 To solve this problem, Descartes draws on his previous research in Optics and reflects on the nature of the particles whose motions at the micro-mechanical level, beyond what can be observed by the senses, produce visible light. Descartes’ method requires that every phenomenon in nature be reducible to the material simple natures of extension, shape, and motion (see  Section 2.2.1  above). What remains to be determined in this case is “what sort of mixture of simple natures is necessary for producing all the effects” of the rainbow (AT 10: 427, CSM 1: 49), i.e., how the extension, shape, and motion of the particles of light produce the precise order of the colors of the rainbow. The transition from the prism to the micro-mechanical level is naturally prompted by the fact that neither the flask nor the prism can be of any assistance in finding the cause of the order of the colors of the rainbow. The prism only provides conditions in which “the refraction, shadow, and light concur in the same way” and yet produce different colors in different places on FGH. The cause of the color order cannot be disclosed by the mere examination of the models.