Hermann grid illusion
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Hermann grid illusion
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An optical illusion first described
by the German physiologist Ludimar Hermann (1838–1914) in 1870. While
reading a book on sound by the Irish physicist John Tyndall,
Hermann saw gray spots in the intersections of spaces among the figures
that Tyndall had arranged in a matrix. Despite the fact that the same intensity
of light is reflected all the way along the white spaces in the Hermann
grid, the intersections appear gray. To explain this, consider two regions
of the retina. One region views an intersection of a white horizontal and
vertical band, while the other views a white band between two intersections
(the region going away from the intersection). Although the two regions
themselves receive the same amount of light, the situation in their neighboring
regions is different. At the intersection, light comes in from all four
sides, but the white band that lies between the two intersections is surrounded
by two dark sides. This leads to an effect called lateral inhibition,
which causes a bright surround to an area appear darker and, conversely,
a dark surround to an area appear lighter.
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scintillating grid illusion
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A similar but more powerful illusion, known as the Lingelbach illusion
or the scintillating grid illusion, was discovered in 1994
by Elke Lingelbach, the wife of a German mathematics professor, and has
not yet been fully explained.1 Curiously, the effect of the scintillation
is lessened by tilting the head through 45°!
Reference
- Schrauf, M., Lingelbach, B., Lingelbach, E., and Wist, E.R. "The
Hermann Grid and the Scintillation Effect." Perception, 24,
suppl. A: 88-89, 1995.
Related category
• ILLUSIONS
AND IMPOSSIBLE FIGURES
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