Caption: "Portrait believed to be of Thomas Harriot (c.1560--1621) (also spelt "Harriott" or "Hariot"), an English astronomer, mathematician, translater, ethnographer, explorer, and Renaissance man apparently painted during his lifetime.'' (Somewhat edited.)
Features:
Galileo's
Moon maps show all these things
too, of course
(see The Galileo Project: The Moon;
file
galileo_moon_map.html).
Actually,
Ibn Sahl (c.940--c.1000)
discovered Snell's law
much earlier
and reported it in his manuscript On Burning Mirrors and Lenses
(984)---but this was
largely unknown until modern
history of science research---maybe
only in 1990
(see Wikipedia: Snell's law: History).
Another actually is that the
effective discovery
of Snell's law
was by Rene Descartes (1596--1650)
in 1637.
It is an effective discovery
because he published it, and so people knew of it.
Snellius
got credit by good luck.