  There are two independent critical exponents that describe the behavior of
systems near their critical point. However, at the critical point only the
exponent $\eta$, which describes the decay of the correlation function, is
usually discussed. We emphasize that there is a second independent exponent
$\eta'$ that describes the decay of the fourth-order correlation function. The
exponent $\eta'$ is related to the exponents determining the behavior of
thermodynamic functions near criticality via a fluctuation-response equation
for the specific heat. We also discuss a scaling law for $\eta'$.
