DOI 10.1393/qsf/i2004-10005-8 QUADERNI DI STORIA DELLA FISICA N. 13 - 2005 Historical notes on the first viewpoint about light diffraction S. Ganci ( ∗ ) Museo di Fisica e Meteo-sismologia “G. Sanguineti - G. Leonardini” Piazza N. S. dell’Orto, 8, 16043 Chiavari (GE ) 1. A general historical survey In the historical development of the theories on light diffraction two differ- ent approaches to the problem are found. A first viewpoint considers the diffrac- tion as an edge effect near or on the line boundary of the aperture; this ap- proach is usually attributed to Thomas Young [1] (in the context of the wave the- ory of light). Probably, it is less known that before T. Young the basic idea of considering diffraction as an edge effect in reducing the diffraction problem to a refraction problem is not at all new and can be found in M. De Mairan and M. Du Tour [2, 3]. This reduction, in the context of the emissive theory, appears as a necessary requirement of agreement with Newtnon’s viewpoints [4]. The ba- sic statement of all Young’s papers is the assumption of the periodic nature of the light and the interference principle which is reformulated with slight differ- ences through all papers [1, 5–7]: dif- fraction was considered as the interfer- ence between a wave originating at the edge and the unperturbed transmitted ( * ) E-mail: salvatore ganci@vodafone.it wave [1]. In Young’s papers [1,5–7] the mechanism of rising of the edge wave is unclear; at first it happens by reflection on the edge and superposition to the “un- perturbed” light [8], afterwards by refrac- tion through an ether atmosphere having a density gradient near the edge itself [1]. Figures 1 (sub fig. 1 and fig. 4) and 2 (sub fig. 29 and 31) illustrate the modelling adopted in diffraction by T. Young and M. De Mairan. In Young’s paper of 1802 [5] diffraction at a hair and others fibres is referred to as the “. . . interference of two portions of light, one reflected from the fibre, the other bending round its op- posite side (?), and in the last coinciding nearly in direction with the former por- tion ; ...”. The mathematical attempt in ref. [1] is totally omitted in the re- publication of this paper [7] and also in the German (free) translation of it [9], the ether distribution hypothesis is not at all mentioned in that paper [5], it is ex- plicitly rejected in the paper of 1804 [6] as pointed out by G. N. Cantor [10, 11] and reproposed in an ambiguous form in the Lectures [7] thus leaving diffraction without any theoretical support. Still re- ferring to diffraction, in the paper of 1804 [6], the fringes in the shadow of a rectilin-