magnetization vanishes, $\langle \vec{m} \rangle = 0$, and one may also use $$\chi = V\beta  \langle m^2\rangle   \mbox{~~~~for~~} \beta <  \beta_c .$$ Since both expressions should scale at criticality like $L^{\gamma /
\nu}= L^{2-\eta}$ a straight line fit in a double logarithmic plot of $\chi / L^2$ versus $L$ gives an estimate of $\eta$. We preferred to visualize $\eta$ as opposed to $\gamma/\nu$ because $\eta$ is the more sensitive parameter. The results for different choices of $\beta_c$ are shown in Fig. 6. We see that our estimate of $\beta_c$ in (\[eq:16\]) seems to be very reasonable, which is also supported by the fact that it results in the fit with the highest $Q$-value. The fact that the curves with $\beta > \beta_c$ ($\beta < \beta_c$) bend upwards (downwards) is easily understood by recalling that in the low-temperature phase $\chi$ scales with the volume $V = L^3$, while in the high-temperature phase $\chi$ eventually saturates at a finite value for any $L$. Close to criticality, as long as the scaling variable $x \equiv (1-\beta/\beta_c) L^{1/\nu}$ satisfies $|x| \ll 1$ and $L \gg 1$, we expect from FSS that the deviations from $-\eta \log L + c_0$ should be simply given by 