Improvements in Spectacles.
Year: 1889

page_text: Date of Application, 14th Aug., 1889-Accepted, 14th Sept., 1889
COMPLETE SPECIFICATION.
[Communicated from abroad by PEROY JAMES EDMUNDS, of 137, Elizabeth Street,
Hyde Park, Sydney, in the Colony of New South Wales, Lecturer on Science.]
Improvements in Spectacles.
I HENRY EDWARD NEWTON of the Office for Patents, 6 Breams Buildings
Chancery Lane in the County of Middlesex, Civil Engineer, 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 :-
The spectacles to be constructed according to this principle differ from all ordinary
spectacles in the following very important particular. Instead of being primarily
intended to correct or make allowance for optical defects, malformations, or other
structural deficiencies in the eye, they may be employed to enable persons with normal
sight to make binocular use of the magnifying and other properties of convex
glasses without strain or injury to the eyes. The principle consists in adding the
effect of prismatic deviation to the effect produced by the convexity alone, by means
of a particular prismatic form combined with lenticular surfaces to be given to
the glasses of spectacles. In order to make the principle clear, it will be necessary
to briefly refer to the effect of the present common form of convex spectacles
upon normal sighted persons.
In the accompanying sheet of drawings, Fig. 1 shows the effect of the convex
spectacles as worn by a normal sighted person. Let A B and A' B be two convex
lenses, E and E2 the two eyes, and P a luminous point. Then, by the well known
optical properties of lenses, the image of P formed by A B is moved a certain
distance farther from the lens, say, to Q. Similarly, the image of P, formed
blurr
a
by A B¹, would appear at Q². 'Since Q₁ does not coincide with Q,
confusion is introduced, produced by the double image, and the normal sighted person
experiences undue muscular strain of the muscles of the eyes.
"
22
or
The object of this invention is to blend the two images Q¹ and Q into a single
image. The manner of attaining this end is as follows. The lenses used are prism-
shaped, with lenticular convex surfaces, the thick edges of the prism being adjacent
to one another, and the prismatic angles pointing outwards. This will have the
effect of lessening the convergence to a degree that will exactly correspond to
the lengthening of the focus, if the lenses be constructed according to a correct
calculation.
In Fig. 2, A B C and A' B₁ C are these prism shaped lenses. P, as before, is a
supposed luminous point. Were it not for the particular prismatic form of the lenses,
there would be two images formed, as before. But the prismatic power of the prisms
is so adjusted, that the principal axis of the rays that strike each lens is bent in
the direction R M and R¹ M' respectively; and it follows that in this manner, both
images are made to blend together into one image at Q. Thus, by the use of such
properly calculated lenticular prisms, it follows that the converging power of the
two eyes may be kept in harmony with the focussing power of each eye, and the
image of P is actually removed, both as regards convergence and focussing, to Q.
What holds for a single point P, will, by elementary principles in optics, approximately
hold for all other points situated similarly.
It is to be understood that the invention does not merely consist in the use of
lenticular prisms. These have been long known. The invention consists in the
binocular application of lenticular prisms constructed and calculated so as to carry
into effect the principle explained, so as to blend together the two images which would
be produced by ordinary lenses only, and thus enable normal sighted persons to
obtain magnification for any purpose without strain to the eyes.

page_text: Spectacles or similar binocular contrivances constructed with lenticular prisms made.
by this formula are intended :-
1. To enable persons who have to observe objects at short distances with both eyes,
to do so with ease and distinctness. This class would include persons engaged in such
trades as engraving, watch-making, &c., &c.
2. To enable students and others who have strained their eyesight, or, are likely to
strain their eyesight, by the nature of their occupation, to read or do their work with
all the henefit of greater apparent distance and the corresponding and accompanying
magnification.
3: For weak sighted persons generally, and many obvious daily uses which might
arise, too numerous to mention, where binocular magnification is required.
Various possible forms which might be given to the lenticular prisms are shown in
Figs. 3. 4 and 5, it being understood in all cases, that the alteration in the apparent
position of the object produced by the lenticular power, is accompanied by a properly
calculated prismatic power, so as to compensate for the double image, and the confusion
which would result, were the prismatic lenses not used.
The following are the mathematical formulæ upon which the amounts of lenticular
and prismatic power requisite are based, and by which the shape of the lenticular
prism is calculated to produce a given effect.
This is the formula by which is found in terms of the known quantities p q and a.
From this it follows, that any radii may be given to the curved surfaces A B and A C,
provided the following formula be satisfied:
-
(n − 1) {
R¹
=
where n means the index of refraction of the glass, and R and R' mean the radii
required. Hence the lenticular prisms may be calculated to assume a biconvex,
plano-convex, or meniscus form.

page_text: =
-
Hence, tand being known, d itself can be found froni tables as required. And
5 since B A C is a small angled prism, then approximately d (n - 1)
xangle B A C (where n means the index of refraction, as before); therefore
It will be understood that whatever be the nature of the curved surfaces, the
prismatic angle always means the outer angle that is formed by the right lines
drawn between the three corners of the lens, i.e., the angle between the straight
lines (shown dotted in Fig. 2) joining B A and CA.
The above formulae (as is customary in optical calculations) are approximate only,
and those which would be used in practice, but the data could, if necessary, be
calculated with other more extended formulae, provided the principle of blending
the images by compensating prisms is followed out.
The following calculations give an example of the way in which the foregoing
formulae are worked out in practice. Suppose a person of normal sight requires
spectacles that will enable him to examine with both eyes an object really at 6 inches
distant from the bridge of the nose, but which, with the aid of the improved glasses,
is to appear as though situated at 10 inches distance, and suppose the distance
between the pupils of his eyes be 2½ inches, -

page_text: An
These formulae hold as they stand for normal sighted and myopic persons.
increase in the lenticular power is however necessary for hypermetropic persons,
where the point Q (Fig. 2) lies within focal power of his eyes. In this case, the
observer may either wear the improved spectacles and his own together, or the focal
distance of the convex surfaces A B and A C may be calculated by the following
formula :-
--
where means the focal length of the convex lens he is accustomed to wear for
obtaining distinct vision at 10 inches distance. For instance, in the former illustrative
problem, if the person naturally required convex spectacles of 20 inches to see distinctly
at 10 inches distance, he would require about a focal distance of 8 inches in the
lenticular surfaces of improved lenses (by formula 4).
The lenses may be mounted in any frame or holder that may be found most suitable
for the purpose for which they are to be used.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, as communicated to me by
my foreign correspondent, I declare that what 1 claim is:-
In binocular spectacles, the use of prisms having convex, plano-convex, or meniscus
lenticular surfaces, the prismatic angle and lenticular surfaces of which should be
calculated by the formulae herein specified (or by equivalent formulae), for the
purposes herein set forth.
Dated this 14th day of August 1889