Nobel Lecture by John L. Hall (45 minutes)John L. Hall held his Nobel Lecture December 8, 2005, at Aula Magna, Stockholm University. He was presented by Professor Sune Svanberg, Chairman of the Nobel Committee for Physics. Summary: A historical account of mechanical clockworks is followed by an in-depth survey of methods to measure optical frequencies with very high resolution. The evolution of the experimental programme at JILA, University of Colorado, is described in detail. The re-definition of metre is discussed.   Credits: World Television Sweden AB (production)