The ratio of two Rayleigh ratios for different angles of observation, i.e. \[z(\theta _{1},\, \theta _{2}) = \frac{R(\theta _{1})}{R(\theta _{2})}\] \[\theta _{1} < \theta _{2}\] The angles must be specified; in @L03525@ it is customary to let \(\theta = 1 - \theta _{1}\) and, most frequently, \(\theta _{1} = 45 ^{\circ}\) and \(\theta _{2} = 135 ^{\circ}\).