Ray equations referenced to the Z axis

$$\begin{eqnarray} M(\theta) = \frac {M_{z}^{3}C^{6} + M_{z}^2 (2M_{x} + M_{xnmo}... ..._{x}^2 C^{2}S^{4} + M_{x}^3 S^{6}} {(M_{z} C^{2} + M_{x} S^{2})^2}\end{eqnarray}$$ (8)