The Ewald and Gross pseudoscope from our HistoLab was developed as part of Hannah Runde's Master's thesis. It is a catoptric pseudoscope that uses highly reflective mirrors to ensure that the right and left eye images are reversed. With suitable bodies, an observer subsequently has the impression that the depth dimension has been reversed, meaning that a voluminous body with positive relief appears to have become the corresponding "funnel" (with negative relief) in the plane of observation. Ewald and Gross also dealt with the differentiation between monocular, binocular and stereoscopic perception and discussed the corresponding boundary conditions for a reversal of relief perception.

The stereoscope is a lens-less wooden construction with four mirror surfaces, in which an object can be placed on a screen at the end of the instrument surface, which can then be observed from the other side of the instrument. The bodies used in the historical experiments ultimately showed that both shapes were equally likely to appear to the observer and it seemed to depend heavily on external circumstances which of the two was actually perceived first. Depending on the lighting and the aspect ratio of the body, there may well still be a favoured variant of perception.