Holmes was working hard over a chemical investigation
Photographic reproduction of ink or watercolor on paper
Illustration for Arthur Conan Doyle's “Adventure of the Naval Treaty,” p. 306.
Passage illustrated: “Holmes was seated at his side-table clad in his dressing-gown, and working hard over a chemical investigation. A large curved retort was boiling furiously in the bluish flame of a Bunsen burner, and the distilled drops were condensing into a two-litre measure. My friend hardly glanced up as I entered, and I, seeing that his investigation must be of importance, seated myself in an arm-chair and waited. He dipped into this bottle or that, drawing out a few drops of each with his glass pipette, and finally brought a test-tube containing a solution over to the table. In his right hand he held a slip of litmus-paper.”