A field emission projection microscope was constructed to investigate the atomic and chemical-bonding structure of molecules using electron in-line holography. Fringes of carbon nanotube images were found to be interferograms equivalent to those created by the electron biprism in conventional electron microscopy. By exploiting carbon nanotubes as the filament of the electron biprism, we measured the transverse coherence length of the electron beam from tungsten field emitters. The measurements revealed that a partially coherent electron-beam was emitted from a finite area.