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Paper Prints (Collodion which otherwise)

of collodion, which otherwise might be washed on to the image by the varnish, and produce a spot.

To Adjust Camera for Chemical Focus.

Take a photograph of a printed sheet with the full aperture of the portrait lens, the central letters being carefully focussed as before. Then examine at what part of the plate the greatest amount of distinctness of outline is to be found. It will, sometimes, happen that whereas the exact centre was focussed visually, the letters on a spot midway between the centre and edge are the sharpest in the photograph. In that case the chemical focus is longer than the other, and by a distance equivalent to, but in the opposite direction of, the space through which the lens has to be moved, in order to define those particulars sharply to the eye. Direct Positives on Glass.

In developing a glass positive, the solution of sulphate of iron should be flowed evenly over the film, and in some quantity, so as to wash off a portion of nitrate of silver into the sink.

Pyrogallic Acid Developer.

The Forty grs. of pyrogallic acid being dissolved in 1 oz. of glacial acetic acid, and 20 minims added to 1 oz. of water when required for use. strong solution becomes black from decomposition in the course of a few weeks, but when diluted it has only a faint yellow tint, and is tolerably effective in bringing out the image. In place of acetic acid strong alcohol may be used as a solvent, a dr. of pyrogallic acid being dissolved in 1 oz. of spirit, and 20 minims of the resulting liquid added to each oz. of acidified water. No attempt must be made, however, to combine acetic acid and spirit in one solution, since abundance of acetic ether would be generated by so doing. These plans of preparing a concentrated developer are useful for a few weeks' keeping, but are not recommended for an unlimited time. To Copy Engravings.

To photograph a full-sized steel engraving on a plate not larger than 7×6 or 5X4, is a very simple operation, and no special directions will be needed. Remove the engraving from its frame (the glass would cause irregular reflection), and suspend it vertically and in a reversed condition, in a good diffused light, placing a black cloth behind it, if any bright reflecting surface be presented to the lens. Point at it a camera mounted with a portrait-lens, and if the image upon the ground glass appears misty towards the edge, make a diaphragm, and place it in front of the anterior glass. Instead of a portrait-lens an ordinary view-lens may be used, and as the field to be covered is small, a comparatively large diaphragm will suffice. Be careful not to over-expose the plate, develop with pyrogallic acid, and fix with hyposulphite.

Copying Prints.

The additional deposit obtained, as above described, will often be found sufficient, and when such is the case the method is to be preferred, because the half-tones of a photograph are easily obliterated by too much intensifying. In the case of large copies of maps taken with orthoscopic lenses of long focus, the iodine method proves insufficient, and the fine lines of the drawing become partly obliterated during the prolonged development. In such a case it is advised to develop the plate only partially in the first instance, and to fix it with cyanide of potassium; then to intensify twice with pyrogallic acid, citric acid, and nitrate of silver, and lastly to treat the plate with the two following liquids: No 1. Iodine, 6 grs.; iodide of potassium, 12 grs.; water, 6 oz. No. 2. Sulphide of potassium, 1 dr.; water, 6 oz.

Apply No. 1, either in the yellow room, or in the daylight, until the whole of the image is converted into iodide of silver, and the deposit appears yellow throughout. Then pour water on the image from a jug, and apply No. 2, which must be allowed to remain until the yellow color changes to a deep reddish-brown. Lastly, dry the plate, and varnish in the usual way.

Stereoscopic Pictures.

Photographs for the refracting stereoscope are taken with small lenses of about 44 inches focus.

For portraits a camera may advantageously be fitted with two double-combination lenses, of 1 inches diameter, exactly equal in focal length and in rapidity of action. The caps are removed simultaneously, and the pictures impressed at the same instant. The centres of the lenses may be separated by 3 inches, when the camera is placed at about 6 or 8 feet from the sitter. Pictures taken with binocular camera of this kind require to be mounted in a reversed position to that which they occupy on the glass, or a pseudoscopic effect will be produced. The negatives may be cut in half, the right half being printed on the left side, or the finished prints may be removed before mounting.

Stereoscopic Transparencies.

Sensitive films prepared by Russell's Tannin Process. Place the negative and the prepared plate in contact, and squeeze them together in an ordinary pressure-frame; not too strongly, however, or it will probably be found, after throwing off a few impressions, that the negative has been scratched. Lay a strip of black velvet behind the sensitive film to absorb stray light.

Fixing Bath.

Take of hyposulphite of soda, 1 oz.; water, 6 oz.

Dissolve without filtering, and preserve the solution in a stock-bottle ready for use.