  The complex optical potential and inelastic form factor given by the folding
model for the $\alpha+^{12}$C scattering are used to calculate the
($\alpha,\alpha'$) cross sections for the known isoscalar states of $^{12}$C in
an elaborate coupled channel approach. The strengths of the form factors for
these states are then fine tuned against the ($\alpha,\alpha'$) data to deduce
the corresponding $E\lambda$ transition strengths. A significant $E2$
transition strength has been obtained for the $2^+_2$ state from the present
analysis of the ($\alpha,\alpha'$) data measured at $E_\alpha=240$ and 386 MeV.
The $E\lambda$ transition strengths of the $0^+_2,\ 3^-_1,\ 0^+_3,$ and $1^-_1$
states were also carefully deduced, and some difference from the results of
earlier analyses has been found and qualitatively understood.
