Moritz von Rohr. Die Binokularen Instrumente. Berlin: Springer, 1920. Pages 88-94: The Binoculars. There has been important work on binoculars during the present decade, but detailed accounts of it are difficult to find. The original patent applications at the French patent office could be a good source of information, especially on the Dutch double telescopes. For example, Hardweiler increased the typical magnification of 4 power or less, to 10 or 15 power by using, in each telescope, a complete teleobjective with a diverging lens. P.G. Bardou (1) connected two terrestrial telescopes to form a double telescope in a similar attempt to increase magnification, using the standard French designs for center focus and interocular distance adjustment. These new instruments resemble the earlier models documented by J.T. Hudson (1) in 1840 (see page 42). The work of the eminent Italian engineer I. Porro, then working in Paris, was much more important for the double telescope. The history of the telescope includes his introduction of reversing prisms, which must have been developed by 1850. E. Abbe’s work allowed the Porro invention to gain its rightful place, though he was at first unaware of this predecessor. Abbe’s contemporaries thought, probably correctly, that the difficulty in obtaining sufficiently clear glass had prevented the production of larger quantities of single - let alone double - telescopes. (* According to K. Fritsch (1), that was the opinion of his French competitor I. Porro in 1851, and the prominent expert Fr. Uchatius shared this view.) According to some sources, Hoffman, the early colleague of Porro, acquired in 1857 a patent limited to the production of single telescopes. However, the time was close when the combination of two Porro glasses attracted the interest of experts. (page 90) A very strange proposal came from the optician A.A. Boulanger in Paris (1), who with M.Ph. Poudrilhe in the fall of 1858 patented an astronomical binocular. Two parallel tubes were fitted at the ocular, each with a pair of [Ableseprismen = read- off prisms]. (Fig 63. A.A. Boulanger’s and M.Ph. Poudrilhe’s prism system for the astronomical double telescope.) The two inner prisms could be separated or approximated to adjust the double tubes to the interpupillary distance of the observer. They could also be turned around a horizontal axis so that the viewing direction into the telescope made possible a more comfortable head position. Connecting these four prisms behind a single objective was also proposed. The limitation this places on the rays will be treated later by LaFleur and Roulot, where the same idea was proposed and also realized. Very soon after that, A.A. Boulanger (2) returned to the Porro design, because he received, as mentioned by G.Witt (1), a patent in August 1959 for a field glass consisting of two Porro prism telescopes (fig. 64), for which he proposed the name 'Neo- Binocular' [Jumelle], or 'Prismatic binocular with upright images'. This field glass could be adjusted for interpupillary distance by rotating the image erecting prisms around the axis of each objective so that the oculars described an arc. As proposed in the patent, by turning a screw, the axes of the oculars could be separated (between 57 and 70 mm, according to a recent measurement), while the center of the objectives remained at a distance of 64 mm. (page 91) The exterior of this binocular clearly showed that the manufacturer based the glass on the Dutch telescope. The instrument was produced by the firm Luquin & l’Hermitte, founded in 1840, which was acquired in 1868 by E. Lacombe, and known since 1895 as L. Lacombe Fils. It would have been better to use Porro telescopes than the Dutch ones, since then his double telescopes, of the same power, would have had the advantages of the Porro monocular telescope: the image quality would have been greater, and the field of view larger. However, these double telescopes were not built in large numbers, because the optical technology of the time could not overcome the difficulties in their execution. It is therefore quite understandable that later on nobody knew about this proposal, especially since it found no real distribution because of the unfortunate openness of the French patents. The endeavors of these craftsmen were accompanied by important theoretical work by scientific leaders, with a goal of increased depth perception through the design of the binocular. Though there was no success in the beginning, the inspiring development of modern binoculars obliges the historians to research the first attempts at an understanding of the effect. W.Hardie was one of the first workers. According to a later account (3), when writing his first paper in 1853 he had already thought about a combination of his telestereoscope with two magnifying telescopes, to achieve an increased depth perception. It was not his fault that this beautiful idea was not practically realised. Adie, the Edinburgh optician that he hoped would construct it, did not see the importance of the new instrument and so fabrication was not attempted. Similar thoughts were expressed by A. Claudet (3) in the same year, when at the beginning of a speech he made the cute pronouncement that a 4 power double telescope gave a view that was flattened by 1/4, and that the depth would be increased when viewing through a double telescope held backwards in front of the eyes. He enlarged upon this idea in a little French book (4) appearing somewhat later, and in 1853 presented the so-called Helmholtz Rule for depth similarity. This stated that a 4 power double telescope should have a separation of the objectives to 4 times the interocular distance. A 63.5 mm ocular distance would mean a 25.4 cm. inter-objective distance. This arrangement could be accomplished by adding prisms. (page 92) In regards to the depth perception this is incorrect, because the flattening is totally independent of the separation between objectives, since it is caused by the change in inclination angle of w’ > w, which is valid for every image. However, that explanation for depth perception is true if the images are at the real distance, with exaggerated depth, instead of near and minimized like a toy. This is how P. Gruetzner (1) described the simple, non-magnifying telestereoscope, and is the convention used by A. Claudet. To explain, if the simple telestereoscope with ‘entrance pupils’ spaced K times the interpupillary distance, can exaggerated the sense of depth; then a truly flattening double telescope of K1 magnification would produce a more realistic depth in the image. As a condition for the correct amount of depth, he gave K= K1, similar to the explanation of J.von Kries (1. 540-543; 548-549) However, these are psychological phenomena, which have nothing to do with the depth of the image in our sense, and which we cannot pursue. It is remarkable that later on, even well known experts in this area adopted ideas similar to Claudet, such as Brewster in 1860 and Helmholtz in 1866. Another claim of invention was raised in October 1859 by J.F.W. Herschel (2. 121) in his article “The Telescope” for the Encyclopaedia Britannica. Herschel gave a complete theory for the telestereoscope with and without telescopic magnification. He knew of the existence of the Helmholtz instrument only in general terms and the details of his description are independent from the publications of his predecessors. At this opportunity he took notice of the invention of the telestereoscope with telescopic magnification, which is supposed to have been done by his son A.S. Herschel in 1855. According to the description, it seems that the instrument was built, but no notice was published. Even if this is true, the priority of A. Claudet and of W Hardie can be upheld. The flattening of the 3d image by such a telestereoscope with telescopic magnification was not emphasized by J.F. Herschel. These ideas and proposals created a real sensation only when H. Helmholtz (1) in 1857 published a description of his telestereoscope (see page 86). (page 93) He had built a terrestrial telescope into each half of his simple instrument. Constructing a double tube instrument from these components was not really new, because J.T. Hudson(see page 42) and P.G. Bardou (see page 89) had already written about such instruments. When writing about the magnifying telestereoscope, he emphasized that the error of flattening was not absolutely eliminated by increasing the distance between the telescopes, only an overall minimization of the flattening in the 3d image was achieved. But it should be possible to get a true 3d image for single objects at a certain distance, if one balanced the flattening effect of magnification with a change in the angle of the outer mirrors; an idea expressed by F.H. Wenham as well (see page 88). It is not known why H. Helmholtz did not put this patently clear and articulate presentation into his ‘Handbook of Physiological Optics’. There is not even a hint (2, 681-82) as to why it was left out. Of course, it does not agree with an often cited phrase of the handbook. (*This phrase is: “Since the magnification is 16 times, the effect of the instrument is as if seeing the object with the naked eye from a distance sixteen times closer.”) That is regrettable, and because of the lack of an explanation for this omission, this sentence became known as the Helmholtz criterion for depth similarity, as used by physics historians, and by extremely frequent repetition it received nearly the same recognition it would have gotten if it were correct. We can surmise that Helmholtz’s description of depth perception was the same as Claudet’s, as just discussed. (*The work of J. von Kries (1, 549) in the first edition, should be mentioned.) But Helmholtz apparently did not always perceive the seemingly overemphasized effect of the simple telestereoscope. A summary of ideas from those times about the magnifying telestereoscope was given by the old hand A. Claudet (12) in 1860 at a meeting in Oxford of English scientists. He demonstrated his experiments with a telestereoscope. At first, he used an opera glass to show the first rows of spectators the difference between the rendering of depth, when viewing through the ocular or through the objective. By placing two reflecting prisms in front of the objectives, to effectively increase the distance between optical axes, a correct perception of depth was given, in his opinion. [p94] Based on these opinions, one can imagine how he manipulated his way to his result, which had already been concluded by W. Hardie (3). But it remains inexplicable why this successful scientific photographer did not mention the recently published Helmholtz Telestereoscope. He could correctly deny Helmholtz’s priority, and so elevate the interest in his lecture. During that decade a wide interest existed in the double telescope, but it had almost no influence on the firms that supplied the markets. It is astonishing to the present day observer that no military organization from any of the European states seriously attempted trials of the magnifying telestereoscope, which Helmholtz had presented to the public. But the seeds must have rested in the field of time, as the first preliminary idea of a stereoscopic distance meter was presented by F.W. Wenham (see page 88), but realised in an imperfect form. It should have become clear in this summary, that the ideas derived from the stereoscope and the rendition of three dimensional images, had quite an effect on the understanding of binoculars, but did not lead to any practical usage. It could be that the lack of glass of a complete transparency partly explains the failure, but part of the cause may have been the negative response of the most important producers. At that time, in German speaking countries there were no workshops under scientific direction for manufacturing terrestrial telescopes. Pages 135-7: Development of the double telescope. The surveyed history of this specific field is as incomplete and unsatisfactory as the preceding time period; and here as well, expert research in the French patent archives would be most desirable, since, in those times, the Parisian craftsmen had the lead in this area. The earlier proposal by Boulanger & Poudrilhe is reiterated by a short remark of J. Watson (1) in 1864, when he proposed a double telescope for astronomical observations. In his case it combined two Herschelian reflectors. [Von Rohr might not mean two telescopes of the Herschelian optical system, but possibly the Newtonian binocular depicted in John Herschel’s book The Telescope, quoted from in an earlier chapter.] Described on page 33 is a combination of reflecting telescopes to form a binocular instrument. The patent from the summer of 1866 from the already mentioned L. Jaubert claimed extensive rights to double telescopes, since he intended to make opera glasses as well as terrestrial and astronomical telescopes. It doesn’t seem that he was aware of the technical requirements in this special area, and his influence on others was rather minor. [p136] His proposals are particularly extensive, but one can hardly imagine that all of the possibilities implicit in the patent application were actually explored. As mentioned, he occupied himself not only with Dutch double telescopes, but also planned double astronomical and terrestrial telescopes. The astronomical telescopes were to include all kinds of reflectors, those of Newton, Gregory and Cassegrain were adapted for binocular observation and devices for adjusting interocular distance were always provided. The terrestrial telescopes were not designed with separated objectives, but just the opposite, some had much of the inner edges of the objectives cut away, because no use was made of Riddell prisms. His designs that used prisms to separate the optical axes, to direct the rays from a single objective to both eyes, sometimes crossed the axes to direct light from the left objective to the right eye and vice versa. Some of his research is not well done. Occasionally his single telescopes, opera glasses, and binocular terrestrial telescopes, have axes with a slight convergence onto an object point of definite distance, and since this device was known to Cherubin d’Orleans, it probably was not quite as new as he thought. The Paris World Exhibition of 1867 allowed ideas and designs of telescopes that had long been known in Paris to spread to German workshops. First there were the Porro reversal prisms, whose application was actively pursued in some areas of the German speaking states, but without great success at that time. In the area of binocular instruments, the eyes of some German manufacturers were opened to the fact that even the common opera glass needed an adjustment at the central axis to accomodate the observer. As a result, the caption of the pertinent paragraph from the Busch price list reads: “Portable Military and Hunting Perspective with distance for both eyes adjustable around axis (called ‘centre’)”, the idea clearly of French origin. One would like to think that the French technical term from 1836 remained in use during the long interval in France, and that opera glasses were produced which permitted an adjustment to the ‘fulcrum distance’, even if made in smaller numbers. (see page 42) [Von Rohr seems to be partly referring to a device for tightening the central hinge to fix a particular interocular distance.] At first E. Busch produced his hinged double scopes with an ‘adjustment assurance’, which was later abandoned, since a tightened hinge was sufficient. But the possibility for adjusting opera glasses was not abandoned, and occasionally in the price lists of those times is the description ‘Family-Theater-Perspective’. Science had at that time lost interest in researching the impression given by three dimensional images from a magnifying telestereoscope. In the sixties the earlier, more logical idea of a spatial image was abandoned. This view was discussed on page 92, noting that it was shared by Helmholtz (page 93), Brewster (page 154) and Claudet (page 114). This regression can be explained by the fact that the increase in apparent depth which was seen in the simple telestereoscope when observing familiar, close objects, was negated by the flattening effect of magnifying telescopes. Overall, in this area, and in contrast to the microscope, no fresh new trend could be observed. Business continued, but the technical progress showed no adherence to the lessons of Science. Pages 170-4: Improvements in the Double Telescope On page 136 of the 1873 - 1875 price list from the Rathenow branch of Schultze & Bartels, one can detect the French influence in the recommendation of 'military, marine and travel telescopes for both eyes'. These are the double terrestrial telescopes of earlier times. (* H. Goltzsch (3. 106) had in 1881 pronounced his astonishment that double telescopes were limited to the double perspective design (Dutch telescopes), and that double terrestrial telescopes were not produced. Since he had great interest in this question and also a certain expertise, one can expect that few sales and no regular production occurred in Germany much before the 80s.) These double terrestrial telescopes had been mentioned by J.T. Hudson (p42) and by P.G. Bardou (p89). Their objective diameters of 27 mm give a 1.7 mm exit pupil at the given 16 power. The hinge for adjusting to interocular distance is expressedly mentioned, and one can suppose a rather short length to the instrument, since only two extensions are described. At about the same time LaFleur (1) and Roulot adopted the idea, already published by A.A. Boulanger (p90) and M.Ph. Proudrilhe, of combining two oculars and a pair of rhomboid prisms with a single objective, for binocular observation. This was not a new invention, but the production of such instruments that followed was important, and is documented in the beautiful collection of Mr. K. Stegmann in Rathenow Fig. 86 is the view across the horizontal axis; showing that if the two small circles are reflected onto the the two adjacent end surfaces of the rhomboid prisms, then through the objective into the object space; moving it into the two object side ‘ray spaces’, one can recognize in each case, how far the the areas of unrestricted and restricted light expand. (page 172) The instrument was introduced with the name Lunette Prismatique, which at least sounds like the name of the Boulanger double telescope (page 90). The Parisian optician C. Nachet (1), whose workshop was already mentioned concerning the binocular microscope, was since 1870 busy with the production of a double telescope with image erecting prisms. Some years later, he applied for patent protection for these, described as Jumelle Prismatique (prismatic binocular). It differed from the Boulanger Neo-Jumelle by a different form of the prism body. Also, the screw for adjusting interocular distance was omitted, and instead one adjusted the interocular distance by rotating an individual ocular. According to the available information, all users with interpupillary distances over 57 mm could use the glass, assuming a distance between objectives of 83 mm. Therefore, in this case an increase in stereoscopic effect was achieved; and for a typical interocular distance the increase was about 1.2 to 1.4 times. But the inventor did not seem to be aware of the advantages of this design, since they were not mentioned in the patent. The importance of Boulanger’s innovation, as described above, is true as well for the binocular of Nachet. The increased distance between objectives was too small to have been noticed by the users, and it is certain that Nachet’s attempt to bring the prism telescope to the market failed because of the disinterest of buyers. At about the same time, W.H. Thornwaite (1) proposed in 1877 an astronomical telescope where also a single objective, a concave mirror, directed the light to a semi-transparent flat mirror, and then to the two oculars. He also touched upon the possibility of using a Wenham prism system (page 132). Here, both eyes of the observer would receive the same image, but under no circumstances could the distant objects in space show depth in any telescope. (page 173) A short time thereafter, in 1881, H. Goltzsch (3) proposed an astronomical double telescope, which was later built, with a few alterations from the original design. The form in which he first proposed it used 74 mm objectives (fig 88). The two main tubes were positioned parallel, and one tube was partly hidden from the viewer, which was necessary so that the interocular distance could be less than the distance between the centers of the objectives. The oculars were placed 60 mm apart, which Goltzsch presumed was the smallest common interpupillary distance. Reading prisms (P) [Ableseprismen] were placed in the exit pupils of the astronomical telescopes, directing the optical axis outwards. These prisms, P in fig. 88, were not in the same plane and therefore were at different distances from the eyes of the observer. To even out these differences he sent the exit pupils through two small astronomical telescopes of different lengths, but of the same magnification (gamma = 1),and then into the eyes, as shown in the diagram. Adjustments for interocular distance were made by rotating the main telescope tubes about their optical axes, so that an interocular distance wider than 60 mm resulted in a convergence of the eyes. This design developed in two directions. First, to use total reflection more efficiently, the simple ‘reading prisms’ were replaced by equilateral prisms, so that the observer did not look downwards at a perpendicular angle, but forward at an angle of 60 degrees. He then proposed reducing the number of lenses, by replacing the Huygenian ocular with a negative lens, and adjusting it to the magnification (gamma = 1) with the small telescope, so that a usable telescope ocular resulted. This last innovation belongs to the history of telescope oculars but must be mentioned here, because H. Goltzsch saw himself forced by the transition from the inverting Huygenian ocular to the simple negative lens which did not invert, to pay attention to the relations which influence the rendering of depth. He did that in an adequate, but quite restricted way, by noting the stereoscopic differences in the image space. In the last case, he created a natural position for the eyes by placing an Amici prism in front of each eye. (page 174) When observing terrestrial objects through his binocular, he did notice a disturbance of the stereoscopic effect, which he correctly explained was a result of the displaced ‘object sided projection centers’ [prism secondaries]. The Boulanger invention was paralleled by the double telescope which Carston Diederich Ahrens (1) applied for patent protection at the end of 1884 in England. However, he did not complete this patent. Judging from the piece in the Zeiss workshop, the failure of this instrument is quite understandable, since image reversal by Porro prisms was not used to shorten the tubes or to augment the quality of the image. The prisms were only used for simplified interocular adjustment, and even then in the same way that Boulanger used, 25 years earlier (page 90). Pages 196-199: The Introduction of the New Double Telescopes In 1893, E. Abbe (6) returned to work on the construction of prism telescopes, which had been abandoned for years. His predecessor I. Porro was made known to him only by a notice from the Berlin Patent Office. Abbe at first concentrated on the production of double telescopes. He far surpassed the Nachet form, by using a lateral displacement of the optical axis to shift the ‘image space’ from its position in the ‘object space’. This characteristic of the Porro prism system allows the possibility of creating a Helmholtz stereoscope with telescopic magnification. By these means the Zeiss field glasses (fig. 98) could have an increased separation of the axes of the objectives, compared to the distance between the eyes. The increase was very small for binoculars, about 1 3/4 to 2 times, but for relief telescopes (fig. 99) it could be increased to more than 5 times. The hinged connection between the two individual tubes that formed the relief telescope allowed its use with ‘outstretched arms’, as well as with ‘closed arms’. In the first usage it acted as a Helmholtz telestereoscope, and in the second case it was a “Polemoskope” for binocular viewing, and in both cases, interpupillary distances between 58 and 72 mm could be accomodated. (page 197) This idea (double telescopes similar to a magnifying Helmholtz telestereoscope, that folded like scissors, and allowed for interpupillary adjustment while in stretched out position of the tubes, as well as in upright position), was important enough to E. Abbe (5) that he asked for special protection for its realisation. At this time, he used two common terrestrial telescopes which were combined with two flat mirrors in the arrangement originally proposed by Helmholtz. Later on he developed this form into the ‘relief pole telescope’ [Relief-Standfernrohr], an instrument without a hinge and with two magnifications. The equipment used for producing the new prism binoculars enabled the workshop to bring these relief telescopes onto the market in the fall of 1894, and sales were soon satisfactory. Of course Zeiss was not the only producer of these for long. All optical workshops in Germany were busy with these profitable instruments, since, because of the predecessorship of I. Porro, Zeiss could protect only the increased separation of the objectives using Porro Prisms, but not their use for image reversal. Typically, the two individual parts of the Porro system were connected so that the distance between the objectives was the same as the distance to the center of rotation. Variations of this instrument were the basis of a proposal by Ed. Sprenger (1) from early 1895 (Fig 100), using a one piece reversing prism invented by A Leman. Compared to the Porro System, the advantages are fewer glass-air surfaces (two), and the large offset of the optical axis, which leads to a significant increase in depth perception. The disadvantages are the decreased amount of shortening of the tube lengths by the prisms, and the difficulty in fabricating the prism. The Sprenger binocular was not successful, and the patent only existed for 2 1/2 years. (page 198) A better fate was in store for the models by H.L. Huet (1). In an especially well executed model, image reversal was achieved by a Goulier pentaprism as modified by A. Daubrese (1). (* Establishing a correct name was rather difficult. Earlier (Von Rohr 8, p177) I believed it to belong to the capable French genius, officer C.M. Goulier, from the year 1864, which J. de Marre (1, p115) cited in 1880. However, later research seemed to indicate I. Porro’s pentaprism, but that may have been a totally different device, and the lack of pictures here makes it difficult. Thus, we continue naming the prism after C.M. Goulier.) The original, non reversing form (fig. 101) had two silvered surfaces, Daubrese had found by 1897 that one silvered surface could be eliminated if a 90 degree roof was added to one plane (fig. 102). This prism would then have to be paired with a ‘reading prism’ [Ableseprisma - appears to mean a simple 90 degree reflecting prism, ‘pick-off prism’] in the instrument. Many experiments with image reversing prisms were done in Paris in the 1850s and later (pages 90 and 172), as cited by P. Bordes (1) in 1898. It cannot be denied that in the mid 1800s, the optical axis was shifted laterally in a conscious attempt to increase depth perception. (page 199) The Daubresse pentaprism became known in Germany through a publication by C. Hensoldt (1) in December 1897. For a long time, Hensoldt Wetzlar produced the prism double telescopes shown in fig. 103, with increased distance between objective axes. The influence of prism glasses was also felt in the development of the double terrestrial telescope [lens erecting systems]. By the summer of 1897, M. Hensoldt (1), Wetzlar, offered a short, 7 power double terrestrial telescope with 25 mm objectives. It did not last long on the market, but the idea was not completely abandoned, for K. Fritsch (2) introduced in 1898 a double terrestrial telescope with zoom magnification. He produced two forms, the smaller one with 18 mm objectives provided between 5 and 15 times magnification and a field of view of about 8 to 3 degrees. The large model had 38 mm objectives, 12 to 36 power, and a true f.o.v. between 3 and 1 degrees. Both could be adjusted to interocular distances between 55 and 75 mm. The understanding of the means of increasing depth perception was aided by the active response from customers of these new resources. Pages 227-229: Non magnifying Binocular Instruments with Uninterrupted Picture. The older Ewald pseudoscope (page 178) was explored by J.R. Ewald (1) and O. Gross in a paper, where it was described how with exercise one could train oneself to see the false image of those objects in space, although they at first resist this depth reversal. Furthermore, a Holmes stereoscope was transformed by the introduction of a pair of mirrors, so that one could comfortably see a pseudoscopic image from common stereograms. Another device produces a disembodiment of the edge of the framework with comfort and safety (see Oppel, p125). There is an especially charming image of two hanging spheres, to be observed with crossed eyes. Very soon thereafter, M. von Rohr (7) wrote about a pseudoscope (fig. 117) that did not change the interocular distance, which he had developed from the design of Hardie (see p85), without knowing that it was essentially the Ewald design of 1889. [Fig. 117. Pseudoscope that does not change the size of the ‘virtual eye position’, by M. von Rohr (7, p415)]. The laterally correct position of the half images in the Brown mirror instrument (p206), led to Th. Brown’s (2) later use of it, and his proposal to observe the stereogram on a screen with a common stereoscope. To make the observation more comfortable and to erect the half images at the same time, he used a mirror chamber, with a mirror that is inclined less than 45 degrees downwards and to the front. This is a simple version of the photographic stereoscope of Brewster (p103), which at that time still needed his ‘telestereoscope’ [Fernrohrstereoskop]. (page 228) One of the Duboscq stereoscopes was also used for this purpose. J. Bischof in 1879 (p178) did related work..Other instruments in this area were produced in order to look at paintings with both eyes to the best advantage. The work of Th. W. Jones (p145) was renewed at this time. E. Berger (3) published in 1901 a ‘plastoscope’ for the binocular observation of single images. According to a later description, it was an arrangement of collecting and refracting lenses along multiple axes, which were placed in front of each eye and directed towards a single image. No further improvement in depth perception could be attained, as was realized with the Jones instrument; and the criticisms of the English judges (p146) are valid here, as well. The comments of M. von Rohr (7, p417) on these instruments led to the better developed theory behind the Javal ‘Eikonoscope’ of 1866 (p133). Without doubt, the most perfected device would be a ‘synopter’ (fig. 118), where the ‘apparent centers of rotation’ [scheinbaren Drehpunkte] coincide in the object space. Unfortunately, to allow adjustment for interocular distance, too many glass-air surfaces were needed, causing reflections that diminish the effect. (page 229) Because of this and the need for economy of production, the Pinakoscope was preferred in Jena at that time, though it is substantially less perfect in its design. The pair of Riddell mirrors used by the left eye (fig. 119) transposes the ‘apparent center of rotation’ of this eye behind that of the right, in a distance which in the drawing is equal to the distance between axes. Thus, a painting will appear to the left eye in a smaller apparent field than to the right eye. From a distance, the difference in size does not disturb the average observer very much. When observing three dimensional objects, this instrument ‘disembodies’. C. Zeiss (4) had this device protected by patent. Among these devices were the life like dolls advertised as the Tanagra Theater, beginning at an unknown date. The installation of Fr. Salle (1) was patented in France in 1907, but in the form shown in the drawing on page 195, offered nothing essentially new compared to the the Horstmann installation. However, it is of interest as an arrangement where the reflection of the person was reversed by a concave mirror. Thereby the right-left reversal of the doll was eliminated, but at the price of image quality. These presentations entertained a larger audience at this date than was possible for the first revival. During this era, the original inventor successfully protested the Salle patent in Germany, and patent protection was eliminated in Germany. Another device of similar purpose was patented by J.G. Bostock (1) in November of 1910. Here the virtual image created by a large refracting lens served as the life like doll of the earlier models. Consequently there is no mirror reversal and the image quality was undoubtedly better. However, it is impossible to put a three dimensional image into a tangible environment, which was after all the main charm of the old Kircher mirror trick. Pages 229-231: The Double Telescopes. After the great progress in this area, as discussed in the previous chapter, the subsequent era appears somewhat sparse. The requirements of the Army, which had already motivated the introduction of scissor telescopes, put more and more emphasis on the increase of depth perception which led to new and different designs. (page 230) The medium size telescopes included the Hypoplast by C.Zeiss (3) (fig. 120), which was based upon R. Straubel’s design, where the two tubes are positioned at a right angle and tilted outward and upward, which increases depth effects and aids the concealment of the observer. According to A. Koenig, increasing the scale, as with the Hyposkop by C. Zeiss (5), permits observation with any symmetrical position of the two arms. As can be seen in fig. 121 , it is possible to gradually increase the height of the objective axis, up to several meters tall, and [by opening the arms] allow a gradual and very marked increase in the depth effect. On the other hand, a diminishing of the distance between the axes of the objectives occurred in the theater glass Fago, with patent application in the spring of 1902. It was produced in the Goerz shop (1) in November 1906 and offered to the public. (page 231) This dainty and handy form (fig. 122) led to a diminished depth perception, but considering the use, the same explanation can be given as on page 216. This example influenced other German shops and dainty theater glasses with diminished axis distance were produced in great numbers during the following years. Even in travel glasses of relatively weak magnification, the axis distance was occasionally diminished to avoid large size. (fig. 113) Since Zeiss’ patent protection for increased distance between the axes of the objectives ran out in July of 1908, according to the ads in the Central Magazine for Optics and Mechanics, binocular designs with this increase in depth perception became the public property of technical optics. 12