  We demonstrate that the Hanle effect can be tuned between magnetically
induced absorption (MIA) and magnetically induced transmission (MIT) simply by
changing the polarization of the input laser beam. The experiments are done
using closed hyperfine transitions of the $ \rm D_2 $ line of ${\rm ^{133}Cs}$
---$ F_g = 3 \rightarrow F_e = 2 $ and $ F_g =4 \rightarrow F_e = 5 $. The
former shows a transformation from MIT to MIA, while the latter shows the
opposite behavior. A qualitative explanation based on optical pumping and
coherences among the magnetic sublevels of the ground state is borne out by a
detailed density-matrix calculation. To increase the coherence time, the
experiments are done in a Cs vapor cell with paraffin coating on the walls. The
observed linewidth is extremely narrow ($\sim 0.1$ mG) compared to previous
work in this area, making this a promising technique for all kinds of precision
measurements.
