John L. Ridgway, The Preparation of Illustrations for Reports of the United States Geological Survey
“ If he is to retouch a number of photographs that have the same local color he may with advantage make up enough of the tint for the entire lot, thoroughly mixing it and seeing that it is not too thin. In making this tint he should use only pigments of the best grade, and if he finds that the Chinese or other white he is using does not photograph well, or that it does not hold its color, he should discard it at once and use another brand. Photographs that are to be retouched should be large enough to permit sufficient reduction to soften the effects of retouching. ”
