Disappearing Illusions happen usually due to focus on some specific area while the surrounding colors trick the human eye into seeing something which is not there.
You may enlarge each picture to verify that what you see is really just an illusion.
Hermann Grid Illusion
In the following Hermann grid illusion the gray spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround. Take a closer look - do you see any grey spots where the black lines intersect? In fact, there is nothing.
Disappearing Yellow Dots
If you stare at one yellow dot for a while, the other yellow dots disappear.
Stare at the black dot in the center and in a few moments the grey area surrounding the dot will disappear.
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