2nd August 1865
Sir,
I am collecting the best portraits that can be obtained of the great men in Science of Great Britain living in 1847 with the view of arranging them together in different groups to form a historical picture of the above period - to be a companion to one I have already Engraved & Published of the great Men of Science living in 1807. Sir John Herschel kindly wrote me the enclosed as an introduction to you asking you to be so kind as to give me a sitting for a good Photograph for the above purpose, & which may I beg you to comply with.
I have the honour to be Sir your obedient humble servant,
Willm Walker.