Sent from a friend of mine, this Java applet starts with a view of the Milky Way at 10 million light years from the Earth and then moves through space towards the Earth in successive orders of magnitude until you reach a tall oak tree just outside the buildings of the National High Magnetic Field Laboratory in Tallahassee, Florida. After that, you begin to move from the actual size of a leaf into a microscopic world that reveals leaf cell walls, the cell nucleus, chromatin, DNA and finally, into the subatomic universe of electrons and protons.
I guess they have a Windows screen saver available as well.
