Cutting Prints Cyanine
a b c is a plane object, next come diaphragm and lens; the rays from b - the point situated on the axis - will come to a focus on the line; but, those rays from a and from c, which are shown in fig. 29 as coming to a nearer focus, are cut off; while those that come to a focus on the lineare allowed to
pass; thus we shall practically have a sufficient approximation to a flat field.
Cutting Prints. This is trimming off the unnecessary part of the print till of the desired size. It should always be done prior to toning, to save the waste of gold in toning unnecessary matter, and it is best done whilst the print is dry. Plate-glass cut, with polished edges, to certain sizes can be obtained com-mercially, but any old negative glass or flat ruler will do as a guiding edge. Numerous and divers knives are sold for trim-ming ; but the author has found a leather-cutter's knife, termed a clicker's knife, the most convenient, and the price is but a few pence. Too many amateurs consider that the finished print should be exactly the full size, when much more pleasing pictures can often be produced by trimming off certain portions of the print.
Cyanides. A class of salts containing Cyanogen (g.v.); the cyanide of potassium being the most important, and highly poisonous. Double cyanides, like the sulphocyanides and ferro-cyanides, although apparently not actually poisonous themselves, should be regarded with extreme caution, as simple cyanides may be very readily produced from them under unexpected conditions.
Cyanine (Ger., Cyanin, or Chinolinbleu; Fr., Cyanine, or Blende quinoleine; Ital., Cianind). Synonyms: Cyanine, Cyanin Iodide, Quinolin, or Chinolin Blue. This is
prepared by the action of iodide of amyl on chinoline; it occurs as a dark blue powder or prismatic crystals, with metallic lustre ; it is not very soluble in water, but more so in alcohol. As cyanine is sensitive to light it must be kept in the dark. It is one of the best sensitisers for red, and for this purpose is used in orthochomatic work, the disadvantage being that the plates are very liable to fog, and therefore Dr. Eder suggests the use of chloro-cyanin, which has not this action, and gives the following