 By the mid @-@ 1950s professional singers were cast in leading roles in most productions ; they included Walter Midgley in Tosca and La bohème ( 1955 ) , Raimund Herincx in Mefistofele ( 1957 ) , Heather Harper in La traviata ( 1957 ) , and Joan Hammond in Madame Butterfly ( 1958 ) . A possibility of strengthening the professional element of the company was mooted in 1958 , when a merger was proposed with the Carl Rosa Company , which was in financial difficulties . The proposal was not followed through and WNO continued independently while the Carl Rosa folded . 
