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