page_title: Geometrical-optical illusions
section_title: Categories of visual illusions
hierarchical_section_title: Geometrical-optical illusions / Categories of visual illusions
caption_attribution_description: English: Geometrical-optical illusions: horizontal/vertical anisotropy
context_page_description: Geometrical-optical illusions are visual illusions, also optical illusions, in which the geometrical properties of what is seen differ from those of the corresponding objects in the visual field.
context_section_description: Visual or Optical Illusions can be categorized according to the nature of the difference between objects and percepts. For example, these can be in brightness or color, called intensive properties of targets, e.g. Mach bands. Or they can be in their location, size, orientation or depth, called extensive.  When an illusion involves properties that fall within the purview of geometry it is geometrical-optical, a term given to it in the first scientific paper devoted to the topic by J.J. Oppel, a German high-school teacher, in 1854.  It was taken up by Wilhelm Wundt, widely regarded as the founder of experimental psychology, and is now universally used. That by 1972 the first edition of Robinson's book devotes 100 closely printed pages and over 180 figures to these illusions attests to their popularity.
caption: Vertical-horizontal Illusion
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