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 3; from the 'Central-Zeitung fuer Optik und Mechanik'
(Journal for Optics and Mechanics), Berlin, 15 Mar., 1896
Translation by Ilse Roberts and Peter Abrahams


In figures 18, 19, 20, and 21 are two extremes of design for 
telescopes with increased distance between objectives.  Figure 18 shows a 
six power double telescope (field glass) at about 1/2 life size, in 
vertical projection and outline; which is based on the design in fig. 4, 
the basic prism system when offset is restricted to the minimal width 
given by the dimensions of the prisms (the diagonal length of the shorter 
sides).  As in the monocular telescope of fig. 8, the individual prisms of 
this basic form can be thought of as a united pair of smaller right angle 
prisms.  The prisms are fastened on ledges inside the metal housing, 
between the objective Ob and the ocular Oc (fig. 18).  These housings form 
the body of the telescope and are connected to each other by a hinge 
(joint) whose axis is parallel to the axes of each telescope.  [The 
placement of the hinge on the binocular body determines the angle that the 
‘axis planes’ (fig. 17b) of the two telescopes take to each other.  
Czapski seems to be pointing out that if the hinge is fastened at the 
point where the two halves are closest to each other, adjusting 
interpupillary distance will rapidly change the angle of the axis planes; 
if fastened at the top or bottom of the housing, adjusting IPD will 
slightly change the angle.]  When the ocular axes are separated by an 
average interpupillary distance of about 6.5 cm., the objective axes are 
separated by 112 mm, or 1.7 times the interocular distance, and the axis 
planes of the two telescopes become nearly parallel to the plane joining 
the ocular axes.

These designs are optimized for portability, but even this relatively 
small increase in objective distance leads to a very visible augmentation 
of depth perception or plasticity of the image.  The capacity for 
distinguishing depth increases proportionally to the product of 
magnification and distance between objectives.  This 6 power glass allows 
the same depth perception as a ‘common’ [Galilean?] binocular of 10 power, 
which has a much smaller f.o.v.  The design shown in fig. 18 allows hand 
held binoculars and also gives an image plasticity equal to common double 
telescopes of greater magnification and therefore narrower f.o.v.  Fig. 19 
shows, at half size, a photograph of this instrument as built by Zeiss.

The double telescope shown in Fig 20 at 2/3 size shows the application 
of the above design principle when portability is sacrificed for the 
widest possible separation between objectives.  [p52]

This instrument uses the Porro prism of fig. 14, which is a variant of 
fig. 5 or 7.  One prism is placed away from the other three by a distance 
of almost the entire focal length of the objective.  The separated prism, 
protected by a special cover, is positioned directly in front of the 
objective of the telescope, and the other three prisms are placed in front 
of and close to the ocular, inside the ocular housing.  Two of these three 
prisms are combined into a large isosceles right angle prism and the third 
is half this size and cemented to its side..The depicted instrument uses 
the prism housing as the hinge between the telescopes, but this connection 
can be made in many other ways.  For portability, hinges are used [instead 
of a rigid tube].  This hinge can be fastened to the telescope bodies so 
that for two positions [rangefinder and periscope], the oculars are spaced 
at normal interpupillary distance, adjustable for individual differences 
by a minor movement at the hinge.  The first position gives the telescope 
its characteristic quality of increased depth perception.  The second 
facilitates transport, because the instrument in its folded up position 
can be fitted in a smaller case.  Both positions permit the possibility of 
observation from cover, if the tubes are long enough.  The observer can 
avoid being visible from the position of the object.  At the outstretched 
position of the tubes, the observer can stand behind a tree, and watch 
around it from both sides.  With folded tubes he can observe over the top 
of an obstacle (a fence, wall, crowd, or other).

The folding tubes can be used in both positions to demonstrate the 
peculiar use and great beauty of the images given when objectives are 
adjacent as well as when they are widely separated.*  [p53] (*It could 
also be made so that the objective spacing is adjustable, without moving 
the ocular spacing that is fixed to an interpupillary distance.  This did 
not seem to meet a practical need and was not produced.)

Figures 21a and 21b show at half size an 8 power Zeiss relief 
telescope with 20 mm diameter objectives, and about 335 mm between 
objectives, i.e. about 5 times larger than normal interpupillary distance.

(These, as well as the other designs mentioned earlier were shown 
during the lecture as demonstration models and as ready to use specimens.)

Previously I have given general explanations of the thinking which 
guided the technical and scientific collaborators at Zeiss during the 
construction of these particular telescopes.  I would now be guilty of not 
telling the truth if I didn’t also tell you what came to light, even if 
much later, about the newness and the historical priority of what we 
thought were inventions.  During the lengthy preparations for the 
inventions, we were not able to find any indication what so ever of prior 
use of these ideas; although as simple and obvious as they seemed, there 
was the possibility of other such designs.  The imperial patent office 
made us aware that in the widely distributed textbook of physics by 
Eisenlohr, one of the designs discussed here was already described as 
derived from Porro.  Indeed, the prism systems shown here in figures 8 and 
9 were shown and described in two separate editions of Eisenlohr.  Thus, 
both major types of systems, as well as the idea of shortening the 
telescope by separating the prisms, and finally the scheme of unifying the 
prisms with the lenses by grinding spherical surfaces instead of planes, 
were all invented by the famous engineer, surveyor, and optician Porro, 
and were considered his intellectual property. 

This discovery motivated further research and brought to light the 
original papers about the Porro telescopes, from 1853 and 1856, in the 
French weekly “Cosmos”, published by Moigno.  The design shown in Fig 8 is 
described as “longue vue cornet ou telemetre” [long view horn or 
telemeter] (Cosmos, vol 11 page 222).  ‘Telemetre’ refers to an instrument 
for measuring or estimating distances.  His inventions are supposed to 
have taken place long before those of Dove in 1851 (or 1849, according to 
other sources).  Cosmos vol. IX, page 401, describes the glass in fig. 8 
as “Lunette Napoleon III”, including spherical planes ground on the 
prisms.  This research and reports from other sources were able to 
establish that the inventions of Porro (the shortening of the instrument 
in connection with the erection of the image by the prisms) were limited 
to the above described arrangements.  Porro and his successors neither 
combined these telescopes into a double telescope for binocular use, nor 
augmented the offset of the objective and ocular axes to allow seeing 
“around a corner” with a monocular or for telestereoscopic viewing with 
binocular instruments.  The independent re-inventors of the ideas of Porro 
number number quite a few, but all of them invented less than Porro, 
namely only the system shown in Fig 6 (for example the physicist Pohl in 
Vienna, who worked in connection with Voigtlaender, the optician Grubb in 
Dublin, and another Englishman, T.J. Potter).  Even Professor Abbe, who as 
a member of the staff at Zeiss has now been involved in the details of the 
designs presented here, had already in 1873 invented the systems shown in 
figures 6 and 7, and assisted with a telescope built using the latter.  
Further developments to be derived from these forms, which give the 
invention its real value, including those of Porro, escaped him as well.

This confession - which I feel obligated to make - surely will raise 
doubts as well as the question of how it came about that those early 
inventions, and especially those of Porro as the oldest and most developed 
ideas, were forgotten, so that presently - and perhaps for the last twenty 
years - no optician is using them.  If these arrangements indeed have the 
many advantages I ascribed to them, it must seem strange and questionable, 
that they did not find recognition, and received no distribution.  These 
doubts & questions, as you can imagine, occurred to us, too, immediately 
upon discovering those historical facts.  But the answer to them was not a 
difficult one and fortunately for us also not an unfavorable one for our 
endeavours. This was confirmed on inspecting the actual telescopes, sent 
to us by friends, including one by Porro himself, one by his main 
successor and pupil the well known optician Hofmann in Paris, and others 
made by various reinventors.  [p54]  Since the participants in these 
inspections have an interest in the outcome, because the long term success 
of the invention is at stake, I will say a few words about it.

Inspecting or executing these simple designs in a practical manner 
must overcome significant technical difficulties, and requires the 
simultaneous implementation of several conditions.  At that time, this was 
not possible.  A detailed explanation of this necessity would lead into 
the narrower area of practical optics, and the basics are easily 
comprehensible, so the discussion will be brief.

First, for these designs, the typical simple collimation [centering] 
procedures do not suffice.  An optical instrument consisting only of 
lenses has an optical axis that is clearly defined, and the central (co-
axial) sequence of the lenses can be easily machined on the lathe.  
Deviations of the axis are obvious, even with an assembled instrument.  
However, in these prism designs, there is no mechanically unified optical 
axis, except perhaps in the simplest and least effective types shown in 
figures 6 and 7.  The essence of most of these designs is the frequent 
shifting and rotating of the axis back and forth.  Collimation therefore 
requires an incomparably more difficult procedure.  Otherwise, the ocular 
views an off-center part of the image given by the objective, or views it 
from a deviated direction, to ruin the quality of the image.

The achievement of a satisfactory quality of image in the monocular 
Porro telescope also requires precise execution of the prisms in the 
telescope.  The hypotenuse planes, which meet the light at an average 45 
degree entry angle and function by reflection, must be flat to a degree 
that is extremely close to an ‘absolute’ (as Moigno emphasized in the 
above mentioned description of the Porro telescopes, Cosmos, vol IX page 
404).  The required precision increases when the prisms are closer to the 
objective of the telescope and with stronger magnification. Those who know 
practical optics are aware that there is no more sensitive test for the 
quality of a mirror, and especially for its flatness, than the test that 
is ‘involuntarily’ applied here, the observation through a telescope of a 
reflected image, encompassing a large incident angle of the rays.*  (*See 
Oertling, Verh. d. Ver. z. Bef. d. Gewfl. vom J. 1843; and H. Schroeder, 
Central-Ztg. f. Opt. u. Mech. vol. 2, page 7, 1881; and also my Theory of 
Optical Instruments, page 77).  This critical test occurs no less than 4 
times in each tube, and requires a high level of technical ability from 
the maker.  The consequence of an error which exceeds the permitted 
tolerance* is astigmatism across the entire image.  (*The well known limit 
for prisms close to the objectives is that the reflecting surface of the 
prism may not deviate more than 1/4 wavelength of light (0.0001 mm) from 
the ideal mathematical plane.  The other prism surfaces are refracting and 
must be worked to the same tolerance, but can be spherical surfaces if of 
sufficiently long radius, unless they are intentionally given stronger 
curvatures as mentioned on page 26.)

Third, in the introduction, concerning the intensity of light given by 
the telescopes (R.5 f.), I emphasized that not only is the size of the 
cross section of the exiting ray bundles important, (which depends solely 
on the size of the objective and the magnification); but also that the 
weakening of the intensity of the light by reflection and absorption in 
the lenses, mirrors, and prisms of the instrument must be considered.  
Generally, the number of reflections in a Porro prism telescope causes no 
worse an effect than terrestrial telescope optics.  But without any doubt, 
the absorptions of the Porro prisms were a source of inferiority which 
caused Porro and Hofmann to fail.  The ray path through the prisms is much 
longer than through (for example) the image reversing lenses in a 
terrestrial telescope.  The absorption and weakening of the rays from 
their passage through the prisms of Porro (or Zeiss) is incomparably 
greater.  This can be prevented by using for the prisms a glass of such 
perfect transparency that a weakening does not occur when the light passes 
through.  But this was not possible for Porro and his successor Hofmann, 
for the simple reason that in their time a glass that satisfied the 
requirements did not exist.  Glasses of such purity were only produced in 
the “Glasschmelzerei fur optische und andere wissenschaftliche Zwecke” 
[Glass Factory for Optical and Other Scientific Purposes]** founded by 
Abbe and Schott in Jena.  (**See the lecture by Dr O. Schott under the 
above title in these papers from 1888, page 162).  Thus, only very 
recently could this part of the task be accomplished and the requirements 
for the construction of prism telescopes be met.***  (***About 100 optical 
glasses are regularly produced by the Jena factory, but only two satisfy 
these requirements for transparency, and only one is usable because the 
other cannot be made to other standards of purity (elimination of small 
bubbles in particular).  An instrument made by Porro himself, which I had 
the opportunity to see, thanks to the kindness of H. Haensch, left much to 
be desired in that regard: the glass of the prisms was streaky as “freshly 
made sugar water” and the rest of the technical construction was extremely 
unsatisfactory as well.  The instruments made by Hofman that I have seen 
were without fault in this regard.)  For this reason alone, these early 
attempts were not successful.

[p55] In my opinion, the instruments made by Porro and Hofmann were 
not successful for an additional reason, namely because Porro intended his 
telescopes to be used for telemetry.  They couldn’t be used by the general 
public, because the absolute as well as relative magnifications were much 
too large.  Absolute, because the magnifications were 15 to 18 power; and 
those are far too large for a handheld telescope, where one half the power 
chosen by Porro would be more than adequate.  These magnifications make 
the instrument practically useless because of the limited objective f.o.v. 
of a high power glass, and even harmful to acuity because of the physical 
impossibility of hand holding such a strong glass sufficiently steady.  
The relative magnifications were too high, because in spite of the 
comparatively large objectives (about 25 mm), the exit pupil is hardly 
more than 1 mm in diameter, so the images produced become very dim and are 
only usable with a very glaring illumination, as was shown by practical 
testing of these instruments.

A vital shortcoming of the Porro-Hofmann telescopes, and therefore a 
more essential advantage of the ones produced by Zeiss, was that they were 
made as monocular telescopes only.  Binocular viewing allows stereoscopy, 
greater ease of use, and better illumination.  Both Porro and Hofmann did 
not advance to the production of double telescopes from two single ones, 
which may have had a good cause in the fact that this unification in a 
practically satisfying manner caused too many great technical difficulties 
which they probably couldn’t have overcome.

For an instrument created for binocular viewing to serve usefully and 
comfortably, other requirements besides those already considered for 
monocular instruments must be fulfilled.  These have already been 
mentioned (pg 30), and their fulfillment for such prism telescopes 
presents quite specific difficulties. The reason for this is quite 
obvious.

The qualitative efficiency of the reflecting surfaces has just been 
discussed, and now their quantitative performance must be examined, 
concerning which they are equally “sensitive” and difficult to handle.  A 
glass of medium index refracts a ray by about a third of the entrance 
angle, from its original direction towards the entrance axis or away from 
it.  Because of the reflection within the prism, it is actually deflected 
by double this angle and a change of inclination of the reflecting surface 
by any angle changes the size of the deflection by double this angle.  
Thus, the change of direction caused by the reflection is about six times 
as efficient and as “sensitive” as that of the refraction.  The position 
of a reflecting surface must be six times as exact as a refracting 
surface, if its effects on the deviation of light are to be equally 
controlled.  A mistake in correction causes six times as strong an effect 
as an equal mistake in the correction of a refracting surface.  In each of 
the two telescopes, there are four reflecting surfaces, and thus in the 
whole double telescope there are eight, greatly increasing the 
requirements for exactitude in the execution of a double telescope with 
reflecting prisms for image erection, both optical and mechanical, if the 
conditions of comfort and usability are to be fulfilled.  (At the lecture, 
a demonstration of the inner structure needed to obtain and guarantee long 
lasting adjustments.)

You will find it understandable after all this, that even a relatively 
well equipped workshop, such as the one of Zeiss, did not come to a 
satisfactory result right away, but that it needed lengthy theoretical as 
well as technical preparation and research, before the success 
corresponded to the expectation and before devices were created which 
guaranteed a long lasting, good performance of every instrument that 
leaves the workshop.






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