  The group delay incurred by an optical wave packet depends on its path
length. Therefore, when a wave packet is obliquely incident on a planar
homogeneous slab, the group delay upon traversing it inevitably increases with
the angle of incidence. Here we confirm the existence of isochronous
`space-time' (ST) wave packets: pulsed beams whose spatio-temporal structure
enables them to traverse the layer with a fixed group delay over a wide span of
incident angles. This unique behavior stems from the dependence of the group
velocity of a refracted ST wave packet on its angle of incidence. Isochronous
ST wave packets are observed in slabs of optical materials with indices in the
range from 1.38 to 2.5 for angles up to $50^{\circ}$ away from normal
incidence.
