Three-dimensional structure data-base searches

A probe assembly is disclosed carrying a grid grating and an index grating adapted to generate a Moire pattern upon relative movement in either or both of two orthogonal directions in a common plane. The index and grid gratings are supported for precise orthogonal motion with respect to each other by means of a pair of grating support assemblies mounted so that the correct index and grid grating spacing and orientation are precisely maintained, including a cross-slide assembly provided to allow movement in a plane while maintaining the proper orientation between the grating assemblies.