Mike McCants writes: > I can only push my stopwatch button at a rate of once every 0.2 seconds. > This was a little longer than the separation between the flashes. So > perhaps the separation was about 0.15 seconds. I'm told by Leith Holloway that the "persistence of vision" is about 1/7 of a second. So would the actual period be about .30 seconds? Or do I misunderstand? Cheers. Walter Nissen dk058@cleveland.freenet.edu