”The first setup augmenting imaging data registered to an object was described in 1938 by the Austrian mathematician Steinhaus. He described the geometric layout to reveal a bullet inside a patient with a pointer that is visually overlaid on the invisible bullet. This overlay was aligned by construction from any point of view and its registration works without any computation. However, the registration procedure is cumbersome and it has to be repeated for each patient. The setup involves two cathodes that emit X-rays projecting the bullet on a fluoroscopic screen (see Fig. 2). On the other side of the X-ray screen, two spheres are placed symmetrically to the X-ray cathodes. A third sphere is fixed on the crossing of the lines between the two spheres and the two projections of the bullet on the screen. The third sphere represents the bullet. Replacing the screen with a semi-transparent mirror and watching the object through the mirror, the third sphere is overlaid exactly on top of the bullet from any point of view. This is possible because the third sphere is at the location to which the bullet is mirrored. Therefore, the setup yields stereoscopic depth impression. The overlay is restricted to a single point and the system has to be manually calibrated for each augmentation with the support of an X-ray image with two X-ray sources.”