If we draw dashes on the parallel lines we get the impression as if those lines were oblique.

         If we draw two parallel lines in a ‘net’ made up of the lines coming out from one point the parallel lines seem to be bent.

         Squares drawn and graded bring about the impression of being three-dimensional. We can see 3 or 5 cubes, depending on the way we look at the sheet of paper.

         A disc made up of the concentric circles seems to spin when we move the drawing or the head.

         These two segments seems to be of different lenght, although they are really the same.

         Analogically the illusion of perspective makes us think the figure at the back is bigger than the figure at the front. A ruler or a pair of compasses would prove something else.

         These illusions are not caused by any eyesight defect. They are the result of our tendency to refer the features of the whole picture to its elements. Thus we could also say they result from the way our brain functions.