The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Printing Surfaces and Blocks
Pyrogallic Acid
the print from the two negatives as to show not the slightest trace of the use of two negatives. (For special information as to printing process, the reader must refer to such articles as Bromide Paper, Artigue's Process, Carbon Process, Cyano-type, Gum-Bichromate, Pizzic.helli Process, Platinotype, Mariotype, Collotype, Photogravure, Ozotype, Automatic Printing, Aux deux Crayons, etc., etc.)
Printing Surfaces and Blocks. See references under Photomechanical Printing Processes.
Printing Types. See Types.
Prism, Reversing. In using the reflecting prism for the purpose of obtaining those reversed negatives which are required in several of the photomechanical processes, care should be taken not to allow too wide angled a cone of rays to enter the prism, otherwise a disturbing reflection from the exit face of the prism will step in. Many fail in the use of the prism by attempting to include too wide an angle. See Reversed Negatives.
Process Blocks. See Chemigraphy, Fish Glue Process, Galvanography, Zincography, Types, Woodburytype, Leim-
" Psychic " Photography. See " Spirit " Photography.
Psychography. See Retinal Impression.
Pyrocatechin Developer. See Development and De-velopers.
Pyrogallic Acid (Ger., Pyrogallussaure; Fr., Acide Pyro-gallique). C6H603 = 126. Is not actually an acid, its proper chemical name being pyrogallol; it is prepared by sublimation from gallic acid. In the presence of alkalies in solution it absorbs oxygen from the air, turning black, a carbonate and acetate of the alkali used being formed. It is a powerful deoxidiser and reducing agent, and is more extensively used than any other agent for developing. As pyro is very easily oxidised it is extremely difficult to preserve in solution, many substances being recom-mended for that purpose ; the following are some of them, which are enumerated in the order of their value for that purpose: - A mixture of glycerine and alcohol, metabisulphite of potash,