Folding the Optical Path

How could a terrestrial telescope be made more compact than comparable conventional designs without sacrificing useful magnification or an upright image? In 1857, drawing on Porro’s prism system of 1854 [1], the Paris-based optical instrument maker J. G. Hofmann patented a solution (Figure 1): the lunette-Hofmann bi-prismatique sans tirage, literally the Hofmann bi-prismatic telescope without draw tubes [2]. As the name suggests, instead of relying on the long series of draw tubes commonly found in portable terrestrial telescopes, Hofmann folded the optical path inside the instrument by means of a prism system.

Hofmann’s letterhead and commercial documentation made clear that the telescope was not merely a patented proposal: it had been manufactured and offered for sale [3]. But did any example survive that could still be examined? The search prompted by this question led to a specimen preserved at the Deutsches Museum in Munich (Figure 2) under inventory number 27871 [4].

Examining the Hofmann Telescope

At first sight, the instrument is quite compact. When assembled, it measures approximately 350 mm in length and consists of three cylindrical sections (the eyepiece tube, the adjacent tube and the objective tube) arranged around a broader central body. The Munich device is mounted on a small folding brass tripod, which allows the telescope to be tilted and rotated. Yet compact does not mean lightweight: the complete instrument weighs approximately 1.4 kg, including the tripod, and around 0.9 kg without it. Its substantial metal construction combines portability with the mechanical stability needed to preserve the alignment of the optical system.

When disassembled, the device shows the key elements of its design. Light entering through the objective tube does not travel directly along a single straight tube towards the eyepiece—whose position can be adjusted for focusing by sliding the eyepiece tube backwards and forwards. Instead, it is redirected within the body by an arrangement of two prisms (Figure 3), so that the effective optical path is longer than the instrument’s total length.

Comparison with three other surviving Hofmann telescopes—two from private collections and one preserved at the Natural History Museum of the Accademia dei Fisiocritici in Siena—provided a broader basis for interpreting the Munich specimen. In particular, one example, whose interior was more accessible, helped confirm that the concealed prism arrangement in the Munich instrument closely corresponds to Hofmann’s patented design. More generally, despite some differences in dimensions and external finish, the four instruments share substantially the same optical and mechanical arrangement. Taken together, these similarities and variations suggest that Hofmann was producing a recognisable family of telescopes based on a common design, with some variation in workshop execution between individual examples.

Moreover, the objective consists of an achromatic doublet, designed to reduce chromatic aberration and minimise coloured halos in the field of view. The telescope has an objective aperture of about 45 mm and an estimated magnification of approximately 30.

Comparison with the patent drawing also reveals differences between Hofmann’s proposed design and the surviving instrument. In the patent, the eyepiece tube appears to be extended by means of a helical mechanism. By contrast, in the Munich specimen, as in two other surviving examples, it is adjusted simply by sliding the tube backwards and forwards.

The Munich telescope also preserves evidence of Hofmann’s manufacturing and commercial activity. The external inscription identifies his Parisian workshop, while the number “67” suggests that the instrument probably formed part of a numbered production rather than being an isolated prototype (Figure 4 and Figure 5).

Hofmann advertised this model for 100 francs and promoted it particularly to sailors and soldiers, for whom compactness and rapid deployment would have been valuable [5].

Unfolding the History of the Munich Specimen

The examination of the Munich specimen also makes it possible to reconstruct part of its trajectory. Its worn leather covering and surface abrasions indicate that it was handled during its working life. However, no document has yet been found that identifies either its actual users or the person who originally purchased it from Hofmann’s workshop.

The final stage of its journey is much better documented. Six letters preserved at the Deutsches Museum record negotiations conducted between January and February 1911 with the Viennese instrument maker and dealer Carl Nic. Richter [6]. Richter offered the museum a group of scientific instruments, including the Hofmann telescope. The museum selected three objects from the shipment and purchased them together for 150 Marks. At the beginning of February 1911, the telescope arrived in Munich and passed from commercial circulation into the Deutsches Museum’s collection.

The patent, the surviving instruments and the archival correspondence therefore illuminate different aspects and stages of the same history. Taken together, they show not only how Hofmann folded light to make a terrestrial telescope more compact, but also how a patented design was translated into workshop practice, realised in individual examples with some variation, and circulated as a commercial instrument. At the same time, the Munich specimen preserves traces of its own individual trajectory: from its manufacture in Hofmann’s workshop and apparent working life to its acquisition in 1911 and preservation by the Deutsches Museum as a material witness to nineteenth-century optical instrument making.

References

[1] Ignace Porro, ‘Perfectionnements apportés aux instruments d’optique et d’astronomie’, 1854, Archives historiques de l’INPI, Patent number: 21192 (Patent type: 15-year invention patent. Filing date: 24 October 1854. Patents sealed: 10 December 1854).

[2] Jean Georges Hofmann, ‘Perfectionnements apportés dans la construction des lunettes’, 1857, Archives historiques de l’INPI, Patent number: 31856 (Patent type: 15-year invention patent, Filing date: 24 April 1857, Patents sealed: 17 June 1857).

[3] Luisa Lovisetti, ‘Le conte de J.G. Hofmann – The Tale of J.G. Hofmann’, Bulletin of the Scientific Instrument Society 165 (2025), pp. 16-27. 

[4] Jean Georges Hofmann, ‘Porro’s telescope with tripod’, c. 1860, Digital Archives of the Deutsches Museum, Inventory No. 27871.

[5] Jean Georges Hofmann, ‘Annonce publicitaire’, Le moniteur de l’armée (1 July, 11 July and 21 July 1859).

[6] Carl Nic. Richter, ‘Correspondence between Carl Nic. Richter and the Administration of the Deutsches Museum’, 1911, Deutsches Museum Archiv, Verwaltungsarchiv, VA 1105/6, Astronomie. Korrespondenz. O–Z, 1911.