Huygens on Descartes Awareness of Snell's Law

  In his Dioptrica, 1703, Huygens indicates that Descartes had come into contact with Snell's manuscript containing the ratio of sines for the angles involved in diffraction. Snell is most likely to have used the particle impulse model to arrive at his results. Huygens states the following,

"Hæc autem omnia, quæ de refractionis inquisitione volumine integro Snellius exposuerat‚ inedita mansere; quae & nos vidimus aliquando, & Cartesium quoque vidisse accepimus, ut hinc fortasse mensuram illam, quae in sinibús consistit, elicuerit; quá in explicanda iride & vitrorum figuris investigandis felicissimè est usus.

Cujusmodi vero sit illa Refractionis in sinubus proportio, cum radius ex aere in aquam, vitrumve, aut alia corpora diaphana desertur, id vel prismate, ut Cartesius praecipit, inquiri potest, vel aliis modis; quos, qui praecedentia intellexerit, non difficulter inveniet."


"All these things, about the refraction question in the work Snell completely explained, but it remained unpublished; what both we've seen for some time and Descartes had seen and heard so here possibly is the measure for it; which he concluded depends on the sines; which in explaining the rainbow of glass figures investigated is most successfully employed.

However with the preceding having been understood, it will not be difficult to discover that the Refraction is in the proportion of sines when a ray has been conveyed from air into water; glass; or any other transparent body; thus or for a prism or in other ways as Descartes has informed us, it is possible to inquire into."