In a study comparing variances of two normal populations, we use the ratio $F = \frac{s_1^2}{s_2^2}$. If we want to test $H_0: \sigma_1^2 = \sigma_2^2$ against $H_a: \sigma_1^2 \neq \sigma_2^2$ at $\alpha = 0.10$, how do we determine the rejection region?