Figure 4.

A stereogram where one eye's image is replaced with its photographic negative, redrawn from Helmholtz⁴⁷ (Plate IV, Figure Q). When these images are fused divergently, the central pentagon should appear in front, in accordance with the disparity of the lines, whereas the contrast mismatch produces the impression of a ‘crystal … of some dark shining substance like graphite'.⁴⁷