This takes patience and the effect may seem too subtle. It is not an optical illusion proper, but it does show that we have a sense to immediately appreciate amounts, or “numerosity”.
What to see
In the experiment on the right there are two large light-grey discs. They are filled with a number of blobs, colored or grey. When the blobs are colored, there are many on the left, few on the right. When the blobs are grey, they are more equally distributed.
What to do
Your task is to look at the central down-counting digit and with your inner eye(!) compare the number of grey dots, whenever they briefly appear. If you feel the number of grey blobs differs between left and right, press one of the two bottom buttons to increase one side (the other decreases; the effect will only be apparent when the grey blobs appear again). Do this for a number of cycles, you may need to press the bottom left button repeatedly. At all times try to steadily fixate the central down-counter. When you're satisfied, press the “stop” button on the top left.
The number of grey dots (now visible above the “increase” buttons) initially is set to 30:30. However, after looking at the display for a number of cycles, and increasing the left grey blob count, for me they appear equal at around 40:20.
There is a rule in psychophysics: “what is adaptable, exists”. Thus this experiment suggests that we actually have specific perception units for “numerosity”. Trivial explanations do not explain the rather strong effect: the position of the dots changes every cycle, thus precluding afterimages, and the effect even generalises across color.
The experiment allows to set the number of blobs for the adaptors and the test. Setting the adaptors to, say, 100:100 cancels any effect. You can also try 20:400 to convince yourself that it's not a trivial left field (or right hemisphere) effect.
If you want to know more: The sources below go into scientific depth. The present experiment was inspired by Burr & Ross’s demonstration, adding the “sawtooth” adaptation technique (Bach & Ulrich 1994) to drive into deep adaptation.
Burr DC, Ross J (2008) A visual sense of number. Current Biology 18:425–442
Ross J, Burr DC (2010) Vision senses number directly. JOV 10(2):10, 1–8