So I had a line of Geiger counters about the cloud chamber, so that the expansion process was initiated only with a single particle going through the Geiger counters and entering the chamber. The mechanism was marvellous. It had been devised by Blackett and as soon as the expansion took place, then this mechanism would clear the chamber and make it ready again for the next expansion. I had the recording was done on an endless strip of film, which I wound round a great frame and then went to the dark room and impatiently looked at it before it was fixed, to see if there were any particles on it. I then in this basement room in the... what was then the new physics building, measured the... I projected the results both of the incoming particle and the outgoing cascade of electrons onto a screen, measuring the curvature and thereby derived the energy from which a single particle would... of the single particle and of the energy of the emerging electrons and positrons. My paper on that, I think, was published on... I think it was called Shower Production by Penetrating... by Energetic Penetrating Particles, and I doubt, I think, other things had happened and I was occupied with something entirely different when that paper was published, I think, in 1940.