These are taken from negatives, and may be obtained of the same size, or larger, or smaller than the original, as desired. For copies of the same size as the negative, the operation can be effected by placing the negative in a printing frame, in contact with an ordinary dry collodion plate. The negative used should be very clear in the lights, and have transparent shades. To obtain a good negative for this process, use a more acid silver nitrate bath than for ordinary negatives, and do not continue the development so long. By daylight, the exposure required will be a few seconds, but gaslight may also be used, when the exposure must be extended over several minutes. In developing, pyrogallic and citric acid give a blue-black tint; pyrogallic and acetic acid a brown-black tint. If intended to be hung up as a transparency, varnish and protect the collodion side with a ground-glass backing; bind round the edges to keep out dust. If the transparency is required of a ditferent size to the negative, the camera and lens must be used.

One means of doing this is to work in a dark room, allow the light to enter through the negative only, and proceed as usual with the exposure and developing. (See also iv. 384.)