In photographic science, the Eberhard effect, named after Gustav Eberhard, is a special case of two Mackie lines when narrow areas of high and low densities (usually lines approaching within 1 mm. thickness) cause the two Mackie lines to meet which results in an increased density of this small area.
Eberhard published his findings in 1912 or in 1926, 1931.
The Eberhard effect belongs to the family of photographic edge effects.
References
Eberhard effect Wikipedia(Text) CC BY-SA