Red notebook labelled on cover 'Phos.' Contains numerous photographs of alpha-particles, probably for work in collaboration with E.P. Hudson on 'The elasticity of the collision of alpha-particles with hydrogen nucleii'. A 2pp. manuscript letter from Blackett to hudson, 13 April 1926, is loosely inserted at the back of the book. The notebook also contains 2 loose pages of notes on 'New linear radiation formula', and 3 sets of photographs of cosmic radiation. On the verso of one set is a Leningrad address, indicating that perhaps Blackett was giving these photographs by a colleague. 1926-1927