About New Kinds of Telescopes, Especially for Handheld Use.
Ueber neue Arten von Fernrohren, insbesondere fuer den Hangebrauch.
Lecture held at the session of the 
Society for Advancement of Industrial Activity on January 7, 1895
by Dr. S. Czapski, 
Scientific Colleague of the Optical Laboratory of Carl Zeiss in Jena
Continuation, Number 5; from the 'Central-Zeitung fuer Optik und Mechanik'
(Journal for Optics and Mechanics), Berlin, 01 Mar., 1896
Translation by Ilse Roberts and Peter Abrahams


The purpose of such a field glass is twofold.  It allows the observer 
to remain behind cover, viewing ‘around a corner’ with only the objective 
unprotected and visible to the object or vulnerable to a bullet.  If the 
offset of the objective from the ocular is sufficiently large, the head 
and body of the observer can be completly hidden without limiting the 
observation.  For simple telescopes, this is the only advantage, but it 
alone should make these instruments useful for military purposes, and 
there are many other circumstances in which they could be useful.

Fig. 16 shows a simple telescope of this type in three views.  The 
sequence of prisms, derived from the first principle of the prism system 
(fig. 6), is similar to fig. 13.  There are 4 prisms, P is an isosceles 
right angle reflecting prism, just behind the objective O.  The remaining 
three prisms are shown as a larger reflecting prism q with two reflecting 
surfaces, and another smaller prism p attached to it.  They are installed 
in the housing G that holds the ocular o of the telescope.

The second purpose of such a telescope is more important, and concerns 
its function when joined with another for binocular use.  Such binoculars 
are desirable for esthetics alone, especially for instruments with 
erecting prisms, which have an asymmetrical appearance as monoculars, with 
an accentuated offset between the objective axis and the ocular axis.  
There have been attempts to cover up this asymmetry with a symmetrical 
housing that does not resemble its interior, but in our opinion these have 
been less attractive than a tightly fitting housing.  If two of these 
asymmetrical bodies are connected in a symmetric way, the transformed 
instrument does not offend the eye, and even gives the opportunity for 
pleasing forms of execution.

By connecting two telescopes into a binocular, the opportunity arises 
to increase or decrease the distance between the objectives without adding 
any additional optical elements to the system.  The distance can be larger 
or smaller than the distance between the oculars, the minimum being 
determined by the dimensions of the prisms, or it can be enlarged almost 
any amount.  To accomplish this, the user need only adjust the connection 
between the telescopes [separate the objectives by opening the hinge 
between the oculars].  If this connection is designed so that when the 
oculars (oo in fig. 17) are adjusted to the user’s interpupillary 
distance, the tubes form an acute angle towards the objectives, as in 17a, 
the objectives become closer to each other than the oculars are, until 
they touch.  With small objectives, this can give nearly completely 
unplastic, 2 dimensional images, although this is not usually 
advantageous.  In fig. 17, the axes of the telescope tubes EE contain both 
the ocular axis and the objective axis.  If the connection between the 
telescopes is designed so that the axes EE diverge to the outside, when 
adjusted to interpupillary distance (fig. 17b), the distance between the 
objectives becomes larger than the interpupillary distance.  This can give 
the user an enhanced perception of depth or space, an increased plasticity 
of the images in comparison to a binocular of similar specifications with 
objectives spaced the same distance as the oculars.  This increase can be 
measured in comparison to a common binocular, and is exactly equal to the 
ratio of the objective distance to the ocular distance.  The increased 
plasticity in comparison to a naked eye view equals the linear 
magnification times the increase in objective distance.

This phenomenon, and its qualitative and quantitative relations, is 
not a new discovery.  Helmholz, the recently deceased leader in physics 
and physiology, recognized all this long ago, pursued all the theoretical 
and practical consequences, and described it in his famously clear and 
precise manner.  Any explanation here would be only a repetition of his 
research in Physiologic Optics (1st edition, especially pp. 647 and 681), 
and in another paper (Pogg. Ann. Bd. 102, pp.16a--175, 1857).  These 
papers are recommended to anyone interested, for they are an opportunity 
for instruction and much enjoyment, to those who read them.

Helmholtz called the instrument which he constructed the 
“Telestereoscope”.  We prefer “Relieffernrohr” [relief telescope], for 
common use.  These use reflections of the image by arranging the (already 
required) optical elements of the telescope along its longitudinal axis.  

The Helmholtz invention was little used in the years since its 
publication, which astonishes anyone who has experienced the charm of 
these instruments.  I believe that inadequate execution, especially the 
adjustment of the axes of each telescope by the mechanics and opticians 
entrusted with it, is to blame.  If this is so, that these instruments 
cannot provide fusable images to the eyes, then the eyes are forced to 
adjust to the optics, which soon causes disgust with such instruments.  
However, with correct adjustment, the highly plastic images produced by a 
Telestereoscope of any variety, give a special charm to the landscape from 
the increased relief they provide.

It is often observed, that a first time user of these instruments is 
unaware of their pecularity and hardly feels their charm.  With increased 
use of these optics, an increase in receptive feeling for their images 
occurs, which increases the enjoyment and also the capacity for (or the 
consciousness of) plastic seeing, even with the naked eye.  Perhaps the 
capacity to see stereoscopically has atrophied for many people from lack 
of exercise, so that it is slowly strengthened and awakened by practice.  
Perhaps the sense of depth, like the sense of color, varies with the 
population and with attention and practice.  If this is so, the present 
era could be the entry to an age of better development of that sense.  At 
the least, the inclination to use the stereoscope for photographic images, 
and the efforts to give these instruments a functional design, is quite 
noticeable to those who follow photographical literature.  The binocular 
telescope here described will make its contribution by granting an 
increased enjoyment of viewing colorful, moving, and real nature; like the 
stereoscope gives for photographs.

(*Esthetic enjoyment requires less magnification (larger f.o.v.) and 
increased distance between objectives.  Increased plasticity does not give 
distant objects an appearance of greater magnification, but the observer 
seems to see smaller plastic details in them, as described in Helmholtz.)  
This ‘sense of depth’ is as worthy of attention and development as any.  
Nature has given us equipment to view our world with two eyes, from two 
different viewpoints at one time.  The ability to unify these two distinct 
images and to infer the spatial arrangement of the objects in the images, 
is a source of much esthetic pleasure and also a valuable skill for 
orientation.  The differences between the two images from the two eyes are 
seen as characteristics of the various distances of the objects in view, 
and are seen with a sensitivity of perception that is unrivalled.  
Binoculars with increased distance between the objectives can heighten 
this sense, as compared to binoculars of the same power, and of normal 
configuration.  This is obvious to any attentive observer using these 
instruments.  Landscapes, that appear as a uniformly extending plane 
through a telescope or even a common binocular; when observed through a 
relief telescope reveal at first glance diverse stratifications, waves, 
slopes, precipices, and cracks.  The user can almost see the air which 
separates the features of the terrain, that are projected onto each other 
in a simple telescope.  The study of a terrain according to its depth 
formation is possible, adding to the information on width and height seen 
with common telescopes.

To obtain these advantages, it is necessary to adjust these telescopes 
to provide images that are magnified to exactly the same size, with 
exactly parallel optical axes with no divergence.  An adjustment to 
individual interpupillary distances is required of the connection between 
the telescopes, and a focusing mechanism for each side is needed.  The 
degree of plasitcity of the fused image depends to a great degree upon the 
resolution with which the two individual images are seen.  Since one eye 
of a person often has small differences from the other eye, the focusing 
requirement is indispensible.  These requirements have been met in the 
Zeiss instruments. 

Other than meeting the required stability, the type of mechanical 
connection between the telescopes is immaterial.  They can be joined by a 
hinge or another joint which permits adjustment to interpupillary distance 
without losing parallelism of the optical axes.  They can also be set in a 
rigid connection, without adjustment for interocular distance; or with 
other forms of adjustment.  Required is a mechanical connection that 
maintains the large distance between the objectives when interocular 
distance is adjusted.





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