In 1672, inspired by the wave theory of Ignace Gaston Pardies, Christiaan Huygens made his first attempt to explain the sine law of refraction, but in 1673 he abandoned his plans owing to difficulties concerning double refraction. Huygens was able to explain double refraction on 6 August 1677 after his discoveries of the axis of symmetry of the crystal and of ‘Huygens's principle’. On 6 August 1679, he wrote: ‘I have found the confirmation of my theory of light and of (...) refraction’, and then described an ‘experimentum crucis’ to decide between his theory and a rival hypothesis. A study of caustics may have led Huygens to the discovery of ‘Huygens's principle’. (shrink)