMPLETE SPECIFICATION.
Improvements in Bifocal Lenses.
I, JOHN LOUIS BORSCH, of 1324, Walnut Street, in the City and County of
Philadelphia and State of Pennsylvania, United States of America, Optician, do
ereby 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:-
My invention relates to lenses, employed in spectacles and eye glasses, of the
special character known as bifocal, that is to say, having two independent focal
points located in the vicinity of each other, each occurring in one of the two regions
into which the lens is divided, which respective regions are formed of such material
and ground to such configuration that their respective coefficients of refraction are
of dissimilar power.
In practice bifocal lenses are employed by persons whose vision is of such a
character that a lens which enables them to see distant objects does not enable
them to see objects or print at close range, such persons being under the necessity
either of employing two pairs of glasses, one for distant and one for near vision, or
of employing lenses of the character above set forth having distinct regions of
dissimilar powers of refraction.
Heretofore bifocal lenses have been frequently formed by matching and uniting
edge to edge two pieces of lens glass, each constituting but part of a complete
lens, and respectively suitably ground, the one for distant and the other for near
vision, and various forms have by different constructors been given to the
respective elements or sections of the lens, the only fixed requirement as to such
sections being that they should when united present as to their combined outer
edges the usual oval outline of a lens.
In whatever forms the respective independent sections or elements of a bifocal
lens of the foregoing character have been made, however, they have been united by
bringing the respective edges of said sections or elements into contact and
cementing the abutting edges by any suitable balsam or uniting medium, or
maintaining them in their assembled position by an enclosing lens frame.
0 This construction has been objectionable, however, by reason of the fact that
however carefully the sections are assembled and cemented, a minute cement-filled
space exists between the abutting edges, and the cement which is present of course
on both surfaces of the lens, in time becomes slightly worn away under the action
of heat and the attrition to which it is subjected in the cleaning of the lens, with
5 the result that the permanence of the union between the elements or sections is
impaired; furthermore, the line of connection between the two sections of a bifocal
lens as heretofore constructed as described is always visible, and not only detracts
from the appearance of the lens but is annoyance to the wearer; furthermore, no
matter how carefully the sections or elements are united it is practically impossible
10 to prevent one of the meeting edges of the sections from projecting slightly above
the opposing edge of the other section, with the result that in the cleaning of the
lens the projecting edge is very liable to be chipped.
When, instead of uniting edge to edge two pieces of lens glass to form a bifoca

page_text: lens, such lenses have been constructed by placing one piece of lens glass upon the
face of the other, the objections referred to are the more pronounced.
Broadly stated, it is the object of my invention to produce a bifocal lens of
an attractive, efficient, and durable, character, in which the objections herein-
before stated to the existing forms of such lenses shall be entirely obviated.
Generally expressed, my invention comprehends the provision of a lens of usual
outline, the body of which is composed of ordinary lens-material, such as crown
glass, and the faces of which are of any usual curvature or form to produce any
selected magnifying power or corrective effect to suit the eyes of the wearer, and
herein termed the major lens,--within a suitable recess in the interior of which
lens is located a smaller lens,-herein termed the minor lens, and which is preferably
formed of flint glass to constitute an element of index different from that of the
major lens, and the refractive power of which minor lens combined with the
refractive power of that portion of the major lens which overlies it, produces a net
coefficient of refraction distinct from that of the remaining portion or field of the
major lens, and, of any desired strength.
In the usual form in which my lens will be constructed, as I contemplate, the
major lens will be a lens for distant vision and the minor lens will constitute a
correction of said lens (of an area equal to its own diameter) for near vision.
The major and minor lenses may, of course, be of such form and relative
proportions as the constructor or user may prefer, and each may, as heretofore, be
of any desired coefficient of refraction, their respective powers of refraction being
entirely independent of each other and capable of being made to conform to the
requirements of any usual formula.
In following a prescribed formula to produce a given strength of refraction
in the field of the minor lens, and which is composed as will be understood, of said
minor lens and the overlying portions of the major lens bounded by the outline of
said minor lens, a compound calculation, so to speak, will be required on the part
of the lens grinder, and it will be necessary for him to calculate the refractive
power to be possessed by that portion of the major lens which overlies or is opposite
to the minor lens, and to then conform or grind the inset minor lens to such shape
that its refractive power plus the refractive power of the ground glass in the
overlying regions, will produce the aggregate refractive power prescribed.
The calculation referred to is one which any optician can readily make in
accordance with known rules.
In the preferred construction of my improved bifocal lens I form the major
lens, whether it be the near lens or the distance lens, as two halves, each preferably
of approximately half the thickness of an ordinary lens, and carefully grind or
conform the opposing faces of the respective halves so that when brought into
contact with each other they shall accurately fit, and I also produce in one or both
of said opposing inner faces, according to the character of the lens employed, a
recess of such form as to be adapted to receive the smaller or minor lens which is
to be enclosed between the two halves of the major lens.
When, then, the two halves of the major lens are, with the minor lens within
the recess as stated, brought together and secured by balsam or otherwise, the
result will be a compound lens of such conformation as to possess as to a selected
portion of its area, a given coefficient of refraction, and the remaining portion of the
area of which is, by the presence in the interior of the structure of a distinct
lenticular body of different index, of a distinct and different coefficient of
refraction.
As will be understood, the respective halves of the lens may be of any preferred
shape as to their faces.
In the accompanying drawings,
Figure 1 is a vertical, transverse, sectional, elevation of a bifocal lens embodying
my invention.
Figure 2 is a view similar to Figure 1, the parts being slightly separated to
more plainly exhibit their construction.

page_text: Figure 3 is a view similar to Figure 1, illustrating, however, a lens grown
fferent form.
Figure 4 is a view similar to Figure 3, the parts being slightly separated to
ore plainly exhibit their construction.
Figure 5 is a view in front elevation of a bifocal lens embodying my invention.
Similar letters of reference indicate corresponding parts.
In the drawings,
A A indicate the respective halves or portions of the major lenses, and B the
ecess formed in the inner faces of the same, said recess is in Figures 1 and 2 shown
s produced in the inner faces of both halves of the major lens, and in Figures 3
nd 4 as produced altogether in one of said halves. C is the minor lens.
As will be understood, a lens embodying my invention is uniform in curvature
nd integral in structure throughout all portions of each face, so that not only
s the presence of the enclosed minor lens not visible except on very close
nspection, but the presence of a joint or union between the edges of the respective
elements on the surface of the lens is entirely avoided, with the result that the
stated disadvantages incident to the use of the well known forms of bifocal
enses are completely avoided.
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
1 claim is
1. A bifocal lens consisting of a major lens having faces which are each of
selected curvature or form and of exteriorly uninterrupted structure and a
relatively smaller minor lens contained within its interior, substantially as set
forth.
2. A bifocal lens, consisting of a major lens having faces which are each of
selected curvature and of exteriorly uninterrupted structure, and a relatively
smaller minor lens contained within the interior of said major lens, and the
refractive power of which is combined with the refractive power of the overlying
portions of the major lens to produce in the region which it occupies a correction
of the refractive power of the said major lens, substantially as set forth.
3. A bifocal lens consisting of a major lens composed of two plates adapted to
be placed face to face and secured together, each ground as to its outer face to
any selected curvature, a recess between said plates, and a minor lens disposed
within said recess, substantially as set forth.
4. A bifocal lens consisting of a lens having faces which are each of selected
curvature or form and of exteriorly uninterrupted structure, and having within
its interior a relatively smaller lens, the glass of the smaller lens being of index
different from that of the larger, substantially as set forth.
5. A bifocal lens consisting of a major lens composed of two plates adapted to be
placed face to face and secured together, each ground as to its outer face to any
selected curvature, a recess between said plates, and a minor lens disposed within
said recess, said major lens and minor lens being formed of glass of different index,
substantially as set forth.
نت
1000
Dated this Third day or Aray
1.1.
JOHN LOUIS BORSCH,
Witnesses:
FREDERIC NORMAN DIXON,
THOMAS KEYS LANCASTER.