Assistant to Roger Fenton in the Crimea. In 1856 a book was authored by W. Sparling "Theory and Practice of The Photographic Art; including it's Chemistry and Optics" (London: Houlston and Stoneman, 1856) and on the title page it states that he was "Assistant to Mr. Fenton, Honorary Secretary to the Photographic Society" so it is presumably the same person.
Genealogy of Marcus Sparling
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Roger Fenton, 1855, May-June, The photographer Roger Fenton's assistant, Marcus Sparling, sitting on Roger Fenton's photographic van, Imperial War Museum, LL/6550