Law of the constancy of interfacial angles
From Online Dictionary of Crystallography
Loi de la constance des angles dièdres (Fr). Gesetz der Winkelkonstanz (Ge). Ley de la constancia de los diedros (Sp). Legge della costanza dell'angolo diedro (It)
The law of the constancy of interfacial angles (or 'first law of crystallography') states that the angles between the crystal faces of a given species are constant, whatever the lateral extension of these faces and the origin of the crystal, and are characteristic of that species. It paved the way for Haüy's law of rational indices.
The law of the constancy of interfacial angles was first observed by the Danish physician Nicolas Stenon (he was the personal doctor of the Grand Duke of Florence) on quartz crystals (De solido intra solidum naturaliter contento, Florence, 1669), and also by Domenico Guglielmini (Riflessioni filosofiche dedotte dalle figure de Sali, Bologna, 1688), but it was generalized and firmly established by Jen-Baptiste Romé de l'Isle (Cristallographie, Paris, 1783) who measured accuretely the interfacial angles of a great variety of crystals, using the goniometer designed by Arnould Carangeot (1783).