image credit: Jason McNeill; image source; larger image
The bright spots in these images are nanoparticles, consisting of about 20 polymer molecules, which fluoresce--give off light of a different frequency--when illuminated with laser light. The image on the right shows these nanoparticles implanted in macrophages (part of the vertebrate immune system), where their fluorescence will be used to track their movements in the cell.
For the track of a single molecule, see this micrograph by Christoph Naumann.
Einstein on Brownian Motion
Einstein's 1905 explanation of Brownian motion settled the issue of the existence of atoms and molecules, as described in David Cassiday's essay.
(This feature was updated on October 15, 2011.)