Consider a pencil of light rays in a homogenous medium that is perpendicular to some surface. Pass the pencil of rays through an arbitrary amount of reflections and refractions, then let it emerge in some other homogenous medium. The theorem states that the resulting pencil of light rays is still perpendicular to some other surface.
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^W. R. Hamilton, Theory of systems of rays, Part First and Part Second. Part first: Trans. Royal Irish Academy, 15, pp. 69–174. Part Second: manuscript. In Sir William Rowan Hamilton mathematical Works, vol. I, chapter I, Cambridge University Press, London, 1931