  We discuss an experimental technique to visualize the motion in the bulk of
superfluid $^4$He by tracking micron-sized solid hydrogen tracers. The behavior
of the tracers is complex since they may be trapped by the quantized vortices
while also interacting with the normal fluid via Stokes drag. We discuss the
mechanism by which tracers may be trapped by quantized vortices as well as the
dependencies on hydrogen volume fraction, temperature, and flow properties. We
apply this technique to study the dynamics of a thermal counterflow. Our
observations serve as a direct confirmation of the two-fluid model as well as a
quantitative test of the normal fluid velocity dependence on the applied heat
flux.
