'purification of knowledge and vision', is the last of the 7 purifications and a name for path-knowledge (maggañāna), i.e. the penetrating realization of the path of Stream-winning, Once-returning, Non-returning or Arahatship. Vis.M. XXII furnishes a detailed explanation of it (s. visuddhi, VII).
In A.IV.41 ñānadassana apparently means the divine eye (dibbacakkhu, s. abhiññā), being produced through concentrating the mind on light.