\[h_{\eta}^{2}=\left(\frac{\partial x}{\partial\eta}\right)^{2}+\left(\frac{%
\partial y}{\partial\eta}\right)^{2}+\left(\frac{\partial z}{\partial\eta}%
\right)^{2}=\frac{c^{2}(\xi^{2}-\eta^{2})}{1-\eta^{2}},\]