It remained for Willebrord Snell, a Dutchman, about the year 1621, to discover the law in question, and for Descartes, a little later, to formulate it.
Descartes, indeed, has sometimes been supposed to be the discoverer of the law.

There is reason to believe that he based his generalizations on the experiment of Snell, though he did not openly acknowledge his indebtedness.

The law, as Descartes expressed it, states that the sine of the angle of incidence bears a fixed ratio to the sine of the angle of refraction for any given medium.

Here, then, was another illustration of the fact that almost infinitely varied phenomena may be brought within the scope of a simple law.

Once the law had been expressed, it could be tested and verified with the greatest ease; and, as usual, the discovery being made, it seems surprising that earlier investigators--in particular so sagacious a guesser as Kepler--should have missed it.
Galileo himself must have been to some extent a student of light, since, as we have seen, he made such notable contributions to practical optics through perfecting the telescope; but he seems not to have added anything to the theory of light.