Musicians who served as `` second conductor '' included the composer Cornelis Dopper , Evert Cornelis and Eduard van Beinum .
During Mengelberg 's time the `` first conductors '' included Karl Muck ( 1921-1925 ) , Pierre Monteux ( 1924-1934 ) , Bruno Walter ( 1934-1939 ) , and Eugen Jochum ( 1941-1943 ) , and the second conductors included the composer Cornelis Dopper , Evert Cornelis and Eduard van Beinum .