PHOTOGRAPHIC CORRESPONDENCE.
Washing Collodion Pictures—Test for Lens.—As I was indebted to the kindness of Dr. Diamond, amongst other friends, for my original initiation into the mysteries of photography, it may appear somewhat presumptuous in me to differ from one who has had so much more experience in a point of practice. I allude to that of washing the collodion negative after developing, previously to fixing with the hyposulphite of soda; but, probably, the reasons I urge may have some weight. As the hyposulphite solution is intended to be used repeatedly, it appears to me not advisable to introduce into it any free acid (which must occur if the negative be not washed, although the quantity at each operation may be small), because it causes a decomposition of the salt, setting free sulphurous acid, and also sulphur; which last is slightly soluble in the hyposulphite of soda, and thus the sulphur is brought in contact with the reduced silver, and forms a sulphuret of that metal. But the change does not stop here: for, by the lapse of time, oxygen is absorbed, and thus a sulphate of silver is formed, and the colour changed from black to white. That sulphur is set free by the addition of an acid to the solution of hyposulphite of soda, is fact so easily demonstrable both to the eyes and nose of the operator, that no one need remain long in doubt who is desirous of trying the experiment.
A correspondent desires to know how to test the coincidence or otherwise of the visual and actinic foci of a combination: this is very readily accomplished by the aid of a focimeter, which can be easily made thus:
Procure a piece of stout card-board, or thin wood covered with white paper, on which draw a considerable number of fine black lines, or cover it with some fine black net (what I believe the ladies call blond), which may be pasted on. Cut up the whole into a dozen good-sized pieces of any convenient form, so that about four square inches of surface at least be allowed to each piece. Paste over the net a circular or square label about the size of a shilling, bearing a distinctly printed number one on each piece, from 1 upwards; and arrange the pieces in any convenient manner by means of wires inserted into a slip of wood; but they must be so placed that the whole can be seen from one point of view, although each piece must be placed so that it is one inch farther from the operator than the next lowest number. Having placed the camera eight or ten feet from the cards, carefully focus to any one of the numbers, 4 or 5 for instance and observe, not that the number is distinct, but that the minute lines or threads of the net are visible: then take a picture, exposing it a very short time, and the threads of the card bearing the number that was most perfectly in focus visually ought to be most distinct; but, if otherwise, that which is most distinct will not only show whether the lens is over or under corrected, but will indicate the amount of error. If under corrected, a lower number will be most distinct; if over corrected, a higher.
Geo. Shadbolt.
Test for Lenses.—I beg to submit to a Country Practitioner the following very simple test for the coincidence of the chemical and visual foci of an achromatic lens:
Take a common hand-bill or other sheet of printed paper, and having stretched it on a board, place it before the lens in an oblique position, so that the plane of the board may make an angle with a vertical plane of about thirty or forty degrees. Bring any line of type about the middle of the sheet into the true visual focus, and take a copy of the sheet by collodion or otherwise. Then, if the line of type focussed upon be reproduced clearly and sharply on the plate, the lens is correct; but if any other line be found sharper than the test one, the foci disagree; and the amount of error will depend on the distance of the two lines of type one from the other on the hand-bill.
J. A. Miles.
Fakenham, Norfolk.
Improvement in Positives.—I have great pleasure in communicating to you an improvement in the process of taking positives, which may not be uninteresting to some of your readers, and which ensures by far the most beautiful tints I have yet seen. I take three ounces of the hyposulphite of soda, and dissolve it in one pint of distilled or rain water; and to this I add about one or one and a half grains of pyrogallic acid, and seventy grains
of chloride of silver; which must be squeezed up between the finders facilitate its solution and separate the lumps, which, in their dry state, are tough, and not easily pulverised. The whole is then to be set aside for a week or two in a warm place. The solution, at first colourless, becomes brown, and ultimately quite opaque; in this state it is fit for use, and the longer kept the better it becomes. I generally use French paper for this process, and, according to the time of immersion, obtain fine sepia or black tints; the latter requiring long over-exposure to the light, and proportionately long exposure to the action of the liquid; which however will be found, particularly when old, to have a more rapid action than most other setting liquids, and has the merit of always affording fine tints, whatever the paper used. I imagine the pyrogallic acid to possess a reducing influence on the salts of silver employed; but this effect is only produced by its combination with the hyposulphite of soda and chloride of silver. I may add, that in any case the pictures should be much overdone before immersion, as the liquid exerts a rapid bleaching action on them; and when the liquid becomes saturated, a few crystals of fresh hyposulphite will renew its action.
F. Maxwell Lyte.
Florian, Torquay.
P. S.—In answer to a Country Practitioner, he will find great assistance in choosing his lens by laying it on a sheet of blue wove post paper, when he will immediately perceive the slightest yellow tinge in the glass, this being the fault which frequently affects many well-ground and well-made lenses. Of course, for sharpness of outline he must be guided entirely by experiment in the camera; but where weakness of action exists, it most frequently arises from this yellow colouration, and which the manufacturers say is very difficult to avoid.
[Mr. Lyte having sent with his communication a positive prepared in the manner described, we are enabled to corroborate all he says as to the richness and beauty of its tints.]
Cheap Portable Tent.—M. F. M. inquires for a cheap and portable tent for working collodion out of doors. I have been using one lately constructed on the principle of Francis's camera stand. It has a good size table, made like the rolling patent shutters; and it is not necessary to stoop, or sit down at your work, which is a great consideration on a hot day: you may get them of any respectable dealer in photographic apparatus; it is called Francis's Collodion Tent.
H. D. Francis.
Rev. Mr. Sisson's New Developing Fluid (Vol. vii., p. 462.).—The Rev. Mr. Sisson's developing fluid for collodion positives, the formula for which was published in the last Number of "N. & Q.," is merely a weak solution of the protonitrate and protosulphate of iron. It does not, as he seems to think, contain any lead; for the whole of the latter is precipitated as sulphate, which the acetic acid does not dissolve even to the smallest extent: and Mr. Sisson will find that an equivalent proportion of the nitrate of baryta will answer equally as well as the nitrate of lead.
I have myself for a long time been in the habit of using a weak solution of the protonitrate of iron in conjunction with acetic acid for positive pictures; for, although I do not consider it so good a developer as that made according to the formula of Dr. Diamond, it produces very good pictures; occupies very little time in preparing, and will moreover keep good for a much longer time than a more concentrated solution would.
J. Leachman.
20. Compton Terrace, Islington.