For Turner and Domenichino in the Louvre, see especially folio 75 verso of this sketchbook (D04371), his copy of the Hercules and Cacus. In his notes on the picture, Turner confused it with its companion, Hercules and Achelous, of which however he made separate notes on the verso of this leaf (DO4373).
Turner’s comments here read:
This by Dominicino [Finberg: Dominicio] Hercules & Achelous | Grand as to composition The lights and shade broad but of one surface as the | whole background hill is all of one tone | generally The trees below somewhat lighter | and diversify’d but the large tree is of the | same tone as the hill therefore I think | although of a fine rich or deep tone yet | the picture is monotonous. The figures are | not sufficiently diversify’d as to colour | or to create any interest but are | rather so many pieces of half light and | Achelous is miserably drawn although fine as to | disposition. Likewise Hercules. The sky plain | but of a fine tone.