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This is Heinrich Rudolf Hertz, from Oliver Heaviside: Sage in Solitude (ISBN 0-87942-238-6), p. 111. The image is reprinted courtesy of the Burndy Library (as stated in the credits in the back of the book, p. 318), but its age implies that it's public domain. (It must have been made in 1894 or earlier.) It was scanned on an Epson Perfection 1250 at 400dpi, cleaned up (some text was showing through the back) in Photoshop, reduced to grayscale, and saved as JPG using the 'Save for Web' optimizer.
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