Abstract

In the second half of the eighteenth century a lively debate was going on in Germany about the nature of light. One important contribution to this discussion, namely a paper by Nicolas Béguelin, is studied in this article. In his essay, Béguelin compared the Newtonian emission theory of light and the wave theory of Leonhard Euler. Whereas others opted for one of the two theories by invoking arguments or authorities, Béguelin made a systematic search for experiments which he hoped would settle the dispute. Two of these experiments were most original. The first, which Béguelin himself performed, concerned light rays grazing a glass surface. For several reasons it did not have the impact it deserved. The second one was a thought experiment which was meant to illustrate a major tenet of the wave theory, that is, the analogy between light and sound. An analysis is given of these two experiments, and it is shown that neither of them brought the debate to an end