 The manner in which these balls tend to rotate depends on the causes that determine them to do so. When they are refracted by a common surface, “all the refractions which occur on the same side [of the balls] cause them to turn in the same direction” (ibid.), but they do not necessarily have the same tendency to rotational speed. Particles of light can acquire different tendencies to rotational speed after refraction, depending on the bodies that surround them, 