"
In the River of Consciousness" is a fascinating essay by physician and author Oliver Sacks--really one of those extended book reviews--at
The New York Review of Books' website. Below is a small image of the Necker cube illusion that Sacks mentions.
A larger, curiouser version of the Necker cube (as an animated Java applet) can be found
here, and a bit more on the cube's history and mathematical nature
here.
