PHOTOGRAPHIC NOTES AND QUERIES.
Pyrogallic Acid (Vol. vi., p. 612.).—In answer to the Query of your correspondent E. S., I beg to give the following method of preparing pyrogallic acid (first published by Dr. Stenhouse), which I have tried and found perfectly successful.
Make a strong aqueous infusion of powdered galls; pour it off from the undissolved residue, and carefully evaporate to dryness by a gentle heat: towards the conclusion of the process the extract is very liable to burn; this is best prevented by continued stirring with a glass or porcelain spatula. Next, procure a flat-bottomed iron pan, about ten inches diameter and five inches deep. Make a hat of cartridge paper pasted together, about seven inches high, to slip over and accurately fit the top of the iron pan. Strew the bottom of the pan with the gall extract to the depth of three-quarters of an inch; over the top stretch and tie a piece of bibulous paper pierced with numerous pin-holes; over this place the hat, and tie it also tightly round the top of the pan.
The whole apparatus is now to be placed in a sand-bath, and heat cautiously applied. It is convenient to place a glass thermometer in the sand-bath as near the iron pan as possible. The heat is to be continued about an hour, and to be kept as near 420° Fah. as possible; on no account is it to exceed 450°. The vapour of the acid condenses in the hat, and the crystals are prevented from falling back into the pan by the bibulous paper diaphragm. When it is supposed that the whole of the acid is sublimed, the strings are to be untied, and the hat and diaphragm cautiously taken off together; the crystals will be found in considerable quantity, and should be removed into a stoppered bottle; they should be very brilliant and perfectly white; if there is any yellow tinge, the heat has been too great.
I believe that close attention to the above details will ensure success to any one who chooses to try the process, but at the same time I must remind your correspondents that scarcely any operation in chemistry is perfectly successful the first time of trial.
J. G. H.
Clapham.
Stereoscopic Pictures with One Camera (Vol. vi., p. 587.).—In reply to the inquiry of Ramus, allow me to say the matter is not difficult. My plan is as follows:—Suppose a piece of still-life to be the subject. Set up the camera at such a distance as will give a picture of the size intended, suppose it sixteen feet from the principal and central object; by means of a measuring tape or a piece of string, measure the exact distance from the principal object to the front of the camera. Take and complete the first picture; if it prove successful, remove the camera about two feet either to the right or left of its first station (i.e. according to the judgment formed as to which will afford the most artistic view of the subject), taking care by help of the tape or string to preserve the same distance between the principal object and the camera, and that the adjustment of focus is not disturbed. In other words, the camera must be moved to another part of the arc of a circle, of which the principal object is the centre, and the measured distance the radius. If the arc through which the camera is moved to its second station be too large, the stereoscopic picture will be unnaturally and unpleasingly distorted. The second picture is now to be taken.
If the subject be a sitter, it is of the utmost importance to proceed as quickly as possible, as the identical position must be retained movelessly till both pictures are completed. This (in my experience) is scarcely practicable with collodion pictures, unless by the aid of an assistant and two levelled developing-stands in the dark closet; for the time occupied by starting the first picture on its development, and preparing the second glass plate (scarcely less than three or four minutes), will be a heavy tax on the quiescent powers of the sitter. This difficulty is avoided by adopting the Daguerreotype process, as the plates can be prepared beforehand, and need not be developed before both pictures are taken. In this case the only delay between the pictures is in the shifting the position of the camera. This is readily done by providing a table of suitable height (instead of the ordinary tripod), on which an arc of a circle is painted, having for its centre the place of the sitter. If the sitter be at the distance of eleven or twelve feet (my usual distance with a 3¼ inch Voightlander), the camera need not be moved more than ten or twelve inches; and even this distance produces some visible distortion to an accurate observer.
The second levelling stand is required when using the collodion process, because the second
picture will be ready for development before the developing and fixing of the first has set its stand at liberty.
Cokely.
Mr. Crookes' Wax-paper Process (Vol. vi., p. 613.).—R. E. wishes to know the exact meaning of the sentence, "With the addition of as much free iodine as will give it a sherry colour." After adding the iodide of potassium to the water, a small quantity of iodine (this can be proctored at any operative chemist's) is to be dissolved in the mixture until it be of the proper colour.
The paper is decidedly more sensitive if exposed wet, but it should not be washed; and I think it is advisable to have a double quantity of nitrate of silver in the exciting bath. I have not yet tried any other salt than iodide of potassium for the first bath; but I hope before the summer to lay before your readers a simpler, and I think superior wax-paper process, upon which I am at present experimenting.
William Crookes.
Hammersmith.
P.S.—I see that in the tables R. E. has given, he has nearly doubled the strength of my iodine bath. It should be twenty-four grains to the ounce, instead of forty-four; and he has entirely left out the iodine.
India Rubber a Substitute for Yellow Glass.—I think that I have made a discovery which may be useful to photographers. It is known that some kinds of yellow glass effectually obstruct the passage of the chemical rays, and that other kinds do not, according to the manner in which the glass is prepared.
I have never heard or read of India rubber being used for this purpose; but I believe it will be found perfectly efficient, and will therefore state how I arrived at this conclusion.
Having occasion to remove a slate from the side of my roof, to make an opening for my camera, I thought of a sheet of India rubber to supply the place of the slate, and thus obtain a flexible waterproof covering to exclude the wet, and to open and shut at pleasure. This succeeded admirably, but I found that I had also obtained a deep rich yellow window, which perfectly lighted a large closet, previously quite dark, and in which for the last ten days I have excited and developed the most sensitive iodized collodion on glass. I therefore simply announce the fact, as it may be of some importance, if verified by others and by further experiment. I have not yet tested it with a lens and the solution of sulphite of quinine, as I wished the sun to shine on the sheet of India rubber at the time, which would decide the question. However, sheet India rubber can be obtained of any size and thickness required: mine is about one-sixteenth of an inch thick, and one foot square; and the advantages over glass would be great in some cases, especially for a dark tent in the open air, as any amount of light might be obtained by stitching a sheet of India rubber into the side, which would fold up without injury. It is possible that gutta percha windows would answer the same purpose.
H. Y. W. N.
Brompton.
Dr. Diamond's Paper Processes.—We have been requested to call attention to, and to correct several errors of the press overlooked by us in Dr. Diamond's article, in the hurry of preparing our enlarged Number (No. 166.). The most important is in the account of the exciting fluid,—the omission, at p. 21. col. 1. l. 47. (after directions to take one drachm of aceto-nitrate of silver), of the words "one drachm of saturated solution of gallic acid." The passage should run thus: "Of this solution take one drachm, and one drachm of saturated solution of gallic acid, and add to it two ounces and a half of distilled water."
In the same page, col. 2. l. 13., "solvent" should be "saturated;" and in the same article, passim, "hyposulphate" should be "hyposulphite," and "solarise" should be "solarize."