In the "Queste Del Saint Graal", Lancelot can't look directly at the Holy Grail because of his affair with her
[DOC] [TLE] Allegories of the Holy Grail - ChristiansOnNet.org [PAR] Bible scholar has more humble vision of the Holy Grail (980414) .... where 
between 1164 and 1173 he wrote his Lancelot for the Countess Marie.