Mr. Wilkinson's Mode of levelling Cameras.—As you have done me the honour to notice my simple invention for levelling cameras, which I have since had an opportunity of trying in the open air for a week, and find to succeed perfectly, I wish to correct some errors which appeared in the Photographic Journal, from which you copied my remarks, and which arose from the notes being taken down from my verbal observations. The first part is perfectly correct but after l. 9. col. 2. "N. & Q." (Vol. vii., p. 462.) it should read thus:
"The other perpendicular is then sought for; the back or front of the camera being raised or lowered until the thread cuts the perpendicular lines drawn upon the sides of the camera. By this means a perfectly horizontal plane is obtained, as true as with the best spirit-levels, and in less time. By tying three knots in the silk at twelve inches distance from the one bullet and from each other, we have a measure for stereoscopic pictures; and by making the thread thirty-nine inches and two-tenths long from one bullet to the centre of the other, we obtain a pendulum vibrating seconds, which is useful in talking portraits; as it will continue vibrating for ten minutes, if one bullet be merely hung over any point of suspension."
Thus we obtain a levelling instrument, a chronometer, and a measure of distances, at a cost considerably under one penny.
The above will more fully explain to your correspondent Φ. (Vol. vii., p. 505.) my reasons for the length of thread stated; and with respect to the diagonal lines on the ground glass, it is not material what may be the distance of the principal object, whether six feet or six hundred: for if the cross lines, or any other lines drawn on the glass, cut the central object in the picture at any particular part—for example, the window of any particular house, or the branch of any tree,—then the camera may be removed to higher or lower ground, several feet or inches, to the right or to the left, and the same lines be made to cut the same objects, previously noted; the elevation will then be the same, which completes all that is required.
In most stereoscopic pictures, the distances are too wide. For a portrait, two inches and half to three inches, at nine or twelve feet distant, is enough; and for landscapes much less is required than is generally given, for no very great accuracy is necessary. Three feet, at three hundred yards, is quite enough; and four to six feet, at a mile, will do very well. Let experiment determine: for every photographer must learn his profession or amusement; there is no royal road to be depended on. But a small aperture, a quarter of an inch diameter, may be considered a good practical size for a lens of three and a quarter inches, depending on light and time: the smaller the aperture, the longer the time; and no rules can be given by any one who does not know the size and quality of the lenses employed. Every one can make a few trials for himself, and find it out; which will be more satisfactory than any instructions derived from books or correspondence. I obtain all the information I can from every source, then try, and judge for myself. At worst, you only spoil a few sheets of paper, and gain experience.
I perfectly agree with Dr. Diamond, that it is much better not to wash the collodion pictures after developing; but pour on about one drachm of sat. sol. hypo. at once, and then, when clear, plenty of water; and let water rest on the surface for an hour or more, before setting on edge to dry.
Henry Wilkinson.
Collodion Negative.—Can you inform me how a collodion negative may be made? that is, how you can ensure the negative being always of a dense enough character to print from. This is rarely the case.
F. M.
Developing Collodion Process.—I use to develope my collodion pictures M. Martin's plan, i. e. a solution of common copperas made a little acid with sulphuric acid. This answers very well and gives to the pictures, after they have been exposed an hour or two to the atmosphere, a silver-like appearance: but this copperas solution seems to destroy the glass for using a second time, inasmuch as a haziness is cast upon the glass, and its former enamel seems lost, not to be regained even by using acids. The hyposulphite also seems to be affected by this manner of developing the