SECTION IV.

On a mode of Printing enlarged and reduced Positives, Transparencies, etc., from Collodion Negatives.

To explain the manner in which a Photograph may be enlarged or reduced in the process of printing, it will be necessary to refer to the remarks made at page 52, on the conjugate foci of lenses.

If a Collodion Negative be placed at a certain distance in front of a Camera, and (by using a tube of black cloth) the light be admitted into the dark chamber only through the Negative, a reduced image will be formed upon the ground glass; but if the Negative be advanced nearer, the image will increase in size, until it becomes first equal to, and then larger than, the original Negative; the focus becoming more and more distant from the lens, or receding, as the Negative is brought nearer.

Again, if a Negative portrait be placed in the Camera slide, and the instrument being carried into a dark room, a hole be cut in the window-shutter so as to admit light through the Negative, the luminous rays, after refraction by the lens, will form an image of the exact size of life upon a white screen placed in the position originally occupied by the sitter. These two planes, in fact, that of the object and of the image, are strictly conjugate foci, and, as regards the result, it is immaterial from which of the two, anterior or posterior, the rays of light proceed.

Therefore in order to obtain a reduced or enlarged copy of a Negative, it is necessary only to form an image of the size required, and to project the image upon a sensitive surface either of Collodion or paper.

A good arrangement for this purpose may be made by taking an ordinary Portrait Camera, and prolonging it in front by a deal box blackened inside and with a double body, to' admit of being lengthened out as required; or, more simply, by adding a framework of wood covered in with black cloth. A groove in front carries the Negative, or receives the slide containing the sensitive layer, as the case may be.

In reducing Photographs, the Negative is placed in front of the lens, in the position ordinarily occupied by the object; but in making an enlarged copy, it must be fixed behind the lens, or, which is equivalent, the lens must be turned round, so that the rays of light transmitted by the Negative enter the back glass of the combination, and pass out at the front. This point should be attended to in order to avoid indistinctness of image from spherical aberration.

A Portrait combination of lenses of 2-1/2 or 3-1/4 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 2-1/4 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.

The employment of a concave reflector, which may be purchased for a few shillings, ensures parallelism of rays, and is a great improvement. The lamp is placed in the focus of the mirror, which may at once be ascertained by moving it backwards and forwards until an evenly illuminated circle is thrown upon a white screen held in front. This in fact is one of the disadvantages of printing by a naked flame--that the light falls most powerfully upon the central part, and less so upon the edges, of the Negative.

The picture must be exposed for a longer or shorter time (about ten seconds will be an average) according to its behaviour during development (see p, 224); this process, as well as the fixing, is conducted in the same manner as for Collodion pictures generally.

Some adopt the plan of whitening by Corrosive Sublimate, and again blackening by dilute Ammonia, as an improvement to the colour of the dark shadows (see p. 113).

If this mode of printing upon Collodion be conducted with care, the Negative being separated from the film by the smallest interval only, the loss of distinctness in outline will scarcely be perceived.

Stereoscopic transparencies may also be printed by the dry Collodion process described in Chapter VI., or by the Collodio-Albumen process. Mr. Llewellyn recommends the employment of a solution of Oxymel, so dilute that the plate becomes nearly dry, and may be laid in contact with the Negative without fear of injury (see the footnote at page 302).