 In the prism model, the rays emanating from the sun at ABC “cross MN at right angles, or nearly so, so that they do not undergo any noticeable refraction there”, but “suffer a fairly great refraction in coming out through NP” (AT 6: 329–330, MOGM: 335). Furthermore, it is only when the two sides of the bottom of the prism NP are covered by a dark body of some sort, so that the rays could only exit through the narrow opening at DE, that the rays paint all the colors of the rainbow on the cloth or white paper FGH, always producing red at F, and blue or violet at H (ibid.). By comparing differences between the flask and the prism, Descartes learns “that the surfaces of the drops of water need not be curved in order to produce these colors, for those of this crystal are completely flat”. He also learns that “the angle under which they appear need not be any particular size, for it can be changed here without their changing” (ibid.). Here, Descartes is referring to the angle of refraction (e.g., HEP), which can vary between the flask and the prism and yet produce the same effect, and which can also be the same for rays ABC in the prism at DE and yet produce different colors at FGH. He further learns that 