Improvements in Sighting Telescopes.
Year: 1896

page_text: Date of Application, 3rd Feb., 1896-Accepted, 25th Apr., 1896
Improvements in Sighting Telescopes.
I, FRIEDRICH RITTER VON VOIGTLÄNDER of 2 Monumentstrasse, Brunswick,
Germany Manufacturer 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 terrestrial telescopes which have hitherto heen used as sighting telescopes
for fire arms required that the eye should be at a short distance from the last lens.
This excluded the general application of the sighting telescopes because in the
most cases and those the most important, especially for field artillery, a larger
distance of the eye from the last lens is indispensable.
A notable step in this direction was made by the firm Carl Zeiss in Jena who in
the German Patent Specification No. 67,823 have described an eye piece suitable
for sighting telescopes and having a point of sight at a great distance from the
last lens.
In this Specification it is firstly stated that the constructions of telescope eye
pieces hitherto known do not admit of fulfilling this requirement viz. :-that the
point of sight should be at a distance from the eye lens equal to a multiple of the
focal distance of the eye piece. Then the peculiarity of the eye piece with
the distant point of sight is characterized by the fact that a dispersing or divergent
lens takes the place of the collective glass convex lens used in ordinary eye pieces.
This lens enlarges the focal distance of the eye piece, and thus the point of vision
conjugated to the point of convergence of the real image is moved away
correspondingly from the eye lens.
The insertion of a negative lens is however not the only means for shifting the
point of vision without altering to any great extent the dimensions of the eye
piece and its focal distance.
A simple enlargement of the focal distance of the eye piece part of a terrestrial
telescope would firstly produce a greater length of tube and a greater focal
distance and hence give the telescope less enlargement. But if the same
enlargement were to be maintained the objective focal distance and therewith also
the length of the tube would have to be increased.
These defects are obviated by the arrangement forming the subject of this
invention. This arrangement is characterized by the fact that the focal distance
of the inverting system or tube in a terrestrial telescope is shortened so much that
it not only inverts the objective image but also enlarges it to a substantial extent.
And one has to go so far in this sense, that the inverting system also fulfils the
main function of the eye lens, viz. to greatly enlarge like a magnifying glass the
image produced by the objective and inverting system while the eye lens takes
the function of projecting the point of sight (as that of the point conjugated to the
point of convergence of the real image) to a greater or smaller distance from the
last surface of the eye lens.
On the drawing
Fig. 1 illustrates the ordinary function of the eye lens. A is the real picture
produced by the objective and inverting system or collective, B is enlarged virtual
image produced by the eye lens C and P is the point of sight.
But by the new arrangement Fig. 2 the image B is very slightly enlarged so
that the point of sight P moves far away from the eye lens C. Fig. 3 shows the
course of the rays in the whole telescope.

page_text: Thus in the telescope now to be described as an example the focal distance of
the objective is 38 mm., and the focal distance of the inverting system 4 mm. 80
that the objective image is enlarged 9-5 times. Then the focal distance of the eye
lens may be greatly enlarged, in this case up to 75 mm. without lessening the
numeral of enlargement below the size of 2 to 2.2 desired for ordinary sighting
telescopes. By the greater focal distance of the eye lens the point of sight, as the
point conjugated to the point of congruence of the real image, is moved corre-
spondingly further away from the eye lens.
But in order to obtain an undistorted, that is to say an evenly enlarged image,
the eye lens must comply with the known condition, that a constant relation subsists
between the tangents of the angles of inclination of the outgoing rays towards the
axis and the section heights of the ingoing rays in the plane of the real image.
A lens of two parts cemented together and of the form now to be described can
fulfil this condition, also for a rather large opening.
The data for such a terrestrial telescope with a point of sight, which by an
enlargement of 2-2 has a distance from the last lens of 58 cm. are as follows:
In Fig. 4 the biconvex lenses are of crown glass with np = 1-517 and the two
negative lenses are of flint glass with n = 1.62.
=216 73 = ∞
T₁ = rs = 2.3
T₂ = ∞ : T₁
=35: T
= 25
Such a simple mode of construction gives the following advantages without
detriment to the quality of the images hitherto obtained with sighting telescopes.
1. Increased strength of light because of the smaller number of the separated
refracting surfaces.
2. Almost any desired enlargement of the eye distance.
3. Greater facility of centering of the system and the maintenance of it.
4. Favourable optical conditions for the amplification of an undistorted image
field by simply enlarging the diameter of the eye lens.
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
claim is
A terrestrial telescope in which the inverting lens or the inverting system s
greatly enlarges the objective image that the eye lens, instead of as heretofor
enlarging the image as a magnifying glass assumes only the function of projecting
the point of sight far away from its last surface, substantially as set forth.
Deted this 3rd day of February 1896.
Such a simple mode of construction gives the following advantages without
detriment to the quality of the images hitherto obtained with sighting telescopes.
1. Increased strength of light because of the smaller number of the separated
refracting surfaces.
2. Almost any desired enlargement of the eye distance.
3. Greater facility of centering of the system and the maintenance of it.
4. Favourable optical conditions for the amplification of an undistorted image
field by simply enlarging the diameter of the eye lens.
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
claim is
A terrestrial telescope in which the inverting lens or the inverting system so
greatly enlarges the objective image that the eye lens, instead of as heretofore
enlarging the image as a magnifying glass assumes only the function of projecting
the point of sight far away from its last surface, substantially as set forth.
Dated this 3rd day of February 1896.
JENSEN & SON,
77 Chancery Lane, London, W.C., Patent Agents.