  Another aspect of this integrated approach to optics was a reliance on internal perceptual phenomena to account for external phenomena regarding appearances, illusory or otherwise. This was not a universally-held view, even in the age of the optics of light and vision. In addition to Gassendi's interest in parhelia, he also sought to explain apparent discrepancies in the size of the sun and moon at different hours by reference to visual experience produced by light phenomena (De apparente magnitudine…, 1642). Thus, these bodies appear larger on the horizon than at their apogee because the pupil dilates from the differential exposure to the light at the horizon. (In fact, all such apparent differences are produced by the distance effect of surface features of the area from which the celestial body is viewed.) One key driver of such an appeal to the interaction of light behavior and our visual apparatuses is Gassendi's view that some explanatory role must be played here by the common atomic structure underlying the images intromitted into our eyes and the light rays cast by celestial bodies which are understood as creating such images.