Improvements in Terrestrial Telescopes.
Year: 1897

page_text: Date of Application, 13th Feb., 1897-Accepted, 24th Apr., 1897
COMPLETE SPECIFICATION.
Improvements in Terrestrial Telescopes.
I, CARL PULFRICH, of No. 1 Botz Strasse, Jena in the Grand-duchy of Saxe-
Weimar, German Empire, Doctor of Philosophy 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 :-
Porro has substituted in the telescope named after him to the combination of
lenses which, in the so called terrestrial telescope, restores the inverted image to
an erect position, a combination of prisms whereby the advantage of a considerably
shorter telescope is obtained. This effect of Porro's combination of prisms for
inverting an image or erecting an inverted image is caused by the circumstance,
that the optical axis of the telescope is broken by the prisms four times each time
through an angle of 90° in such successive directions, that fractions of it are
parallel to each other and at the same time are arranged not behind one another
but one next to the other. For certain purposes however, Porro's telescope cannot
replace the terrestrial telescope, because its magnifying power can be altered only
by the eye piece being replaced by another of a different focal length, whereas in
the ordinary terrestrial telescope the magnifying power can be altered simply by
moving the combination of lenses used for erecting the image in the direction of
the optical axis.
According to the present invention the last named advantage of the ordinary
terrestrial telescope and the advantage of a more compact form of Porro's
telescope are combined in one instrument by adding to the optical elements of the
first telescope a combination of four prisms which like Porro's combination by four
times at angles of 90 degrees deflecting the optical axis causes parts of the said
axis to lie parallel to and side by side with each other but which unlike Porro's
combination does not invert the image.
Figures 1, 2 and 3 of the accompanying drawings illustrate the three com-
binations each of four prisms I, II, III and IV which satisfy these requirements.
Figure 4 represents a single telescope,
Figures 5 and 6 represent a binocular telescope, and
Figure 7 represents another single telescope, each separate telescope being
provided with a system of prisms of the above kind.
The systems of prisms shown in Figures 1, 2 and have the property which is
advantageous in practice, that they leave the optical axis in one plane, which
property constitutes a second difference between the three primary forms of the
combination of prisms according to the present invention and the two primary
forms of Porro's combination. The primary form shewn in Fig. 1 also has a third
different feature which for some purposes is very advantageous, this third feature
being that the direction of the axial ray entering the first prism coincides with
that of the axial ray coming out from the last prism. It is however sufficient to
move in Fig. 1 one of the prisms I and IV away from the double prism II, III in
the direction 2-1 or 3-4 for obtaining the lateral displacement-characteristic
or Porro's combination-of the rays coming out from the combination of prisms,
elatively to the entering rays, that is to say to obtain an eccentricity of the axis
of the eye glass relatively to the axis of the object glass. This latter property
an therefore be given to a terrestrial telescope with any of the three primary
orms of the combination of prisms, and as the combination of two telescopes into

page_text: one double telescope having a greater distance between the object glasses than
between the eye glasses and by consequence having an increased stereoscopic
effect is dependent on this property, it is possible to combine two terrestrial
telescopes provided with either of the three primary forms of the new combination
of prisms in such a double telescope.
From each of the three primary forms indicated in Figs. 1, 2 and 3, new and
different forms may be developed by moving separate prisms in the manner already
described with reference to Figure 1. Such adjacent elements of the combination
which are in contact may be secured to each other or made in one piece.
A single telescope of the new kind provided with a combination of prisms which
is developed from the primary form of Figure 1 is illustrated in Figure 4 in
section taken in the plane of the optical axis. A double telescope in which the
distance between the object glasses is greater than that between the eye glasses
provided with a combination of prisms which is developed from the primary
form of Figure 2 is shewn in central section in Fig. 5 and in plan in Fig. 6.
A single telescope provided with a combination of prisms which is developed from
the primary form of Fig. 3 is shewn in Fig. 7 in section through the axis.
The single telescope in Fig. 4 comprises an object glass A and an eye glass B
with a lens C for erecting the image, and a condenser D. Of the four elements
of the combination of prisms in Fig. 1, the prism I is arranged in front of the
object glass A, the prisms II and III made in one piece are placed between the
object glass and the condenser D and the prism IV is placed between the erecting
lens C and the eye glass B. The arrangement of the series of prisms relatively
to the series of lenses as shewn in Figure 4 presents practical advantages, but of
course if the consecutive order of the lenses and the consecutive order of the
prisms be maintained the order in which these optical elements are arranged
in the telescope may be variously altered without affecting the object of the
invention. The same applies to the two other constructions hereinafter described.
The condenser D is cemented unto the adjoining surface of the prism III but of
course it may be mounted separately. The instrument is held to the eye in the
vertical position indicated in the drawing, its lower end being used as a handle
and its upper end resting with its pad P against the forehead. By means of a
stud co secured to the mounting of the lens C and guided in a slot e¹ of the casing
said lens C may be moved in the direction of the optical axis for the purpose of
altering the magnifying power.
The binocular telescope (Figs. 5 and 6) has in each member the same lenses as
the telescope just described, the series of prisms used corresponding to the primary
form represented in Figure 2. The prisms I and II and also the prisms III
and IV are made as double prisms and the former of these double prisms (I II)
is placed between the object glass A. and the condenser D and the second prism is
placed between the condenser lens D and the erecting lens C. The condenser D
is shewn as being cemented unto the double prism III IV. The observer can
alter the distance between the eye glasses to make it equal to the distance
between his eyes by turning the tubes or telescopes about their common hinge
bolt e. By this hinge the instrument is also capable of being folded like a book.
The hinge bolt is further provided with a device for simultaneously moving
both erecting lenses C which consists of the nut c³ screwed on a threaded part of
the bolt and having a circular groove c' which engages the two studs co secured to
the mountings of the said erecting lenses C.
The single telescope represented in Fig. 7 combines the usual lenses-the object
glass A, the eye glass B, the erecting lens C and the condenser D-with a com-
bination of four prisms which is developed from the primary form illustrated in
Fig. 3. The prism I is placed between the object glass A and the condenser D,
the prisms II and 111, which are combined into a double prism, between the
condenser D and the erecting lens C and the prism IV between the lens C and the
eye glass B. The mounting a of the object glass A acts as a shade in consequence
of the great distance of its outer aperture from the lens.

page_text: Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
A terrestrial telescope (i.e. a telescope provided with a combination of lenses for
erecting the image) having four reflecting prisms (Fig. 1 to 3) arranged so as to
deflect the optical axis four times at 90° in the same plane and in such a way, that
the position of the image is not altered and parts of the broken axis are situated
side by side and parallel to each other, essentially as and for the purpose set
forth.
Dated this 13th day of February 1897.
JOHNSONS & WILLCOX,
47 Lincoln's Inn Fields, London, W.C., Agents.