Excite for Positives.—Excite with No. 1.; blot off: lay it in your press, place the negative face downwards: expose to the light from ten seconds to half a minute, or more, according to the light (not in the sun), and bring out with No. 3.; and when it is nearly developed add No. 1.; then take it up and pour water upon it, and then place it in hyposulphite of soda (cold) until the iodine is removed; after which put it into allum water, about half a teaspoonful of powdered allum in two quarts of water; this will readily remove the hyposulphite, and also fix the positive more particularly; it will also take away any impurities which there may be in the paper, after which put it into clean cold water, and change two or three times.
I have been thus particular in describing the process which I have adopted, more especially for beginners; and with great cleanliness and care in each process, and especially in keeping all the bottles with the chemicals free from dirt of every kind, the foregoing will lead to favourable results.
W. J. Newton.
I have been making some experiments in preparing the iodized paper in the following manner, more especially in consideration of the present price of iodide of potassium:—60 grains of nitrate of silver; 60 ditto of iodide of potassium, cleaned and prepared as before described, by the addition of three ounces of water,—that is 3 oz. altogether; 60 grains of cyanide of potassium; add a little of this at a time, and shake it up; and I generally find that this quantity is sufficient to redissolve the 60 grains of iodide of silver. Brush the paper over with the above, and when the wet surface disappears, dip it into cold water containing one drachm of dilute sulphuric acid to one quart of water; and then into water for half an hour, changing the water once: pin up to dry. I have not had an opportunity of trying this for negatives, but I have taken some good positives with the paper so prepared.
N.B.—I find that if the paper is allowed to dry with the cyanide of potassium, or that it is allowed to remain in the dilute sulphuric acid water too long, it weakens the paper so much as to be very absorbent. I would therefore wish to know from any of your correspondents whether this arises from taking away the size, or injuring the fibres of the paper? and, if so, whether a paper prepared with starch, instead of size, would be better? as it appears to me that this mode of iodizing might be an improvement. At all events, it is an enormous saving of iodide of potassium; as, for instance, to redissolve the 60 grains, it would take 1½ oz. of iodide of potassium (about four shillings); whereas 60 grains of cyanide would not cost more than one penny or twopence.
W. J. N.
Collodion Film on Copper Plates.—Would any of your correspondents kindly describe the manner in which the collodion film may be transferred to prepared copper plates?
It was noticed by your correspondent H. W. D. in Vol. vi., p. 470.
J. M. S.
Treatment of the Paper Positive after fixing.—1. Is it absolutely necessary for the preservation of the picture, that the size should be wholly removed from the paper? It seems to me that the hot-water treatment materially injures the tone.