The Dictionary Of Photography

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

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Isochromatic Photography
Tailfer's process consists of the use of eosin or erythrosin in conjunction with ammonia, and the same may be added to the emulsion at the moment of formation of the sensitive salt, or the previously prepared and coated plate may be bathed in such a solution ; and these plates are thereby rendered more sensitive to yellow and yellowish-green. Dr. Vogel has introduced com-mercially a process in which " azaline " is used, and this is said to be a mixture of quinoline blue (cyanin) and quinoline red, by means of which the sensitiveness is still further increased for the orange, orange-red, and red rays. The chart of curves, fig. 70, is taken from Meldola's work mentioned above, and is constructed from results given by Bothamley and Abney, and it shows the relative sensitiveness of the film dyed with various colouring matters. (1) Violet dyes ; (2) green dyes ; (3) iodine green ; (4) cyanin; (5) eosin; (6) ammoniacal rose, Bengal; (7) ccerulein ; (8) chrysaniline ; (9) eosin on chloride of silver; (10) eosin and cyanin mixed ; (11) erythrosin on iodide of silver and nitrate of silver; (12) erythrosin on bromide of silver; (13) erythrosin on chloride of silver; (14) cyanin on chloride of silver. Where the haloid is not specified bromide of silver has been used. It will be seen by an examination of the above chart that, although the sensitiveness of the haloid salts is increased towards the less refrangible rays beyond E in the yellow and red, yet the greatest sensitiveness is still in G and H in the blue ; therefore, to obviate this, a coloured screen is used, usually of glass of a more or less deep tint of yellow, which was first suggested by Sir William Crookes, a pioneer of eminence, long before any iso- or ortho-chromatic process was thought of. The glass screens may be used either in front of the lens, between the combinations of a doublet, or behind the lens ; and it should be of absolutely plain glass, with parallel surfaces, so as not to interfere with the definition of the lens; or a really serviceable makeshift may be made according to the plan proposed by Engler, in which glass plates are first of all waxed, and then polished and coated with a plain collodion, as free from structure as possible, and when thoroughly dry are coated with gelatine, stained to the desired tint with aurantia or Manchester yellow. For general use a good lemon tint should be made. When thoroughly dry the gelatine film may be stripped from the glass and cut into pieces, and enclosed between small brass plates or