This is the last of four reports on Robert Hofstadter’s post-Nobel project: construction and operation of total absorption detectors for gamma ray spectroscopy. The earlier reports were given in 1968, 1971 and 1973. The title of this talk, “The Crystal Ball Experiment” refers to experiments performed with a detector in the shape of a ball of crystals. The “ball” is really an icosahedron consisting of more than 700 individual crystal modules enclosing a collision chamber connected to two beam pipes through which the colliding particles enter. The crystals are looked at from the outside by photomultipliers, connected to each other so that gamma rays emanating from the collisions can be detected in coincidence. During the decade since the 1973 lecture, one can say that his project had been a total success. As Hofstadter remarks, the number of physicists working on the Crystal Ball project at the very beginning was 32. It had increased to as many as 80 at the time of the lecture. The Crystal Ball was first set up at the SPEAR colliding beam facility at SLAC, where it in particular looked at transitions between energy levels of charmonium, the quark-antiquark system discovered in 1974 by the 1976 Nobel Laureates B. Richter and S.C.C. Ting. In 1968, Hofstadter used the blackboard for pedagogical explanations, but this time he has so many interesting results that the lecture becomes a slide show and his “next slide please” is repeated N times, where N is a large number. At the end of his lecture he shows pictures from the transport of the Crystal Ball detector from SLAC to the DESY laboratory in Hamburg, Germany, where it was installed at the DORIS colliding beam facility. There both intensity and energy of the beams were larger than at SPEAR and the plan was to study gamma rays from transitions in systems containing the bottom quark. As far as I am aware, the Crystal Ball is still used, but now (2012) it has moved to Mainz. Anders Bárány