Case, de Hoffmann and Placzek1proved a theorem stating that given a uniform medium bounded by a surface B, and surrounded by a vacuum, the angular density inside B does not change if (i) the uniform medium is extended to infinity, (ii) neutron sources outside B are removed, the sources inside B being retained, and (iii) a suitable additional surface source is put on the surface B; for applications and modifications of this theorem see Montagnini and Pierpaoli2: Beauwens and Devooght3; Benoist and Kavenky4: Ribaric5§ V.c.21.