Graham 1965: Illusions are effects of contrasting experiences.
Da Pos 1995: An experience which is conflicting with other experiences, usually with remembered experiences. Only one experience is not enough to be considered an illusion because it does not produce a perceptive mismatch. This mismatch, which is a consequence of two contrasting experiences, is according to me the main factor attracting our curiosity and exciting our aesthetic pleasure.
Colin y Blakemore 1973: A visual illusion is a discrepancy between two independent detector systems in the brain.
Plate1
Plate2
The above images (plate1) shows that the hue of two colors (X&Y) when is overlapped by another transparent hue it changes its dimension (depth of field). While overlapping area X it tends to recede back and on the area Y it comes forward. Physically it is the same hue that is covering X & Y but optically it changes the dimension. In Plate2 when the same image is separated it shows different dimensions.
