Fix your sight on the white dot located on the center of the red circle for about three to four minutes (you can blink, but don’t look away). Little by little you will see a luminous halo form on the edges of the red circle, but keep your eyes fixed on the central white dot.
After a few minutes, throw your head back and you will see a luminous ring around the red circle. The most curious thing is that this ring is from a greenish blue color that is perceptually different from any other color that your computer screen might display. The range of cyan colors that your PC can display is the one shown in the square.