From Frances Dickey’s account 

(January 20, 1931) ... [Eliot] //concludes with an enigmatic remark, saying (in French) that he knows more about potions than she does. He clarifies his reference to potions by reminding her that she once took him to see Tristan und Isolde—a detail that probably sheds light on his quotations from this opera in The Waste Land directly before and after the “Hyacinth garden” lines.//