  We use a confocal microscope to examine the motion of individual particles in
a dense colloidal suspension. Close to the glass transition, particle motion is
strongly spatially correlated. The correlations decay exponentially with
particle separation, yielding a dynamic length scale of O(2-3 sigma) (in terms
of particle diameter sigma). This length scale grows modestly as the glass
transition is approached. Further, the correlated motion exhibits a strong
spatial dependence on the pair correlation function g(r). Motion within glassy
samples is weakly correlated, but with a larger spatial scale for this
correlation.
