He then returned to Batavia and worked at the Museum of the Royal Batavian Society of Arts and Sciences (today the National Museum of Indonesia), and catalogued the museum's large collection of Javanese texts, accumulated over nearly a century. While doing this, he wrote monographs on several groups of manuscripts, containing their Dutch summaries and indexes of names.[3] In Batavia, he also met the future historian of Java, H. J. de Graaf, to whom he gave weekly lessons in Javanese culture and language between 1927 and 1930. De Graaf later considered Poerbatjaraka his "revered teacher" and praised him for "unveil[ing] some of the secrets of your people".[6]