 Descartes next examines what he describes as the “principal difficulty”. Suppose a ray strikes the flask somewhere between K and B, undergoes two refractions and one or two reflections, and upon leaving the flask tends toward the eye at E. Why this ray produces no color, and “only those of which I have spoken […] cause certain colors to appear”, is not clear (AT 6: 329, MOGM: 334). The difficulty here is twofold. First, why is it that only the rays Descartes has identified produce colors? Second, why do these rays produce “certain colors”, i.e.., these colors in this particular order (see Buchwald 2008: 10)? To resolve this difficulty, Descartes 