Vasco Ronchi (1927) "Due nuovi metodi per lo studio delle superficie e dei sistemi ottici" (Two new methods for the study of surfaces and optical systems), Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Series 1, vol. 15, pages 1-50. [Note: This citation is somewhat misleading. Ronchi first published this article in 1923, but it was included in volume 15 of Annali della Scuola..., which wasn't issued until 1927.] Available on-line (in Italian): http://archive.numdam.org/ARCHIVE/ASNSP/ASNSP_1927_1_15_/ASNSP_1927_1_15__A1_0/ASNSP_1927_1_15__A1_0.pdf .