In May 1996 , the National Gallery took on loan Velázquez 's Innocent X portrait and hung it alongside four Bacon paintings ; Head VI , Pope I ( 1951 ) , Pope 1961 and Pope 1965 . Peppiatt believes that Bacon would have disapproved of such a showing with a work he considered one of the finest ever painted , but writes that two , including Head VI , " stood up to it , and even enhanced its authority as one of the most penetrating studies of human nature and human power " .