 How did Descartes arrive at this particular finding? By intervening directly in the model in order to exclude factors irrelevant to the production of the effect (the bright red at D) and cleanly isolate the cause that alone produces it. Whenever he “put an opaque or dark body in some place on the lines AB, BC, CD, or DE, this red color would disappear”, but whenever he “covered the whole ball except for the points B and D, and put dark bodies everywhere else”, then the red color would appear at D. Similarly, in the case of K, he discovered that the ray that produces the red color there comes from F toward G, where it is refracted toward H, and thence reflected toward I, and at I once more reflected, this time toward K, where it is refracted toward E. He concludes: 