  We have observed bottom-charm mesons B_c via the decay mode Bc -> J/psi
lepton neutrino in 1.8 TeV p-bar p collisions using the CDF detector at the
Fermilab Tevatron. A fit of background and signal contributions to the J/psi +
lepton mass distribution yielded 20.4 +6.2 -5.5 events from B_c mesons. A fit
to the same distribution with background alone was rejected at the level of 4.8
standard deviations. We measured the B_c mass to be 6.40 +- 0.39 +- 0.13 GeVc^2
and the B_c lifetime to be tau(B_c) = 0.46 +0.18 -0.16 +- 0.03 ps. We measured
the production cross section times branching ratio for B_c -> J/psi lepton
neutrino relative to that for B+ -> J/psi K to be 0.132 +0.041 -0.037 (stat) +-
0.031 (syst) +0.032 -0.020 (lifetime).
