In Dante's vision of Heaven, souls do not resemble material bodies, as they do in Hell or Purgatory, but are depicted as rays of light. These lights grow ever brighter the higher up one ascends, because the closer one is to God, the more one reflects His light of love, shining down from the Empyrean. Mortals cannot gaze on the souls past a certain point (for Dante, it is in the Seventh Heaven of Saturn) without being blinded as a result. On Judgment Day, the blessed souls will wear their physical bodies again and shine all the brighter for being complete and for meriting more of God's grace. Finally, these souls sometimes form holy symbols in the sky with their light.
The blessed souls' "vision," on which their brilliance depends, is really a measure of how well they understand God's love; thus, one may argue that the contemplative souls (in Saturn) are more worthy than the glorious souls (in Mercury).