The Dictionary Of Photography

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

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Photography in Natural Colour
B.
Water           ............        ...    200 c.cm.
Potassium bromide... ... ...        ...      20 gm.
Ammonia (sp. gr. 0.96 at 18o C.) ...        ...      67 c.cm.
For use mix
Solution A ... ... ... ... ... 10 c.cm.
ยป B............... 20 ..
Water           ............... 70 ..
For chloride bromide plates these developers should be diluted to double the volume. For fixing, a 4 or 5 per cent, solution of potassium cyanide is the best. Valenta points out that under-exposed and under-developed photochromes thus prepared can be intensified with mercuric chloride, followed by amidol and sulphite. Lippmann presented to the Academie des Sciences de Paris on October 24th, 1892, some good reproductions of the solar spectrum on bichromated albumen and gelatine ; these only show the colours when wet. The albumen must first be coagulated with mercuric chloride solution, then well washed before treating with bichromate of potash. Lippmann's theory of the formation of these colours is that in the chromated film series of maxima and minima of interference are formed. The maxima alone make the film insoluble, and in treating the plates with water a laminar structure is formed which is partly saturated with water and partly dry. The index of refraction of the laminae varies periodically, and thus gives rise to the interference pheno-mena and the colours. St. Florent is stated to have obtained photochromes or colour photographs by treating a plate coated with a thin gelatine film with a solution of -
Water           ...............1,000 c.cm.
Ferric chloride         ... ... ... ... 10 gm.
Citric acid ...... .........         5 ,*,
and then exposing under a coloured glass transparency. Valenta writes upon the preparation of emulsions for Lippmann's pro-cess : "The drawback to plates prepared by the Lumiere or Valenta process for colour photographs is that the means taken to secure the grain being sufficiently fine results in the plates being very slow. So slow is this emulsion that a plate exposed in a sensitometer for five minutes to a light of fifty-candle power
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