The Monkey Buddha Archives:
Created by Jacques Ninio, a French researcher in visual perception, this is one of the more striking visual tricks I've seen. The black dots in your peripheral vision disappear as your eyes focus on different areas of the grid.
I first saw it on a Reddit thread, and also found good explanations of the phenomenon:
"Your visual system is built with nerves cells that fire in response to different arrangements of dark and light. You should be able to see a circle in your peripherals, but the criss cross pattern really messes with that ability. It surrounds the dots with information that severely interferes with the visual system's way of signaling "here's a bright edge around a dark circle"
The strength of the grid pattern (at this level of contrast) overpowers the ability of the brain to detect the circles when they are so spaced out. Our mind is basically filling in the blanks with the bold, regular arrangement of lines. When the grid is lightened, the circles become easy to see all at once.
Also for comparison, see this similar pattern, but with the circles in a more tightly arranged, regular spacing: