In an effort to study the way in which particles interact within a granular material (such as sand, salt, or sugar), Robert Behringer at Duke University uses a collection of disks which are set in motion around a rotating table. The inter-disk stresses can be imaged since the stress subtly changes the polarization of light reflected from the disks. The black bars in the figure are superimposed in order to indicate the disks' orientation. See also http://www.phy.duke.edu/~bob. This research is reported by Robert Behringer at the 1998 meeting of the APS Division of Fluid Dynamics.