Good Optical Illusion

Attentively fix the cross at the center of the circle during several seconds. Two spectacular phenomena will occur : on the one hand a yellow spot will be formed ; on the other hand, if you do not blink eyes, the blue spots disappear. This illusion illustrates the phenomenon of afterimage. Indeed, when the retina is subjected to saturation during several seconds with a coloured gleam, it is the complementary color which appears at the time of the desaturation of the photosensitive cells. Thus, blue is transformed into yellow. But where the effect lasts normally only a few seconds (because the retina is rehabilitated quickly), here we are surprised to see this yellow spot turning indefinitely. This is explained by the fact that after each phenomenon of retinal persistence on a spot, another blue spot disappear, leaving place to a new illusory yellow. Concerning the progressive disappearance of the blue discs, it seem that the brain interprets the image like a displacement of a single yellow disc drawing aside the presence of the blue discs.