Gum Dammar Halation
this matter, or as regards the extent to which the platinum must be printed. All such questions have to be determined in rela-tion to the work in hand.
During the past few years very much has been written on the gum-bichromate process, and we may give a list of some few books: - "Photo-Aquatint; or, the Gum-Bichromate Process," by A. Maskell and R. Demachy: published by Hazell, Watson & Viney; " Sepia-Photo et Sanguine-Photo," par A. Rouille-Ladeveze: Paris, Gauthier-Villars ; " Der Gummidruck," von J. Gaedicke : Berlin, 1898, Gustave Schmidt; "Der Gummidruck," von F. Behrens: Berlin, 1898, M. Krayn.
Gum Dammar. See Dammar.
Gum Dragon. See Tragacanth.
Gum Elemi (Ger., Oelbamnharz: Fr., Gom??ie elcmi.) A resinous exudation from Canarium commufie, iTnported from the Philippine Islands, and also obtained from Amyris elemifera in Central America. It is used in varnishes and encaustic paste. It is very soluble in alcohol, insoluble in water, and should have somewhat the colour and consistence of honey, but generally, from exposure to air and impurities, is more yellowish-brown and indurated.
Gum Guaiacum. A resinous exudation from Guaiacu?n officinale, a native of San Domingo and Jamaica, soluble to the extent of 90 per cent, in absolute alcohol, and when triturated with water forms a mucilage of pale greenish hue. It is used in some of the old collodion processes, and was used by Niepce as a sensitive resist for etching on metal plates.
Gum-Ozotype. See Ozotype.
Gun-Cotton. See Pyrqxyline.
Halation. A blurring of the image and an encroachment of the high-lights upon the surrounding shadows or darker portions. It is but too well known as a source of trouble in photographing an interior in which a brilliantly lighted window appears ; or again, when photographing landscapes in autumn or winter, halation is very likely to make its appearance when the leafless boughs of