The intensely bright doublets sometimes observed in field emission microscopes vary in size, shape, intensity, and resolution. It is proposed that these doublets are owing to simple small molecules adsorbed on tungsten protrusions or ridges on the tungsten tip covered with a chemisorbed layer of residual gas. The variation in the doublets is ascribed to variation in the size and shape of the protrusions as predicted by a theory developed by D. J. Rose.