A extract from a letter by Hamilton,
Remarques de M. Hamilton, Directeur de l'Observatoire de Dublin, sur un Mémoire de M. Plana inséré dans le Tome VII de la Correspondance Math. (Extrait d'une Lettre).appeared in the Correspondance Mathématique et Physique, edited by Quetelet, (tome 8 (1834), pp. 27-30).
This letter is available in the following formats:
In this letter, Hamilton responds to an assertion of Plana, of Turin, disputing the theorem that light rays, in an isotropic medium, that undergo successive reflections are orthogonal to a series of surfaces. Hamilton explains how this theorem could be deduced from Plana's analysis, and outlines how the validity of this theorem follows from his own approach to the study of optics.