A helen is a humorous unit of measurement based on the concept that Helen of Troy, from the Iliad, had a "face that launched a thousand ships". The helen is thus used to measure quantities of beauty in terms of the theoretical action that could be accomplished by the wielder of such beauty.
Analysis.
Marlowe sets up an evil parallel of the Christian trinity in the three devils (Lucifer, Mephostophilis, and Belzebub). The devils of hell make an occasion out of winning the single soul of Faustus. Just as Christ is the Good Shepherd, who goes in search of one lost sheep to save it, the devils take great pains even to damn just one soul.
The conjuration of Helen of Troy, in addition to providing occasion for some of the play's finest lines, also resonates strongly with the central themes of the play. The scholars' delight reflects Faustus' old infatuation with the beauty of Greek thinking and literature.
The Old Man offers Faustus yet another chance to repent, and makes clear that Faustus can still be saved. But Faustus chooses instead to take a lover-spirit in the shape of Helen of Troy. His speech is beautiful, but as usual Faustus is all talk. He seems unable, or unwilling, to realize that his poetic praise is only a damned man's fantasy. Helen of Troy is not there: Faustus makes love to a dream.
Even within his fantasies, Faustus reveals his failure. Though he fantasizes about being Paris, the Trojan prince who causes the war by abducting Helen, he chooses not to remember that Paris is traditionally depicted as a coward and moral failure. Faustus speaks of battling for Helen: "And I will combat with weak Menelaus, / And wear thy colours on my plumed crest. / Yes, I will wound Achilles in the heel, / And then return to Helen for a kiss" (5.1.106-7). The language is beautiful, but Faustus has altered his source story. Paris did indeed fight Menelaus, but the Greek king was far from "weak." Only the intervention of the gods saved Paris, and by allowing himself to be saved, Paris doomed his city and his people to destruction. Faustus imagines himself as a Greek hero, with a touch of the chivalric lore. His talk of wearing Helen's colors on his crest was a knightly tradition. But shooting Achilles in the heel was not a knightly act. It was an example of weak man beating a far better one, by exploiting a unique weakness. This speech shows Faustus' problem. He seems to know the Greek stories, and loves their beauty, but he doesn't understand them. Though he rejected the Christian God in part because he thought to aspire to Greek greatness, his understanding of the Greek worldview is selective and shallow.
He loses his last chance at redemption, and he also wastes his remaining time on lechery. He also orders his devils to attack an old man who only tried to help him. But the Old Man's spirit is untouchable, and the wounds to his flesh are insignificant. Faustus, on the other hand, caves quickly when Mephostophilis threatens him with physical violence. By prizing flesh over spirit, Faustus betrays both Greek and Christian values. He escapes physical harm for now, but Faustus, and not the Old Man, is the one who'll know True suffering.
https://www.gradesaver.com/dr-faustus/study-guide/summary-act-v-scene-1#:~:text=Mephostophilis%20tells%20Faustus%20that%20he,love%2C%20and%20Mephostophilis%20readily%20agrees.&text=Sweet%20Helen%2C%20make%20me%20immortal,soul%3A%20see%20where%20it%20flies.
http://www.lib.ru/INOOLD/MARLO/marlowe_faust.txt
http://www.lib.ru/INOOLD/MARLO/marlowe_faust2.txt
https://youtu.be/U2CqhKjGvxs
