Part 6 (1/2)

A curious experiment showing that a photographic dry-plate can be otherwise affected than by light, so as to form an image upon it, is the following:

An image of copper in relief is necessary--a penny will do for this purpose. Place an unexposed dry-plate in a normal pyro developer, and on it lay the copper coin. After about five minutes or so, remove the penny, fix and wash the plate, when a perfect image of the penny will be found on it.

ELECTRIC PHOTOGRAPHS.

Similar experiments to that described above have been carried out by Prof. Fernando Sanford. He placed a coin on a dry-plate and connected it with the terminal of a small induction coil, capable of giving a spark of three or four millimeters, while a piece of tin foil upon the opposite side of the plate was connected with the other terminal of the coil.

Several negatives were made in this way, the accompanying photograph, Fig. 58, being from one of them. With one exception, they all show a fringe around them, due to the escape of the charge from the edge of the coin, which accounts for the formation of the dark ring observed around the breath figures made upon gla.s.s.

Later on he undertook to photograph in the same way objects insulated from the photographic plate, and has since made negatives of coins separated from the plate by paraffine, sh.e.l.lac, mica, and gutta percha. The accompanying photograph, Fig. 59, was made with the coin insulated from the photographic plate by a sheet of mica about 0.04 mm. thick. The mica was laid directly upon the film side of the plate, and the coin was placed upon it and connected to one terminal of the small induction coil already mentioned. A circular piece of tin foil of the circ.u.mference of the coin was placed upon the gla.s.s side of the plate directly opposite the coin, and was connected to the other terminal of the induction coil. The little condenser thus made was clamped between two boards, and was covered up in a dark room. Two small discharging k.n.o.bs were also attached to the terminals of the induction coil, and were separated by a s.p.a.ce of less than a millimeter, so that, when a single cell was connected with the primary coil, the spark between the k.n.o.bs seemed continuous.

[Ill.u.s.tration: FIG. 58.]

[Ill.u.s.tration: FIG. 59.]

The plate was exposed to the action of the waves set up in this condenser for one hour, when it was taken out and the negative image developed upon it by the usual process.

MAGIC VIGNETTES.

These are reversed vignettes, that is to say, the margins round the portrait instead of being white as in the ordinary vignette are black.

A method of making them was recently described by ”Teinte” in _The Photogram_. This was as follows:

Two methods can be adopted. The first of these about to be detailed, though entailing, perhaps, in the first place a trifle more trouble, produces the best results. We require a black background, preferably of black velveteen, large enough for a head and shoulders. As the material is not usually obtainable of a width greater than twenty inches or so, there will have to be a seam, and this must be very neatly done. The seamed velveteen is then stretched taut on a frame, which should preferably be covered first with calico, to prevent ”sagging.” Always, before use, dust the velveteen with a soft brush--say, a hat brush--to remove any adhering dust or fluff. Instead of velveteen, a good paper background can be used, only it must be seen that the surface is smooth and free from cracks or creases, and is _dead black_.

[Ill.u.s.tration: FIG. 60.--MAGIC VIGNETTER.]

We require also a vignetting mask suitable to the subject, with a serrated edge. This has to be fixed inside the camera between the lens and plate.[4] The proper position can be found by trial; the further the card is away from the plate the softer and more gradual the vignetting. No special arrangement for holding this is required beyond what can be prepared by any one who can use his fingers. We take a piece of stout card, the outside of which will just fit into the folds of the camera's bellows, and by a little twisting it can be sprung in between the folds which will hold it. There is an opening in the center, square in shape, about quarter plate size. This acts as a frame to hold the vignetting mask, which has the opening of proper size and shape. By using a frame as described the vignetter can be moved about up and down and from side to side, and when the correct position is found fixed by drawing pins. The frame and vignetter should be blacked all over. For this purpose take some lampblack ground in turps, and mix with it a little gold size sufficient (found by trial) to prevent the lampblack from rubbing off when dry, but not enough to cause the paint to dry s.h.i.+ny.

[4] A vignetter for the purpose, as shown in Fig. 60, has been placed on the market.

A good distance to fix the vignetter is about one-third the extension of the camera when the object is in focus, measuring from the lens.

We adjust the camera so that the image of the figure falls in the correct position on the screen, and the vignette is made of such a size and shape as to give the amount required.

The shadow of the mask protects the edges of the plate surrounding the image, and in development we obtain a negative in which the image is vignetted into clear gla.s.s, and on printing from such the margins print dark. The printing of such a negative should be prolonged until the margins of the picture are quite lost, or they are apt to show after toning.

The sketch shows the arrangement of vignetter inside camera.

The other plan consists in making an ordinary negative, using preferably a dark background. From this is made a vignette in the ordinary manner. When this comes from the frame it is placed on a piece of clean gla.s.s--face up--and another piece of gla.s.s free from flaws placed over it. Now cut a piece of card to the size and shape of the vignetted portion of the print, and fix this with glue to a piece of cork. This piece of cork must vary in thickness with various pictures. Now place the cork on the gla.s.s so that the mask covers the picture and fix with glue to prevent slipping. Place the whole out in diffused light, and allow the darkening of the margins to go on until sufficiently deep. The print is then toned.

The height of the card from the print must be such that no abrupt line is produced between the first printing and the darkened margin, but that one will shade into the other without break.