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The photographic news
- Bandzählung
- 12.1868
- Erscheinungsdatum
- 1868
- Sprache
- Englisch
- Signatur
- F 135
- Vorlage
- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- Hochschule für Grafik und Buchkunst Leipzig
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- SLUB Dresden
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- Public Domain Mark 1.0
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- urn:nbn:de:bsz:14-db-id1780948042-186800009
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- http://digital.slub-dresden.de/id1780948042-18680000
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- oai:de:slub-dresden:db:id-1780948042-18680000
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- LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 529, October 23, 1868
- Digitalisat
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Zeitschrift
The photographic news
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Band
Band 12.1868
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- Titelblatt Titelblatt I
- Kapitel Preface III
- Ausgabe Ausgabe 1
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- Register The Index To Volume XII 619
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Band
Band 12.1868
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- The photographic news
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[Octoeek 23, 1868. of the swing-back, for, in tilting the camera, you will observe, in practice, that the ground-glass, through its whole extent, has to be drawn back a distance equal to the distance at the top which was swung. This causes the con jugate focal length to be slightly greater when the camera is tilted than when the camera is kept level and the ground- glass is swung back; and this little difference produces a disadvantage for the tilting. When the camera is horizontal, the picture is not accu rately in focus, but an approximation to sharpness may be obtained by what is called equalization of focus; that is, by focussing more upon the middle or above the middle parts of the steeple, when both the top and bottom will be nearly in focus. Now let us examine the results of sharp focussing in the two cases of swinging and tilting. In optics it is a well-known fact that the size of the pic ture of an object varies inversely as its distances from a lens ; that is, the greater the distance of the object from the lens, the less the picture. Let this principle be examined in reference to the space on the steeple situated between two parallel lines; since the top of the steeple is more remote from the lens than the point D, the picture of this top space will be narrower than that of the space at D ; that is, the parallel lines must converge towards the top. Hence, in both cases (of tilting and swinging) all parallel lines of objects higher than the lens converge upwards ; and, if the objects are lower than the lens, downwards. But the con vergence is slightly greater by tilting than by swinging the back, owing to the slight difference in the absolute con jugate focal length in the former above alluded to. The consequence of this convergence is that the steeple will appear to be the irustrum of a pyramid ; and if you focus on the point D, the parallel lines of the steeple will con verge in the picture from d to c, and from d to a in both directions, up and down. This will cause the lines of the picture to be barrel-shaped, or, still better, like the sides of a lamp-chimney. This conversion of parallel lines into •convergent lines can be avoided, to a great extent, bv levelling the camera, both in the gallery and in the field, and by equalizing the focus. You now know the facts of the three cases; draw your ■own conclusions, and work accordingly. In this exposition there is no condemnation ; I sometimes avail myself of each case, but I prefer the horizontal when it is feasible. Oorrespoueuce. CLEANING OLD PLATES. Srn,—Many suggestions have appeared in that part of the PiorocnAPItIC News devoted to correspondence, respecting the cleaning of plates, and in removing the varnish and collo- .lion film from all negatives. I have read carefully all that has been stud at various times by your contributors on this subject, and have tried many experiments tending to solve this difficulty. If the method 1 adopt and hero explain is not a complete solution, it is, at all events, superior to any that has hitherto crossed my observation, for its simplicity, inexpensive- ness, and effect. I use Gray’s solution—or, what is commonly called by painters, liquid wash—adding clean water in equal proportion. Let the plates remain immersed in this for twenty-four hours or more. In taking the plates out, bo careful to use a flat stick, or the finger-nails will be marked as though nitre of silver had been used. Have clean water at hand, or flow from a tap, to clear the plates from all superfluous wash ; be pre pared with a pad of cotton or wool to rub both sides of the plate, particularly the edges; half-a-dozen changes of clean water, slightly rubbing each time, will complete the process. Plates that have been considered useless through the presence of paint, varnish, grease, &c., I have thoroughly restored by the above simple method. I would also add that the liquid wash alone will remove varnish films in three or four hours. During two days of the past week I have cleaned upwards of two hundred plates, varying in size from quarter-plates io 10 by 8’s, in order to thoroughly test this process, which I 1ow present to your readers. PS.—Many thanks to Mr. Bovey for his valuable recommen dation of “ sugar in the printing bath for the preservation of sensitive paper.” Enclosed are four specimens of the paper, a week and a fortnight old. No. 1 was in the bath forty-eight hours; 2. 3, and 4 were upwards of twenty-four hours each. To Mr. Nelson K. Cherrill I am also indebted for llis informa tion on fixing and toning at onco. The specimens enclosed aro sent as evidence of the complete success of the above-named gentleman’s communication. Remarks as to tone and the white will oblige,—Yours obediently, S. GARSIDE. Church Street, Ambleside. [The prints enclosed aro excellent.—Ed.] AMOUNT OF SALT USED IN ALBUMINIZING PAPER. Sir,—At the North London Association I made a sugges tion which, with your permission, I will give in the words as delivered, viz.: “ I do not think that any manufacturer of albuminized paper—when asked—over objects to give an approximate idea as to the amount of salt in his paper; but when photographers are so very anxious to ascertain the amount of salt in the paper, they can soak the albumen and salt from a quarter sheet, and precipitate with a standard solution of nitrate of silver.” By a standard solution I mean (in speaking to a photographic public) any known strength- say the floating strengths of 40, 50, 60, 70, or 80 grains per ounce, as the case may bo. Then, with a minim measure, how easy it will bo to find the number of grains of nitrate it will take to satisfy a sheet, and so calculate the quantity required for the daily consumption, where proper instruments aro not employed for the purpose I This I take to be the aim of the few who now and then bring up the subject. If they desire further information, pray let me suggest that they commence at the first of their pro cesses, and ask the collodion manufacturers the particular amounts of iodides, bromides, &c., with their respective bases, and particularly the preparation of the cotton and solvents. Possibly this would aid those who produce indifferent nega tives about as much as the salting of the paper would those who get but indifferent prints, while others, using the same materials, produce the finest results without any trouble. In the report of the meeting in your last I am made to sug gest testing the paper with a standard solution of chloride of sodium. This is clearly a slip of the pen, but ought to bo corrected. Had I accidentally made such a lapsus lingua, it would, doubtless, have been at once corrected in the meeting. Yours, &c., F. W. Hart. 8, Kingsland Green. [The misprint is better corrected, although no one would dream of attempting to precipitate common salt with common salt. Wo have a few words on the general subject in another column.—Ed.] Sir,—-I notice that at the last North London mooting Mr. Hart recommended photographers who wore anxious to know the proportion of salt in albuminized paper to dissolve the salt and albumen oft a quarter of a sheet, and precipitate it with a standard solution of nitrate of silver, I presume, although chloride of sodium, evidently by some error, appears in the report. I wish to ask Mr. Hart how many photographers ho thinks aro prepared with apparatus for such an operation ? How many could conduct it with sufficient delicacy and care to bo ot any service? And, finally, if it is not a little unfair to ask tho photographer to take this trouble to ascertain a fact which tho albuminizer should bo able and willing to give him without any trouble at ail ? I cannot understand why it is that albu- minizers seem to placo themselves in antagonism to photo graphers on this point. The request that manufacturers should give some idea of tho proportion of salt their paper contains has often been made, but has never brought any systematic | compliance with tho request. It has, on the other hand, so often evoked a querulous protest against tho necessity, that one | is tempted to believe some occult reason exists for tho hnbitua
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