John (‘Jack’) Mitchell was a New Zealander who came to Oxford University on a scholarship of the Commission of the 1851 Exhibition. Mitchell’s association with Nevill (later Sir Nevill) Mott FRS during World War II, when they both worked at the Armament Research Department, afterwards led Mitchell to join Mott at Bristol University, where he began the research into the photographic process for which he is best known. His pursuit of an understanding of the mechanism through which a latent image forms led to the important discovery of the decoration by silver particles of individual dislocations in silver halide crystals and the mosaic microstructure of them. In turn the decoration technique provided the first clear experimental evidence of the link between plastic deformation and the creation and movement of dislocations. In 1960 Mitchell was appointed as professor in the Physics Department at the University of Virginia, where, apart from a brief spell in England as the Director of the National Chemical Laboratory, he worked happily for some 40 years. During this time his research group published many papers describing and explaining the mechanisms of plastic deformation in metallic alloys, devising and using state-of-the-art methods, thereby adding much to a wider understanding of mechanical properties and strength. All the time, working alone, Mitchell further developed and refined his photoaggregation theory of the photographic process, gaining worldwide recognition and honours for his effort.
- This publication is © 2011 The Royal Society