Improvements in or connected with Telescopes.
Year: 1894

page_text: Date of Application, 21st Apr., 1894-Accepted, 30th June, 1894
COMPLETE SPECIFICATION.
Improvements in or connected with Telescopes.
I, JAMES YATE JOHNSON of 47 Lincoln's Inn Fields, in the County of
London, Gentleman do hereby declare the nature of this invention (which has
been communicated to me from abroad by the Firm of Carl Zeiss of Jena in the
Grand Duchy of Saxe-Weimer, in the German Empire) and in what manner the
same is to be performed to be particularly described and ascertained in and by
the following statement:-
This invention relates to telescopes and especially to binocular telescopes in
which the two objectives or objective apertures are arranged at a greater distance
apart than are the two eye-glasses for the purpose of increasing the parallactic
difference of the images produced in both component telescopes so as to increase the
perception of the depth of the objects, that is to say to make the combined image
more in relief or in accordance with the image as it would be seen by direct vision.
One feature of this invention consists in improved means for effecting the said
greater distance of the objectives.
Hitherto binocular telescopes of this description (known as " telestereoscopes of
Helmholtz") have consisted of a pair of telescopes, which are constructed in the
following manner. The optical components of an ordinary terrestrial telescope
(that is to say an objective glass and a so called terrestrial eye-piece consisting of a
positive eye-piece and a lens system for erecting the image) are combined with two
plane mirrors or reflecting prisms in such a way, that one is placed before the
objective glass at an angle of 45 degrees so as to direct the axis of the objective
glass perpendicularly to the optical axis of the objective opening or aperture,
through which the rays enter the telescope, and the other is placed between the
objective and the eye-piece in proximity to the latter also at an angle of 45 degrees
so as to direct the axis of the eye-piece perpendicularly to the axis of the objective
glass-which is also the axis of the tube of the telescope-and parallel to the
optical axis of the objective aperture. Both telescopes are connected together in
such a way, that the axes of their tubes form one straight line, the eye glasses
being a distance apart of or about the distance between the human eyes and the
objective apertures being at a greater distance apart.
The same result may be obtained according to this invention without any
supplemental parts and only with the constitutive elements of terrestrial telescopes,
if for erecting the image instead of the inverting lens system of the so-called
terrestrial eye-piece one of two combinations of prisms be used which were
suggested in the French weekly magazine Cosmos vol. II (1852) p. 222 and
vol. IX (1856) p. 401 by the Italian engineer Porro for the purpose of lessening
the length of terrestrial telescopes. Either of these combinations of prisms
combined with an objective and a mere astronomical (positive) eye piece, produces
in addition to the erection of the image a dislocation of the eye glass axis from the
objective axis and it is the main feature of the present invention to make use of
this quality of the Porro-telescopes for constructing improved telestereoscopes.
With this object two equal Porro-telescopes are connected together in such relative
positions, that they form a binocular glass, the objectives of which are at a greater
distance apart than are the eye glasses.
Another feature of this invention consists in improved arrangements of either of
the above said combinations of prisms, in which adjacent elements are separated
from each other for the purpose of increasing the distance between the axis of the

page_text: eye-glass and the axis of the objective glass or of the objective aperture. A
telescope provided with such a modified combination of prisms of Porro is equally
well adapted to be used as a single or monocular glass for some special purposes
and it may be combined with a second one to form a telestereoscope of very great
distance of the objectives.
And in order that this invention may be fully understood I shall now proceed
more particularly to describe the same and for that purpose I refer to the
accompanying drawing.
Figures
of Porro.
and 2 represent perspective views of the two combinations of prisms
Figure 3 represents a modified arrangement of the combination of Figure 1.
Figure 4 represents a modified arrangement of the combination of Figure 2.
Figure 5 represents another modified arrangement of the combination of
Figure 1.
Figure 6 represents a diagrammatic view of a binocular telescope, the objectives
of which are at a less distance apart than are the eye-glasses.
Figure 7 represents another diagrammatic view of a binocular telescope, the
objectives of which are at a greater distance apart than are the eye glasses.
Figure 8 represents a plan and Figure 9 a front view of a binocular glass,
constructed from two Porro-telescopes.
Figure 10 represents a plan and Figure 11 a front view of a binocular glass,
constructed from two Porro-telescopes modified according to this invention.
Figure 12 represents a plan, Figure 13 a front view and Figure 14 an end view
of a modified Porro-telescope, arranged for use as a single or monocular glass.
The first of Porro's combinations of prisms represented by Figure 1 consists of
four rectangular isosceles prisms 1, 2, 3 and 4, constructed from one piece of glass or
connected to form one piece. The entering ray a', which coincides with the optical
axis, strikes at 10 the hypotenuse surface of the prism 1 at an angle of 45 degrees
and being reflected at the same angle relatively to this surface is at this point
deflected by 90 degrees from its original direction. The same effects are
successively produced by the hypotenuse surfaces of the prisms 2, 3 and 4 at the
points 20, 30 and 40 so that the direction of the emerging ray at is parallel to that
of the entering ray a¹ and both lines of direction or optical axes have a distance
equal to that of the points 20 and 30, which is determined by the dimensions of the
° prisms.
ray
The second of Porro's combinations of prisms represented by Figure 2 consiste
also of four rectangular isosceles prisms, I, II, III and IV, deflecting the entering
a four times at an angle of .90 degrees at the points 11, 22, 33 and 44, but
arranged in a different manner from that shown in Figure 1, so that the line of
direction of the emerging ray a' has a distance from the line of direction of the
entering ray equal to the hypotenuse of a right-angled triangle, the small sides of
which are formed by the lines 11-22 and 33-44, this distance being therefor
likewise determined by the dimensions of the prisms.
It is obvious that either of these combinations of prisms may be interposed
between the objective and the eye-glass of a telescope for the purpose of erecting
the image, if the axes of the objective and the eye glass are distanced apart the
same as the entering and the emerging ray in the combination of prisms. Therefor
Porro's telescopes have an objective arranged eccentric to the eye-glass at a
distance depending upon the dimensions of the prisms used for erecting the image.
This is not altered by the fact, that none of Porro's combinations of prisms are
inserted into his telescopes in one piece. His aim being solely the lessening of the
length of the telescopes, he has placed (in the arrangement Figure 1) the
prisms 2, 3 at a distance from the prisms 1, 4, and to compensate for this separation,
of the prisms (by which the passage of the rays between points 10 and 20
and between points 30 and 40 are lengthened) the distance of the objective
aud the eye-glass should be correspondingly shortened. For the same reason in
the arrangement of Figure 2 he has distanced the prisms III and IV from the

page_text: prisms I and II, lengthening thereby the way between 22 and 33 passed by the
rays in a direction contrary to that in which they enter into prism I and emerge
from prism IV.
Two equal Porro telescopes connected together to form a binocular telescope may
be arranged with the distance between the objectives different from the distance
between the eye-glasses. If the objectives have a less distance than the eye-
glasses-and this distance may be lessened by so much that the objectives touch each
other-the images shown by the binocular glass appear less in relief approaching
to mere plane images. If on the contrary according to the present invention the
objectives are at a greater distance apart than are the eye glasses and this distance
may surpass that of the eye-glasses up to the full sum of the distances of the
objective and eye-glass axes of either telescope, the contrary effect is obtained, as
has been above described. Both effects may be obtained with the same binocular
glass, as is illustrated by Figures 6 and 7. The diagram Figure 6 shews the
objectives nearer together than the eye-glasses o, the planes E, which include
the axes of both the objectives O and the eye-glasses o, being inclined to the plane e
including the axes of both eye-glasses o at acute internal angles. By turning each
telescope around the axis of its eye-glass the objectives O may be brought into the
position shewn in Figure 7, in which they are more distant from each other than
are the eye-glasses and the planes E are inclined at acute external angles to the
plane e.
For the purpose of enabling the binocular telescope to be adapted to the various
distances between the eyes of different individuals the single telescopes are preferably
hinged together or otherwise connected so as to allow the eye-glasses to be adjusted
to or away from each other.
Figures 8 and 9 show a binocular glass consisting of two Porro-telescopes
constructed by aid of the combination of prisms of Figure 2. The double
prisms I, II and III, IV are enclosed in a casing G, which carries at the front side
the eye-piece o and at the opposite end the objective O. Both casings G are
connected by means of brackets h, hinged together on a common bolt II, and by
turning them around this hinge bolt the distance between the eye-glasses can be
adapted to the various distances between the eyes of individuals and moreover both
parts of the binocular glass may be closely approached for enabling them to be
enclosed in a conveniently small case. For the double prisms I, II and III, IV
equivalent to the combinations of prisms of Figure 2 and characterizing one kind
of Porro's telescopes may be substituted a double prism 2, 3 and two united
prisms 1, 4 equivalent to the combination of prisms of Figure 1 and characterizing
he other kind of Porro's telescopes with no further alterations of the telescope
than a different shape of the casings G.
In the binocular glass shown in Figures 8 and 9 the distance between the objectives
does not exceed considerably the distance between the eye-glasses. Notwithstanding
this the perception of depth increases remarkably, as may be understood from
considering the optical principle, that for the user of a binocular glass the ability
to distinguish differences of distance is proportional to the product of the rate of
magnification and the distance between the centres of the objective apertures.
Therfor the above described glass shows images, which with regard to the relief
appearance of the objects are equal to images produced by an ordinary binocular
telescope of a higher magnifying power and correspondingly smaller field of sight.
A binocular glass of greater stereoscopical effect may be constructed, as already
has been mentioned by separating the components of the combination of prisms in
such directions, that the perpendicular distance between the ray, which enters the
first prism 1 or 1, and the ray, which emerges from the fourth prism 4 or IV, is
increased, which increase is determined by the dimensions of the prisms.
Of all dislocations of prisms suitable to increase the distance between the
entering ray and the emerging ray the simple and the most important consists in.
dividing the prisms of Figure 1 and Figure 2 into only two parts. In the arrange-
ment shewn in Figure 3 and corresponding to Figure 1 the first part 1 is separated

page_text: from the main part 2, 3, 4. In the arrangement shewn by Figure 4 and corresponding
to Figure 2 the first part I is separated from the main part II, III, IV. In a third
arrangement shown by Figure 5 and corresponding to Figure 1 the prisms 1, 2 are
separated from the prisms 3, 4.
Figures 10 and 11 represent a binocular telescope provided with the arrangement
of prisms shown in Figure 3. Both component telescopes are rigidly connected
together by a common casing Go, which includes both sets of prisms 2, 3, 4 and
carries at its front side the eye-pieces and at either lateral side a tube 9°, at the
extremity of which is arranged the objective casing Ko, which carries at the side
opposite to the eye-piece the objective aperture Po and contains the first prism 1 in
front of the objective glass 0°, the latter being placed in this way within the
described combination of prisms. The distance between prism 1 and prisms 2, 3, 4
is nearly equal to the whole focal distance of the objective. The prisms 2 and 3
form a double prism and the prism 4 is closely connected to them. It will be easily
understood, that similar binocular glasses may be constructed with the combination
of prisms shewn in Figure 4 and the combination of Figure 5. In all these cases it
is unessential, whether the separated part of the combination of prisms be arranged
in front of the objective as in Figures 10 and 11 or between the objective and the
eye-glass as in Figures 8 and 9. With the arrangements of prisms shewn in
Figures 3 and 4 it may sometimes even be of advantage, especially in slightly
magnifying telescopes which often have an objective of less diameter than the ocular
field, to place the one separated prism near the eye-glass and the three remaining.
prisms near the objective for the reason that in consequence of this changed
disposition three small prisms and a large one may be used instead of three large
prisms and a small one."
The modified Porro-telescopes herein described as components of binocular glasses
are also as applied to single telescopes, to be considered as included within the
present invention. The distance of the axis of the objective aperture from the axis
of the eye-glass widely surpassing the corresponding distance in the original
Porro-telescopes enables them to be used for observing at an angle or round the
corner. This application of single Porro-telescopes, modified according to this
invention is of importance for military purposes allowing an observer to remain
behind a cover, whilst observing. Such a result could hitherto only be obtained by
combining with an ordinary telescope an additional set of reflecting prisms, whilst
according to this invention it is obtained without requiring more elements than
constitute a terrestrial telescope.
Figures 12, 13 and 14 represent a single telescope of this description constructed
with an arrangement of prisms corresponding to that of Figure 4. The separated
prism I is placed behind the objective aperture P¹ within the objective casing K¹
and close to the objective-glass O' and the prisms II, III and IV are enclosed in a
casing G¹ carrying at its front side the eye-glass o¹ and connected with the objective
casing K¹ by the tube gl. The four prisms may also be arranged according to
Figure 3 or Figure 4 with no further alterations of the telescope than a different
shape of the casing G'.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is
1. A binocular telescope of stereoscopical effect, or telestereoscope consisting of two
Porro-telescopes connected together with parallel axes and in such relative positions
(rigidly or adjustably) that at a distance of both eye-glasses equal to the distance
of both eyes the plane of the axes of either telescope coincides with the plane of
the axes of both eye-glasses or intersects it at an acute external angle, substantially
as hereinbefore described and shewn.
2. A telestereoscope consisting of two Porro-telescopes arranged as described in the
first claiming clause and modified by separating the components of Porro's combination
of prisms so as to increase the distance between the rays entering into and emerging

page_text: from the same beyond the amount determined by the dimensions of the prisms,.
substantially as hereinbefore described and shewn.
3. A single Porro-telescope modified as described in the second claiming clause,..
substantially as hereinbefore described and shewn.
of Amil 1904
JOHNSONS & WILLCOX,
Agents.