Back ionisation from porous layers of tissue paper on the plane of a point-to-plane corona discharge has been studied by observing light emission from the layer and from the point and by detecting the associated current pulses in the external circuit. A quasi-continuous low level light emission was detected from all paper surfaces. It is proposed that this glow emanates from a discharge which is the source of a steady current of positive ions which is responsible for the increase in Trichel pulse frequency; this in turn accounts for nearly all the increase in the current in the external circuit during back ionisation. Three types of pulsed light emission were also detected. One of these was identified as a space streamer which originated from pinholes in the layer and traversed the point-to-plane gap. This phenomenon was observed at relatively low resistivities and current densities and might account for the low breakdown voltage when any porous layer is present on the plane. Preliminary measurements with dried fly ash on the plane showed that at high current densities the space streamer could become sufficiently frequent to provide a significant contribution to the external current.