The eighth child (of six sons and five daughters) of Welsh cabinet maker and carpenter, later clerk of works on the estate of the Earl of Eldon at Stowell Park, then builder, Edgar William Cosslett (1871–1948) and Anne (née Williams; 1871–1951),[3][4] he was raised at Cirencester and educated at Cirencester Grammar School, the University of Bristol, the Kaiser-Wilhelm-Institut, Berlin-Dahlem, and University College, London.
He was a research fellow at the University of Bristol after completing his PhD there in 1932, having been awarded an H. H. Wills Memorial Fellowship, remaining there until 1935.
[8] Cosslett was elected Fellow of the Royal Society in 1972[2] and won the Royal Medal in 1979 "In recognition of his outstanding contributions to the design and development of the X-ray microscope, the scanning electron microprobe analyser, the high voltage and ultrahigh resolution (2.5A) electron microscopes and their applications in many disciplines.
[8] and was also instrumental in the creation of International Federation of Societies for Electron Microscopy where he was president from 1970 till 1973.
During the Second World War, Cosslett provided accommodation for refugee scientists at his flat in Hampstead; thus he met Viennese physicist and microscopist Dr Anna Joanna Wischin (1912–1969)[12] whom Cosslett married in 1940 following his divorce from his first wife.