Copy letter from Robert William Frederick Harrison, to The Swan Electric Engraving Company Limited, Northumbria House, 116 Charing Cross Road, W.C. Would be obliged if they could put the work of making half tone blocks of Duffield's Spectra in hand at once at the rate of 6d per square inch, as quoted in their letter. The two illustrations should be reproduced at the same size as the original photographs, to form two plates of the size of a 'Philosophical Transactions [of the Royal Society]' page. The lettering will be inserted by the printers. 29 September 1908