Hermann grid illusion. The intersections of the white horizontal and vertical lines in this image appear to be grey. But when looked at directly they are clearly white. This is because of the role played by the edge of the retina. Our peripheral vision is designed to prioritise identifying shapes and edges. To this end the brightness of an image on the edge of the retina is diminished when it is not important to shape identification. At the intersections, a lot more of the area is white light compared to elsewhere on the lines, and there is an effect called lateral inhibition. The brightness is dimmed for this area, and we observe it as grey.