  We investigate the escape of Ly$\beta$ from emission nebulae with a
significant population of excited hydrogen atoms in the level $n=2$, rendering
them optically thick in H$\alpha$. The transfer of Ly$\beta$ line photons in
these optically thick regions is complicated by the presence of another
scattering channel leading to re-emission of H$\alpha$, alternating their
identities between Ly$\beta$ and H$\alpha$. In this work, we develop a Monte
Carlo code to simulate the transfer of Ly$\beta$ line photons incorporating the
scattering channel into H$\alpha$. Both H$\alpha$ and Ly$\beta$ lines are
formed through diffusion in frequency space, where a line photon enters the
wing regime after a fairly large number of resonance scatterings with hydrogen
atoms. Various line profiles of H$\alpha$ and Ly$\beta$ emergent from our model
nebulae are presented. It is argued that the electron temperature is a critical
parameter which controls the flux ratio of emergent Ly$\beta$ and H$\alpha$.
Specifically for $T=3 \times 10^4{\rm\ K}$ and H$\alpha$ line center optical
depth $\tau_\alpha=10$, the number flux ratio of emergent Ly$\beta$ and
H$\alpha$ is $\sim 49$ percent, which is quite significant. We propose that the
leaking Ly$\beta$ can be an interesting source for the formation of H$\alpha$
wings observed in many symbiotic stars and active galactic nuclei. Similar
broad H$\alpha$ wings are also expected in Ly$\alpha$ emitting halos found in
the early universe, which can be potentially probed by the {\it James Webb
Telescope} in the future.
