The Museum also has on display a pocket stereoscopic viewer quite similar to toy viewers, which accompanies a Hilger x-ray spectrograph with twenty-four pairs of photographs for the stereoscopic observation of models of crystals.
This exemplar, modern stereoscopic viewers in general and toys are versions, more or less economical, of the stereoscope based on the one derived from Brewster's in 1866 by Hermann Ludwig Ferdinand von Helmoltz (1821-1894).
The one presented here is black-enamelled plate metal and consists of a rectangular platform (width 12.3 cm, depth 9 cm) at the ends of which are two hinged screens with springs that hold them vertical while in use.
One of the two screens acts as a support for the pair of photographs which are printed side by side on the same card; this is inserted from above between the screen and a frame that delimits and separates the two images, The other screen has at the top two converging lenses placed with their centres at the average distance of 6.5 cm between the irises; below, at the centre, there is an opening for the nose so as to be able to see the two images from the most suitable position, one with the right eye and the other with the left.
When the viewer is not in use the two screens can be folded and, by pressing lightly they can be joined together to make the device thinner (about 0.5 cm).


Bibliografia:
Daguin (1879), T. IV, p. 394
Falk - Brill - Stork (1986), p. 212
Pouillet (1856), T. II, pp. 224-228


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