 Descartes has so far compared the production of the rainbow in two distinct models: the flask and the prism. In both cases, he enumerates variations and invariances in the production of one and the same effect, excludes irrelevant causes, and pinpoints only those that are necessary. Interestingly, the second experiment in particular also raises new problems, problems Descartes could not have been familiar with prior to the experiment, but which do enable him to more concretely define the series of problems he needs to solve in order to determine the cause of the rainbow (see Garber 2001: 101–104 and Buchwald 2008). For example, the colors produced at F and H (see  Fig. 6  below) are different, “even though the refraction, shadow, and light concur there in the same way” (AT 6: 331, MOGM: 336). What causes these colors to differ?