And in the blast there smote along the hall A beam of light seven times more clear than day: And down the long beam stole the Holy Grail All over covered with a luminous cloud. And none might see who bare it, and it past. But every knight beheld his fellow's face As in a glory, and all the knights arose, And staring each at other like dumb men Stood, till I found a voice and sware a vow. - Idylls of the King
Le Morte d'Arthur by Sir Thomas Malory:
"Then anon they heard cracking and crying of thunder, that them thought the place should be destroyed. In the midst of this blast entered a sunbeam more clear by seven times than ever they saw day, and all there were alighted of the grace of the Holy Ghost. Then began every knight to behold other, and either saw other, by their seeming, fairer than ever they saw afore. There was no knight might speak one word a great while, and so they looked every man on other as they had been dumb. Then there entered into the hall the Holy Grail covered with white samite, but there was none might see it, nor who bare it. And there was all the hall filled with good odours, and every knight had such meats and drinks as he best loved in this world. And when the Holy Grail had been borne through the hall, then it departed suddenly, that they wist not where it went: then had they all breath to speak. And the king yielded thanks to God, of His good grace that he had sent them. 'Certainly', said the king, 'we ought to thank our Lord Jesu greatly for that he hath shewed us this day, at the reverence of this high feast of Pentecost.'"
