In La Dioptrique (1637) René Descartes elucidated his derivation of the sine law of refraction by means of a comparison of light rays with the motion of tennis balls. In terms of a mathematical-physical model this comparison was quite problematic, as his critics then and now did not hesitate to point out. However, they misread Descartes’ intention, which was to appeal in a clarifying way to the imagination of his readers in order to render his discoveries comprehensible. These readers were in the first place the circle of supporters around Constantijn Huygens. In his comparisons Descartes drew directly on the cultural meanings and the practice of tennis in the urban culture of the Dutch Republic. The tennis metaphor entered Descartes’ writing in this cultural setting, providing him with a clue to understanding the rainbow. Later, however, in the Météores it figured much less prominently than in La Dioptrique.