A Portrait combination of lenses of 2½ or 3¼ inches diameter is the best form to use, and the actinic and luminous foci should accurately correspond, as any difference between them would be increased by enlarging. A stop of an inch or an inch and a half aperture placed between the lenses obviates to some extent the loss of sharp outline usually following enlargement of the image.
The light may be admitted through the Negative by pointing the Camera towards the sky; or direct sunlight may be used, thrown upon the Negative by a plane reflector. A common swing looking-glass, if clear and free from specks, does very well; it should be so placed that the centre on which it turns is on a level with the axis of the lens.
The best Negatives for printing enlarged Positives are those which are distinct and clear; and it is important to use a small Negative, which strains the lens less and gives a better result than one of larger size. In printing by a 2¼ lens for instance, prepare the Negative upon a plate about two inches square, and afterwards enlarge it four diameters.
Paper containing Chloride of Silver is not sufficiently sensitive to receive the image, and the Print should be formed upon Collodion, or on iodized paper developed by Gallic Acid (see [p. 263]).
The exposure required will vary not only with the intensity of the light and the sensibility of the surface used, but also with the degree of reduction or enlargement of the image.
In printing upon Collodion the resulting picture is Positive by transmitted light; it should be backed up with white varnish, and then becomes Positive by reflected light. The tone of the blacks is improved by treating the plate first with Bichloride of Mercury, and then with Ammonia, in the manner described at pages 113 and 207.
Mr. Wenham, who has written a paper on the mode of obtaining Positives of the life size, operates in the following way:—he places the Camera, with the slide containing the Negative, in a dark room, and reflects the sunlight in through a hole in the shutter, so as to pass first through the Negative and then through the lens; the image is received upon iodized paper, and developed by Gallic Acid, in the mode described in the second Section of this Chapter ([p. 263]).
On printing Collodion transparencies for the Stereoscope.—This may be done by using the Camera to form an image of the Negative in the mode described in the last page; but more simply by the following process:—Coat the glass, upon which the Print is to be formed, with Collodio-Iodide of Silver in the usual way; then lay it upon a piece of black cloth, Collodion side uppermost, and place two strips of paper of about the thickness of cardboard and one-fourth of an inch broad, along the two opposite edges, to prevent the Negative being soiled by contact with the film. Both glasses must be perfectly flat, and even then it may happen that the Negative is unavoidably wetted; if so, wash it immediately with water, and if it be properly varnished, no harm will result.
A little ingenuity will suggest a simple framework of wood, on which the Negative and sensitive plate are retained, separated only by the thickness of a sheet of paper; and the use of this will be better than holding the combination in the hand.
The printing is conducted by the light of gas, or of a camphine or moderator lamp; diffused daylight would be too powerful.