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Orthoc1iromatic or Isochromatic

plate, dye, plates, water, erythrosine and ammonia

ORTHOC1IROMATIC OR ISOCHROMATIC PROCESS.—A process of obtaining photo graphic images of colored objects in true relative tone values. To obtain this, by counteracting the opposite effect that colors have upon the photographic plate to that upon the retina of the eye certain dyes are used.

The facts are these*, that when certain fugitive dyes absorb in a part of the spectrum to which the silver salt is insensitive, or only slightly sensitive, the rays absorbed produce an action on the silver salt in contact with it, either producing sensitiveness or increasing the sensitiveness. If the silver salt combines with the dye the action is readily explained by the ordinary theory, whilst if it does not the dye itself appears to undergo alteration, and to become, as it were, a de veloper, reducing a small portion of silver salt, on which subsequent development takes place.

The dyes generally used in practice are those known as erythrosine, rose bengal, or eosine, usually with an alkali as ammonia. There are two methods of applying the dye to the plate. It may be added to the gelatine emulsion before coating, and orthochromatic or isochromatic plates then prepared are sold commercially, or the plate may be laid in a bath of the dye.

In working with orthochromatized plates every precaution must be taken to prevent fog, as it should be remembered that these plates are sensitive to the yellow or red light.. The very deepest ruby should be used, and the plates covered up as often as possible.

A plate should be chosen with an emulsion containing little or no iodide.t This is first bathed for two minutes in a solution of- Liquor ammonia I part.

Water roo parts.

Then without any previous washing it is immersed in— Dye (eosinet or erythrosine) r part.

Water to,ouo parts.

Ammonia Jo° parts.

Erythrosine or eosineff I part.

Water 1,000 part.

This is kept for a considerable time in the dark The ammonia solution may be a xo per cent, one. Then take— Dye solution (as above) . i part.

Ammonia (10 per cent.) i part.

Water 8 parts.

Ives's process of orthochromatic photography is as follows : In four ounces of absolute alcohol dissolve one grain of erythrosine or cyanine, soak the gelatino-bromide plate in this for a minute, allow it to dry, and wash for a short time in a stream of running water. Dry and use. No alkali is necessary. Cyanine renders the plates so sensitive to red, however that all subsequent operations must be done as far as possible in total darkness.

Captain Abney recently found that color sensitiveness can be given to a plate by coating it with either varnish or collodion in which various sensitive dyes are dissolved. If erythrosine is to be used, varnish is stained to such an extent that when flowed over the film it shows a very light stain of dye. After exposure the plate is soaked in two or three changes of methylated spirit to remove the varnish, and after washing, development takes place in the ordinary manner. Any slight trace of varnish that may be seen, at once disappears if the plate be varnished as is usually done. If collodion be used the film must be first softened by alcohol to which a little ether has been added. After washing, development may proceed. Dyes which are not soluble in water can be made effective on gelatine plates by this method, and are certainly useful* Ives has also introduced a similar process, using chlorophyll as the sensitiser.

The following are some of the best known methods of rendering gelatino-bromide dry plates orthochromatic by bathing :—