v 4.26

$${\delta^\parallel = \pi - 2\arctan \bigg[ \sqrt{\frac{sin^2 \alpha - N_{21}^2}{N_{21}^2 \cos \alpha}} \bigg] \\ \delta^\perp = - 2 \arctan \bigg[ \sqrt{ \frac{\sin^2 \alpha -N_{21}^2}{\cos \alpha }} \bigg ]}$$