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.