The Wundt illusion is a distortion illusion devised by the German 'father of experimental psychology,' and one-time assistant to Hermann von Helmholtz at Heidelberg, Wilhelm Wundt (1832–1920). In the figure, the two horizontal lines are both straight, but they look as if they bow in at the middle. The distortion is induced by the crooked lines on the background, as in Orbison's illusion. The simplest of all distortion illusions – the horizontal-vertical illusion (in which a vertical line looks longer than a horizontal line of equal length) – was also discovered by Wundt.