Copy letter from Robert William Frederick Harrison to Kenneth Cookson, Oakwood, Wylam, SO, Northumberland 'In answer to your letter enquiring concerning papers on the colouring of opals by Stewster and others, I beg to suggest that you consult the Society's Catalogue of Scientific Papers, of which there should be a copy in Newcastle. This will at least give you the titles of all the papers by Brewster, and the other authors you mention, down to 1883. I regret that we have not the time to go through the lists here, but on glancing through the first list (under Brewster) I find a reference to Reports of the British Association 1844 (Pt.2) p.9. For years later than 1883, I can only suggest that you consult the Indexes to Nature, the Chemical News, The Geological Magazine, and other such likely serials. I regret I cannot help you more definitely.' 22 April 1912