 The principal function of the comparison is to determine whether the factors that produce the colors of the rainbow in water can be found in other media. Descartes decides to examine the production of these colors in a prism (see  Fig. 4).  Prisms are differently shaped than water, produce the colors of the rainbow without any reflections, and with only one refraction. These conditions are rather different than the conditions in which the colors of the rainbow are produced in a flask. Clearly, then, the true cause of the rainbow has not yet been fully determined.