Systematic temperature-dependent optical studies of optimally doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (OP Bi2212) single crystals have been carried out. An electron-boson spectral function peaked at 43 meV is derived in the normal state at 100 K, indicating that the most significant spectral feature in OP Bi2212 is a resonance at 43 meV. The optical data below ${T}_{c}$ show that this boson is directly involved in the pair formation in OP Bi2212. Origins of the 43 meV mode are discussed.