With "1+1" and "Two Pages" (composed in February 1969) Glass turned to a more "rigorous approach" to his "most basic minimalist technique, additive process", pieces which were followed in the same year by "Music in Contrary Motion" and "Music in Fifths" (a kind of homage to his composition teacher Nadia Boulanger, who pointed out "hidden fifths" in his works but regarded them as cardinal sins). Eventually Glass's music grew less austere, becoming more complex and dramatic, with pieces such as "Music in Similar Motion" (1969), and "Music with Changing Parts" (1970). These pieces were performed by The Philip Glass Ensemble in the Whitney Museum of American Art in 1969 and in the Solomon R. Guggenheim Museum in 1970, often encountering hostile reaction from critics, but Glass's music was also met with enthusiasm from younger artists such as Brian Eno and David Bowie (at the Royal College of Art ca. 1970). Eno described this encounter with Glass's music as one of the "most extraordinary musical experiences of life", as a "viscous bath of pure, thick energy", concluding "this was actually the most detailed music I'd ever heard. It was all intricacy, exotic harmonics". In 1970 Glass returned to the theatre, composing music for the theatre group Mabou Mines, resulting in his first minimalist pieces employing voices: "Red Horse Animation" and "Music for Voices" (both 1970, and premiered at the Paula Cooper Gallery).