37.

In the last equation the quantity on the left side is independent of the angle of incidence $\theta$ and of the particular kind of polarization; hence the same must be true for the right side. Hence, whenever the value of this quantity is known for a single angle of incidence and any definite kind of polarization, this value will remain valid for all angles of incidence and all kinds of polarization. Now in the special case when the rays are polarized at right angles to the plane of incidence and strike the

bounding surface at the angle of polarization, $\rho =0$, and ${\rho }^{\prime }=0$. The expression on the right side of the last equation then becomes $1$; hence it must always be $1$ and we have the general relations: $\rho ={\rho }^{\prime }\mathrm{\backslash Label}[eqn](40)\text{upshape (40)}$ and $q^2{𝖪}_{\nu }=q^{\prime 2}{𝖪}_{\nu }^{\prime }.\mathrm{\backslash Label}[eqn](41)\text{upshape (41)}$