The telestereoscope according to Helmholtz is a lens-free instrument in which the distance between the right and left eye image is enlarged from an average of 6.5 cm to 108 cm in this case by reflection on two pairs of mirrors. The physiologist and physicist Hermann von Helmholtz presented the instrument to the scientific public in 1857 and described its particular benefit as being able to maintain spatial vision even at very great distances. In addition to this general description, he gave a series of experiments that were intended to demonstrate increased depth perception with lens-free set-ups. Helmholtz's results were hotly disputed during his lifetime.

The pictures show the reconstruction of our former Master's student Sebastian Reder, who was able to recreate this apparatus from a simple floor plan drawing. When analysing the results, he came to similarly critical conclusions as Helmholtz himself, but he also came across deviations that may have been caused by physiological factors. Similar to the contemporary discussion of the 19th century, Helmholtz's statements were at times incomprehensible, at others it was only possible to guess what the physiologist might have meant at the time. In various series of measurements at medium and long distances, however, it was possible to verify an important parameter for stereoscopic observations: the length of the baseline. Even with this simple instrument, a spatial resolution up to distances of 3000 metres was easily recognisable.