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The Muller-Lyer illusion is not strictly a perspective illusion but nevertheless clearly indicates the confusion that may arise when distance, angles, perspective and converging lines are introduced into a scene or a two-dimensional image. In the diagram below it is quite obvious that the red line on the right-hand side, the apparently more distant one, is longer than that on the left-hand side. It is three panels high as opposed to one, and despite appearing more distant is clearly longer. When the eye is moved from the left-hand line to the right-hand line, it senses the angle opening up to accommodate the longer right-hand line.

Experience tells us that distant objects are not actually smaller than nearer ones. We are therefore inclined to see them as larger than their retinal size - an effect known as size constancy. In the diagram above, perspective leads us to believe that the right-hand line is more distant and hence larger than its retinal size. This is known as misapplied size constancy.

Well, now measure the length of the two red lines!


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