Ordnance Survey. Glass negative is retouched to remove blemishes before it goes to the "helio" department for reproduction. Minute spots in the emulsion must be filled in with opaque paint, the retoucher working on a light table. Retoucher are highly skilled; they must be able to scratch name in the emulsion (in reverse, as this is the negative which will be indistinguishable from the printed letter. This is sometimes necessary when a small correction has to be made, and saves the cost of changing the enamel and re-photographing it. Retouching also cuts costs when small revisions have to be made later. June 15, 1948. (Photo by Pictorial Press).