Defined by the equation: \[D_{\theta }=\frac{t_{\theta }}{\frac{\partial (f(\theta ,\mathit{\Phi }))}{\partial \theta }\ \sin\,\theta }\] where \(f\left(\theta ,\mathit{\Phi }\right)\:\sin \,\theta \ \text{d}\theta \ \text{d}\mathit{\Phi }\) is the fraction of particles whose axes make an @A00346@ between \(\theta \) and \(\theta +\mathrm{d}\theta \) with the direction \(\theta =0\), and have an azimuth between \(\mathit{\Phi}\) and \(\mathit{\Phi} + \text{d}\mathit{\Phi}\); \(t_{\theta }\ \mathrm{d}\mathit{\Phi }\) is the fraction of particles having an azimuth between \(\mathit{\Phi}\) and \(\mathit{\Phi} + \text{d}\mathit{\Phi}\) whose axis passes from values \(<\theta \) to values \(>\theta \) in unit time. The axis whose @R05410@ is considered has to be clearly indicated.