 He designs a model that will enable him to acquire more observations about of the behavior of light when it acts on water. Since water is perfectly round, and since the size of the water does not change the appearance of the arc, he fills “a perfectly round and transparent large flask with water” and examines the behavior of light when it acts on the water in the flask. Already at this early stage, delicate considerations of relevance and irrelevance are clearly on display, and these considerations allow Descartes to narrow down and more clearly define the problem. Once he filled the large flask with water, he 