Multiplying Negatives.—In reply to M. N. S. (Vol. ix., p. 83.) I would suggest the following mode of multiplying negatives on glass, which I have every reason to believe would be perfectly successful:—First, varnish the negative to be copied by means of Dr. Diamond's solution of amber in chloroform; then attach to each angle, with any convenient varnish, a small piece of writing-paper. Prepare a similar plate of glass with collodion, and drain off all superfluous nitrate of silver, by standing it for a minute or so on edge upon a piece of blotting-paper. Lay it flat upon a board, collodion side upwards, and the negative prepared above upon it, collodion side downwards. Expose the whole to daylight for a single second, or to gas-light for about a minute, and develope as usual. The result will be a transmitted positive, but with reversed sides; and from this, when varnished and treated as the original negative, any number of negatives similar to the first may be produced.
The paper at the angles is to prevent the absolute contact and consequent injury by the solution of nitrate of silver; and, for the same reason, it is advisable not to attempt to print until the primary negative is varnished, as, with all one's care, sometimes the nitrate will come in contact and produce spots, if the varnishing has been omitted. Should the negative become moistened, it should be at once washed with a gentle stream of water and dried.
I have repeatedly performed the operation above described so far as the production of the positive, and so perfect is the impression that I see no reason why the second negative should be at all distinguishable from the original.
I am, indeed, at present engaged upon a similar attempt; but there are several other difficulties in my way: I, however, entertain no doubts of perfect success.
Geo. Shadbolt.
Towgood's Paper.—A. B. (Vol. ix., p. 83.) can purchase Towgood's paper of Mr. Sandford, who frequently advertises in "N. & Q." With regard to his other Query, I think there can be no doubt of his being at liberty to publish a photographic copy of a portrait, Mr. Fox Talbot having reserved only the right to paper copies of a photographic portrait. Collodion portraits are not patent, but the paper proofs from collodion negatives are.
Geo. Shadbolt.
Adulteration of Nitrate of Silver.—Will any of your chemical readers tell me how I am to know if nitrate of silver is pure, and how to detect the adulteration? If so with nitrate of potash, how? One writer on photography recommends the fused, as then the excess of nitric acid is got rid of. Another says the fused nitrate is nearly always adulterated. I fear you have more querists than respondents. I have looked carefully for a reply to some former Queries respecting Mr. Crookes's restoration of old collodion, but at present they have failed in appearance.
The Reader of Photographic Works.