gma, it is a bit hit and miss but on this occassion I exposed Pan F at 50 ISO and developed in Rodinal for 5 mins 1:25 @ 20 deg C re-exposing the negative to ambient light halfway through its development time for a second or so before continuing development without agitation. Slower films slightly overdeveloped seem to work best. Give it a go, it is a bit of a lottery though.
Thanks for the detailed explanation. I think that this technique works well only with sheet film developed in a tray. On a reel the additional short second exposure would not be uniform enough for a smooth effect. Correct? Also with sheet film you can risk a single sheet at a time.
My method actually uses roll film rather than sheet film. Using sheet film would be far to expensive, Man Ray must of had very deep pockets, because the failure rate is very high. By shooting 1 roll of film of the same subject I generally get 1 useable image per roll. I must say they can be very tricky to print because the negs are very dense. Recopying the image would help I guess. Do have a go, it's great fun - you never know what you're going to get until you print!