The Dictionary Of Photography

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

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Photography in Natural Colours.
scarcely shows the first number. Valenta finds, however, that the addition of one gramme of sodium sulphite to 300 of emulsion with subsequent heating at 380 C. results in five minutes in an emulsion showing the fourth number on the sensitometer, or the eighteenth number in an hour. Krone has formulated the following conditions or rules, which must be complied with for the successful working of Lippmann's process. (1) It is essential that the sensitive film be homogeneous, and that a reflecting surface be in contact with the film, the rays from which reflecting surface must interfere with the direct rays and thus produce stagnant or stationary waves in the film. (2) If the thickness of the film exceeds a certain thickness, the colours are not obtained, or not the correct colours. Where the regular film encloses a particle of dust this appears particularly notice-able. (3) The faithful reproduction and appearance of the
colours in correct position is not absolute, but depends upon the following without exception: - (a) an absolutely accurate con currence in the film of the finest divided silver haloid with the colour sensitiser and the correct amount of the latter ; (b) on the temperature in drying the film ; (c) on the duration of exposure ; (d) on the development. It may thus happen that with less favourable concurrence of the above-mentioned factors, green may appear in the place of blue, and yellow in the place of red. This is also the reason why many appearances of anomalous colour-dispersion appear, and that many colours are wanting, etc. (4) An increased or diminished dampness of the results alters the colours. (5) The altitude of the sun at the time of the exposure has considerable influence on the results, as shortly before sunset or after sunrise, although all the colours may appear vigorous in the spectroscope, yet the more refrangible rays show much less actinic power, so that finally, with prolonged