 \duck\whale\queen-bee\giraffe \xi ^ 2 = \left ( \frac { \varepsilon _ 1 - \varepsilon _ 2 } { \varepsilon _ 1 + \varepsilon _ 2 } \right ) ^ 2 = \left ( \frac { \mu _ 1 - \mu _ 2 } { \mu _ 1 + \mu _ 2 } \right ) ^ 2 ,