The American annual of photography (1892) (14597853399)

Identifier: americanannualof1892newy (find matches)

Title: The American annual of photography

Year: 1892 (1890s)

Authors:

Subjects: Photography

Publisher: New York : Tennant and Ward

Contributing Library: Harold B. Lee Library

Digitizing Sponsor: Brigham Young University

Text Appearing Before Image:

n is totally destroyed in five or six hours. At 212 deg.Fahr. the destruction is almost immediate. It follows that any formula giving more than 2 per cent, of ammonia must be worked with great caution. A trace of fixed alkali, caustic potassa, at ordinary temperatures produces no fog, but at 212deg. Fahr. even this trace acts very energetically and reduces the silver bromide to metallic silver. The carbonates of the alkalies act much more mildly. An emulsion may even be boiled for ten or fiften minutes with \of a per cent, of ammonium carbonate without fear of fog.Carbonate of soda acts more quickly, but even 1 per cent, can be used at a temperature of 100 deg. Fahr. for an hour with-out giving fog. It i3 evident from the foregoing that all gelatines should be tested for alkali. The interchangeable factors in the production of fog are time, heat, alkali; and it is only by a harmonious adjustment of each one to the other that the dangerous chemical fog can be prevented. R. K Van Oieson, M.D.

Text Appearing After Image:

AND PHOTOGRAPHIC TIMES ALMANAC. 183 DIAPHRAGMS, DENSITOMETER NUMBERS, AND EXPOSURES. It is seldom easy to estimate the exact exposure requisite for an interior, and the difficulty is much increased when the operator is obliged to use unfamiliar lenses and untried plates.It is therefore hoped that the following account of the method pursued in determining the proper exposures for certain recent photographs of the large astronomical instruments at the United States Naval Observatory may not be without interest. The plates used measured 5x8 inches, and the lenses avail-able were a Dallmeyer 1AA wide angle rectilinear of 4.44inches equivalent focus, capable of covering the entire plate;and a pair of Dallmeyer rectilinear stereoscopics of 3.28 inches equivalent focus, each capable of covering half the plate. The first step was to determine the intensity ratios obtainable with these lenses, or, in other words, the series of quotients1:7W, where m—f\d\f being the equivalent focal distance of the

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1892

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