This illustration shows perhaps the discovery of a nefarious crime. That must be Mr. Holmes with the lantern and Watson peering through the trap door to discover some nefarious deed . . .
In the process of painting the presumed Dr. Watson became Guy Fawkes and Holmes an anonymous lantern bearer there to provide a Promethean flame to the androgynous prophet bathing in the Roman bath below.
Image taken from page 130 of 'The Memoirs of Sherlock Holmes' Collection of the British Library.
View illustration on British Library's Flickr Photostream
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