IMPROVEMENTS IN THE CON- STRUCTION OF TELESCOPES
Year: 1878

page_text: LETTERS PATENT to Thomas Johnson Potter, of Clapham, in the County of
Surrey, Master of Arts, for the Invention of "IMPROVEMENTS IN THE CON-
STRUCTION OF TELESCOPES.”
Sealed the 17th December 1878, and dated the 21st June 1878.
PROVISIONAL SPECIFICATION left by the said Thomas Johnson Potter at
the Office of the Commissioners of Patents on the 21st June 1878.
THOMAS JOHNSON POTTER, of Clapham, in the County of Surrey, Master of Arts.
"IMPROVEMENTS IN THE CONSTRUCTION OF TELESCOPES."
The object of my Invention of improvements in the construction of telescopes is
to diminish the length of these instruments so that they will be properly balanced
when held in the hand; also to increase the field of view, and prevent the loss of
light by diminishing the number of lenzes.
In carrying out my Invention I dispense with the lenzes usually employed for
erecting the inverted image, and in place thereof I make use of two rectangular
prisms, the vertical height and width of which is by preference half that of the
base or hypotheneuse. These two prisms are cemented together at their bases, at
right angles to each other, end to end, thereby leaving one-half of the area of the
base of each prism free to receive a lens. This system of prisms, together with
their lenzes and an additional lens in front of the second lens from the object glass,
forms the eye-piece of the telescope.
A ray of light after passing through the object glass passes along the axis of the
telescope through the lens on the base of one of the prisms, and is reflected at right
angles from one of the inclined faces of the prism to the opposite inclined face of
the same prism, then at right angles again from that face through the base of the
other prism on to the inclined side of this latter, and again at right angles to the
opposite inclined side, and through the base and the lens cemented thereon, after
"

page_text: which it reaches the eye of the observer. The image, which has been inverted by
the object glass, as is well known, is by this arrangement of prisins erected as in
ordinary telescopes, and that within little more than half of the length usually
required in ordinary telescopes.
Instead of the prisms being cemented together at their bases they may be
adapted to the opposite ends of a set of three parallel tubes arranged side by side,
so that the ray of light from the object glass may pass along the axis of the first
tube, then be reflected by the prism at its end down and along the axis of tho
second tube on to the second prism at the opposite end of this tube. From this
prism it will be reflected along the axis of the third tube to the eye-piece at the
end.
The combination of prisms and lenzes first described may be applied to binocular
as well as to monocular telescopes, race glasses, or opera glasses; and a very simple
screw motion may be used to adjust the width between the eye-pieces to suit
different persons without moving the bodies of the two telescopes.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Thomas Johnson Potter in the Great Seal Patent Office on the 21st
December 1878.
THOMAS JOHNSON POTTER, of Clapham, in the County of Surrey, Master of Arts.
"IMPROVEMENTS IN THE CONSTRUCTION OF TELESCOPES."
This Invention consists in improvements in the construction of telescopes, single
or binocular, by greatly diminishing the length, enlarging the field with less loss of
light in proportion to the power of the instrument, and by providing it with two
readily adjustable powers.
The re-inversion of the image formed by the object glass is effected by two right-
angled prisms cemented together by their hypothenuses, end to end, at right angles
to one another, as shown in Figure 1 of the Drawings annexed.
The enlargement of the field is effected by cementing a plano-convex lens of long
focus to the half of the prism facing the object glass, as shown in Figure 2. A
second plano-convex lens of short focus is cemented to the half of the second prism
facing the eye, Figure 3.
The prisms are fixed in a cylindrical metal box, the cap of which is movable
round the box. Into the cap are screwed the tubes containing the eye lenses, each
of which can be, brought into use without removing the other by simply turning
the cap round.
A combination of two lenses is used instead of one for the higher power.
The cap is shown in Figure 4 with perforations for the eye tubes.
The lens cemented to the second prism is made of thick glass to bring it nearer
to the eye lens, Figure 3.
Figure 5 shows the telescope with cylindrical box, lenses, and eye-piece without
the prisons.

page_text: In the binocular form the prism boxes are provided with a scrow motion for
adjusting the eye-pieces to the width between the eyes. The two pairs of prisms
must be cemented in opposite ways, as in Figures 2 and 3.
Having described the nature of my Invention, and the manner of performing the
same, I declare that what I claim as my Invention to be protected by the herein-
before in part recited Letters Patent is,-
The entire arrangement of lenses whereby a greater field and less loss of light,
and two different powers are secured; the first lens, or that nearest the object glass,
both enlarges the field and assists in rendering the system of lenses achromatic, and
allows the second lens to be plano-convex, and so to be cemented to the second
prism; thus both these two lenses being cemented to the prisms a considerable loss
of light is avoided.
I also claim as new the application to the telescope of two different powers by
merely turning the cap of the cylindrical box.
In witness whereof, I, the said Thomas Johnson Potter, have hereunto set my
hand and scal, this Nineteenth day of December, in the year of our Lord
One thousand eight hundred and seventy-eight,
T I DOTTER (TS)