called No. 40, no matter what focus lens was used. Any lens is measured for its actual focus by measuring the distance of its burning point from the back of the lens in complete quarter-inches neglecting fractions. Suppose a lens was found to measure 20 quarter-inches ; 20 x 20 = 400, striking off the last figure, its area number would be 40. Any other lens would be measured in the same way in quarter-inches. Thus it will be seen that every stop has its own number, and that whatever relation that number has to that of the lens it is used with gives at a glance the exposure required. It need hardly be pointed out that this method is subject to grave inaccuracies.
Argentometer. An instrument constructed on the principle of a Hydrometer (q.v.), and marked with a scale to show the number of grains per oz. of nitrate of silver in a silver bath. This is, of course, only applicable when the solution is pure.
Argentotype. A name applied to one particular make of gelatino-bromide paper.
Aristogen. Under this name Liesegang has introduced a concentrated hydroquinone developer, specially designed for developing gelatino-chloride prints. Its formula is said to be
Hydroquinone (10% alcoholic sol.) ... 10 parts.
Sodium sulphite cone. sol. ... ... 10 ,,
.. acetate 20% sol.......... 5 ,,
Citric acid, 20% sol. ... ... ... 5 ..
Water ............... 100 ,,
Aristotype. A name applied to one particular make of gelatino-chloride paper, but occasionally applied as a general term to printing-out papers with a base of collodion and gelatine. (See Gelatino-Chloride, and Collodio-Chloride.) The fol-lowing books deal specially with the aristotype process: - "Aristotypes and How to Make Them," by Walter E. Wood-bury, Gew York, 1893, Scovill & Adams Co. ; " LAristotypie" par le Commandant V. Legros, Paris, 1897, Societe" d'Editions Scientifiques; "Les Epreuves Positives, surPapiers Emulsiomies" par E. Trutat, Paris, 1896, Gauthier-Villars ; " Der Positiv-process auf Gelatine-Emulsionspapier" von Dr. E. A. Just, Vienna, 1885, published by the author.