 Descartes’ discovery of the law of refraction is arguably one of his most celebrated scientific achievements. We have already encountered the law of refraction in Descartes’ discussion of the anaclastic line in Rule 8 (see  Section 3).  There, the law of refraction appears as the solution to the penultimate problem, “What is the relation (ratio) between the angle of incidence and the angle of refraction?” We also learned that the law of refraction depends on two other problems, “What is a natural power?” and “What is the action of light?” In Rules, Descartes proposes solving the problem of what a natural power is by means of intuition, and he recommends solving the problem of what the action of light consists in by means of deduction or by means of an analogy with other, more familiar natural powers. In Rule 9, analogizes the action of light to the motion of a stick. Similarly, in Optics II, Descartes deduces the law of refraction from analogies (or comparisons) and suppositions about the reflection and refraction of light. As in Rule 9, the first comparison analogizes the action of light to the transmission of motion from one end of a stick to another, and is meant to illustrate how light travels instantaneously from one part of space to another: