  The novel isotope and magnetic effects on two level system echoes in glasses
observed by Nagel et al.(Phys.Rev.Lett. 92, 245511 (2004)) provide unique
experimental insight into the nature of tunneling systems in amorphous solids.
We show their observations are consistent with tunneling systems being extended
systems having of order 100 moving units, as predicted by theory
(Phys.Rev.Lett. 87, 195901 (2001)). The numerous nuclear quadrupole
interactions within a tunneling center explain the magnitude of the magnetic
and frequency dependence observed in deuterated glycerol. The extended nature
of two level systems explains why a small residual effect can still be observed
in regular glycerol even with its low abundance of quadrupolar nuclei.
