Detecting Forgery

Suppose you have two absolutely identical drawings, of two equal black squares, for instance. In the stereoscope they appear as one square which is exactly like either of the twin squares. If there is a white dot in the middle of each square, it is bound to show up on the square in the stereoscope. But if you shift the dot on one of the squares slightly off centre, the stereoscope will show one dot, however, it will appear either in front of, or beyond, the square, not on it. The slightest of differences already produces the impression of depth in the stereoscope. This provides a simple method for revealing forgeries. You need only put the suspected bank-bill next to a genuine one in a stereoscope, to detect the forged one, however cunningly made. The slightest discrepancy, even in one teeny-weeny line, will strike the eye at once appearing either in front of, or behind, the banknote. When an object is very far away, more than 450 meters distant, the stereoscopic impression is no longer perceptible. After all, the six centimeters at which our eyes are set apart are nothing compared with such a distance as 450 meters. No wonder buildings, mountains, and landscapes that are far away seen flat. So do the celestial objects all appear to be at the same distance, though, actually, the moon is much closer than the planets, while the planets, in turn, are very much closer than the fixed stars. Naturally, a stereoscopic pair thus photographed will not produce the illusion of relief in the stereoscope