First let me state that the spiral is animated and moving by design. The spiral is not an optical illusion. That said let's see how we can transfer the animation from the moving spiral to the fish scene. Focus on the spiral center exclusively for at least 60 seconds, then shift your focus to the fish. It's amazing how the fish actually float in the ocean currents.
The original instructions were "Stare at the center of the spiral for about a minute. Then look at the back of your hand.It will appear to be twitching and moving."
I found this illusion on several sites on the internet. One semi-popular link farm spent 6 pages of comments discussing this optical illusion. Many were confused about what this optical illusion was all about so let's start off with the following. (If all goes well you might see something like the small image to the right).
Pachinko illusion
Hold on to your seats here is yet another illusion that has to be prefixed with the statement that this is not an animated file. If this looks like it is in motion to you (as it does to me) then you might be suffering from an Optical Illusion
The picture below is another illusion that appears to be in motion. I want to point people to the caution at the top of this page. Please read and understand this caution prior to moving forward. Remember, If it looks like it is in motion to you then you might be suffering from an Optical Illusion.
The optical illusion picture below is not animated. I find that starting off by saying that helps because some optical effects do use an animated graphic to emphasize the after effects of the motion. This image though appears to move on its own with only your sight and brain providing with the power of motion. Below we see a rippling effect.