  In a recent letter (PRL 79, 2117 (1997)), Bobroff et al. presented novel
$^{17}$O NMR measurements for YBa_2(Cu_{1-x}Ni_x)_3O_{6+y} which offer a new
probe of the momentum dependence of the static spin susceptibility $\chi'(q)$.
They analyzed the $^{17}$O line width by assuming a Gaussian form for $\chi'$,
and came to the conclusion that the correlation length $\xi$ is independent of
temperature $T$. Furthermore, they claim that a Lorentzian form of $\chi'$
yields the same result. We will demonstrate that the Lorentzian and Gaussian
form of $\chi'$ actually yield qualitatively different results for the $^{17}$O
line width and its dependence on $\xi$. We show that their data when combined
with $T_{2G}$ data from Takigawa require that $\xi$ be T-dependent.
Furthermore, assuming a Gaussian form of $\chi'$ leads to the unphysical result
that $\xi$ increases with $T$. In contrast, the Lorentzian form of $\chi'$
yields that $\xi$ decreases with increasing $T$ and is thus fully compatible
with the experimental results.
