  Based on a newly advanced phenomenological understanding of the high-field
insulator- Hall liquid transition in a composite fermion picture, we extend its
composite boson counterpart to the analysis of the low-field insulator-Hall
liquid transition. We thus achieve a comparative study of these two
transitions. In this way, the similar reflection symmetries in filling factors
in both transitions are understood consistently as due to the symmetry of the
gapful excitations which dominate $\sigma_{xx}$ across the transitions, and the
abrupt change in $\sigma_{xy}$ at the transitions. The substantially different
characteristic energy scales involved in these two transitions can be
attributed to the differences in critical filling factors $\nu_c$ and the
effective masses. The opposite temperature-dependences of the critical
longitudinal resistivities are also well-understood, which can be traced to the
opposite statistical natures of the composite fermion and the composite boson.
We also give a tentative discussion of the zero-temperature dissipative
conductivity. The above results are supported by a recent experiment
(cond-mat/9708239).
