Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XXIV.
COLLODION POSITIVES ON GLASS BY TRANSMITTED LIGHT.

Transparent Positives.

THIS kind of picture is used more especially for stereoscopic slides. Its application to church-windows, etc., for which it is so well adapted, has not yet been introduced to any great extent. A transparent positive may be produced either by means of the camera, or by direct contact of the negative. By means of the camera the proceeding is as follows:

In the first place we require a good orthoscopic lens, or, in fact, any lens that will produce with an inserted diaphragm a clear, well-defined picture of a page of print, without distortion of the marginal lines. Ascertain the length of the equal conjugate focus of the lens, that is, half the distance between the object and its image, when these are of the same size. Then construct a square cylinder of thin wood, in which the camera can slide; let the inside be blackened with a solution of ink, laid on twice. At the end in front of the lens, cut out an aperture of the size of the negative, leaving a ledge of three sixteenths of an inch all round on which the negative can rest. Fix the negative by means of a tack or small pin in each corner. It is inverted laterally, that is, the sides have changed places, left being right, and right left; and the collodion side is inwards, or facing the lens. This compound camera is now pointed either to a white cloud, or directly to the sun. Focus the image on the ground glass with great accuracy; it is much more difficult to obtain the right focus in such work than in ordinary portraiture, and a microscope is invariably required to obtain a sharp and correct copy. It facilitates the operation of focussing to find some small point, or mark; or wrinkle, and then to slide the camera in the cylinder backward and forward, until you think yon lave got the sharpest definition, and afterward to make the final adjustment with the microscope. Inasmuch as the lens is within the cylinder, all the focussing leas to be performed by means of the sliding of the camera; and when once the right focus has been found, the cylinder and the camera are firmly fastened; and a mark is made by which at any time afterward the adjustment can be quickly made, without resorting to an independent system of focussing on each occasion when a transparent positive has to be taken.

With the bright rays of the sun, and an orthoscopic lens, probably as much as from one to three minutes' exposure will be required; whereas, with an ordinary well-corrected portrait lens, the time will vary from a quarter of a minute upward. It is supposed, of course, that a small stop is used, so as to obtain a sharp and undistorted picture. With a large diaphragm, naturally a much shorter exposure would be quite sufficient. All the rest of the operation of collodionizing, developing, and fixing is the same as that already described. The picture is developed near the pane of glass which admits light from below. A bright, transparent picture is particularly required in this operation; there must be no fogging, and the shades must be pretty deep and distinct.

Such is a general outline of producing transparent positives on glass, by means of the lens and camera; but there are specialties that demand our attention. One of these refers in particular to the nature of the negative. A bright, transparent, and clear negative, somewhat less opaque in the shadows than for the common printing process on paper, is best adapted for the purpose in question. If a negative had to be specially prepared for producing transparent positives, I would recommend its preparation as above described, only giving a trifling less exposure, and using a slightly stronger developer. The reduction, too, must be stopped the very moment there is the slightest tendency to veiling. Finally after the negative is fired, supposing it to be already sufficiently intense not to require any redevelopment, (which is a very desirable condition,) it is flowed with a solution of iodine in iodide of potassium for a few moments, taking care to keep the fluid in motion; this operation must be very short in duration. Pour off the solution; wash, and again fix with cyanide of potassium. This operation may be appropriately termed the Clarifying Operation,, for the negative becomes quite clear and transparent, from the fact that in those parts where there was a tendency to a veil or fog, the reduced silver that produced it has been converted into iodide of silver, and dissolved by the cyanide in the second fixing. This clarifying operation must be employed with extreme care, lest the minute details might be carried off at the same time. Varnishing, it is true, will also reduce the amount of density in the shadows, but it does not remove any of the fogging, and besides this it increases the opacity of the transparent parts; in short, it tends to diminish contrast. On this account it is preferable not to varnish the negative.

By fixing the negative in the holder with the collodion side next to the lens, the positive collodion picture will be on the right side of the glass, erect and free from lateral inversion. If it were fixed otherwise, then the positive would be on the under side of the glass, and would not appear so brilliant when mounted.

Another specialty to be observed, refers to the color of the positive. The shadows, after reduction with the protosulphate of iron, are grayish or silver-white. For viewing by reflected light, if they were in their proper place, they would be endowed with a very pleasing aspect; but viewed by transmitted light, the contrast is by no means agreeable; the shades are too gray. The object, therefore, is to communicate to them a rich black hue. We effect this by pouring over the film a sufficient quantity of a saturated solution of bichloride of mercury free from acidity. AS soon as the film is black, pour off the mercury, and wash the plate in rain-water.

The next operation is to flow over the plate a saturated solution of cyanide of silver in cyanide of potassium.

Formula No. 1

Nitrate of silver solution, (50 grains to the ounce,) as long as the precipitate is dissolved.

This solution, after filtration, is ready for use. Or a solution of cyanide of copper may be substituted for the silver salt.

Formula No. 2.

Nitrate of copper solution as long as the precipitate is dissolved by shaking. Filter as before, and use.

The image when flowed with either of these menstrua assumes an intense black hue. The solutions can be used over and over again until exhausted.

The plates are now washed carefully and thoroughly, and again fixed with solution of hyposulphite of soda, but not with cyanide of potassium, because it reduces the silver to a white film main. This mode of blackening the silver film may be used also as an intensifier.

When this operation is complete, the plate is washed and dried, also varnished, unless the slide has to be mounted with a glass before it, when the varnishing may be omitted. Previous to mounting,, it may be colored either on the picture side or on the back, by which a very rich effect is produced. When positives are thus colored, they are mounted with a plate of ground glass behind them, and thin transparent glass in front.

For the magic lantern, the slides must be preserved as transparent as possible; consequently no ground glass is used behind. The coloring, too, must be laid on, either before varnishing, or afterward, very lightly and artistically, so as to impede the passage of the light as little as possible.