  An attempt is made to conceptualize the derivation as well as to facilitate
the computation of Ohtsuki's rational invariants $\lambda_n$ of integral
homology 3-spheres extracted from Reshetikhin-Turaev SU(2) quantum invariants.
Several interesting consequences will follow from our computation of
$\lambda_2$. One of them says that $\lambda_2$ is always an integer divisible
by 3. It seems interesting to compare this result with the fact shown by
Murakami that $\lambda_{1}$ is 6 times the Casson invariant. Other consequences
include some general criteria for distinguishing homology 3-spheres obtained
from surgery on knots by using the Jones polynomial.
