ALBUMEN-DIA-POSITIVE PROCESS ON GLASS.
Positives obtained by this process are intended to be viewed by transmitted light. The manipulation is so nearly identical with that of the albumen-negative process, described in the foregoing article, that it is only necessary to point out the difference between them.
The negative to be copied is placed either in a copying camera, (see" Copying Camera,") or in direct contact with the sensitive plate in a pressure-frame. In the latter case the plate must be used dry, and the exposure to diffused daylight, or artificial light, only occupies a few seconds ; in the former case, the plate may be either dry or wet, and the exposure is considerably longer. The wet process is the least troublesome, and yields the best results, because the operations of exciting, exposing, and developing, may then succeed each other at once, and less time is allowed for a combination to take place between the silver and albumen, which causes the lights of the picture to assume a yellow tint. The development is also a much quicker operation in the wet process, more nitrate of silver being allowed to remain on the plate, and for this reason also the lights are less likely to assume a yellow tint.
The difference between this and the negative process consists chiefly in the employment of a gold toning-bath, in order to vary and improve the tint of the finished picture. Thus, before fixing
the picture, pour over it a little of the following solution :-1 grain of sel d'or, 20 drops of hydrochloric acid, and two minces of distilled water. Allow this to remain on the plate a short time, until the shadows assume a purple tint ; then wash it off, and fix as before.
Dia-positives on glass should be viewed with the plain side of the glass next the eye ; and against the film the rough side of a finely ground glass should be placed, the two glasses being bound together at the edges with a strip of tape or paper pasted over them. In this way the print is protected from injury, and has a proper semi-transparent background.
The chief use of this proc,ess is for printing transparent slides for the stereoscope. In this operation it must be remembered that the picture taken from the right station must be viewed by the right eye, and vice-verdi ; and also that the objects in the view must not be reversed as regards right and left. It may therefore be necessary to place the negative in the copying frame with its back to the lens. Matters of this kind must be carefully considered by the operator ; and his ingenuity will suggest the proper way of proceed ing in every case. No general rules need be laid down in this place.