Two-photon production processes at high energy. III

We discuss several two-photon processes in the energy region covered by the new $e^{+}{e}^{-}$ colliding-beam machines at DESY and SLAC. In particular we study the observable cross section for ${\eta }_c$ (and ${\eta }_b$) production using (1) its two-$\gamma$ decay mode and (2) missing-mass techniques. We also discuss the singlephoton production of new heavy leptons in the presence of a two-photon background via a measurement of the visible energy. Other methods for reducing the two-photon contamination are discussed.