Uranium Toning Varnish
Silver nitrate ............25 grs.
Distilled water ......... ... 1 oz.
This will give a greyish image, which can be toned after washing in a combined toning and fixing bath.
Chloride of gold............ 1 gr.
Distilled water ............ 1 oz.
This should be brushed over the image, and gives a purplish-black. The prints after development should be washed in a bath of hydrochloric acid, 1 to 80, and then again washed thoroughly.
Uranium Toning. See Bromide Paper and Platinotype.
U.S. or Uniform System. See Diaphragms.
Vapography. A term applied, like atmography, to various effects on photographic plates by emanations or vapours. Dr. Russell has recently shown that the oily vehicle of printing ink, and also clean zinc, often so reacts with atmospheric oxygen and moisture as to produce sufficient hydrogen peroxide to affect the sensitive plate. Hence a developable image will usually be obtained if a sensitive plate is allowed to remain in contact with clean zinc or recently printed letter-presses.
Varnish. A solution of resinous bodies in a volatile solvent, used for covering the film of a negative with a coating of matter impermeable to air and damp.
The following is a table of the principal varnishes of