(René Descartes’ diagram describing the arc of the rainbow)

Along with the Meditations, the most commonly read work by René Descartes is his Discourse on the Method, which contains the famous assertion: “I think, therefore I am.” In the Discourse, Descartes laid out his rules that, he claimed, could be used to solve any problem that the mind might encounter. This was quite a large claim, but Descartes went a long way toward proving it in the essays that followed. (Originally, Discourse was only meant as an introduction to these technical works.) In one of these essays, “The Geometry,” Descartes solved the “Pappus Locus Problem,” a complicated geometrical puzzle that was first formulated thousands of years before Descartes’ birth, and never solved before he applied himself to it. (Incidentally, it was in solving this problem that the great Enlightenment philosopher invented “Cartesian coordinates.”) In another essay, “The Optics,” Descartes discovered the Law of Refraction. And in the final essay, “The Meteors” (what we would now call “meteorology”), Descartes used the new Law of Refraction to make an important discovery about the rainbow.

As today’s entry in the Devotional points out, you can never stand under a rainbow, because it is an optical illusion caused by sunlight passing through raindrops. Descartes used his discoveries in optics to make this point more precisely. As he discovered, the angle at which light passes through the edge of a rainbow at its center – or its “angular radius” – is exactly 42 degrees. As impressive as the Discourse itself was, the fact that Descartes could use its rules to make three major discoveries is far more impressive still.

Read Descartes’ Discourse on the Method here.

For more on Descartes, read the entry for Week 15, Day 6.

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