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The photographic news
- Bandzählung
- 24.1880
- Erscheinungsdatum
- 1880
- Sprache
- Englisch
- Signatur
- F 135
- Vorlage
- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- Hochschule für Grafik und Buchkunst Leipzig
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- urn:nbn:de:bsz:14-db-id1780948042-188000001
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- http://digital.slub-dresden.de/id1780948042-18800000
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- Bandzählung
- No. 1139, July 2, 1880
- Digitalisat
- SLUB Dresden
- Strukturtyp
- Ausgabe
- Parlamentsperiode
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- Wahlperiode
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Zeitschrift
The photographic news
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Band
Band 24.1880
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- Register Index 631
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Band
Band 24.1880
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- Titel
- The photographic news
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July 2, 1880.J THE PHOTOGRAPHIC NEWS. 321 the above described position, the point C will be on that line in the plan. The same can be done from 13, and consequently the intersection of these two lines from A and 13 determines the position on the plan of point C. To make the method theoretically correct it would be necessary in all cases to take vertical projections of the points on a horizontal line, and not the points themselves on the photograph. We have thus the means of determining the position of any object which can be seen in two photo graphs, and in this manner all the roads, rivers, houses, &c., might be drawn on the map, and the work completea. The first question asked by a photographer undertaking such work would be, What lens shall I use—a long focus lens, or a wide angle lens ? If a wide angle lens is used, the distance looks so small that different points could not well be distinguished and recognised in two photographs ; but if a long focus lens is used, it may be necessary to take as many as ten photographs at each station, in order to get the complete panoramic view. As rapidity is one of the chief points claimed for surveying by photo graphy, a wide angle lens must be chosen, when the panorama might be completed in five photographs. Besides which, it must be remembered that three fixed points are wanted in each photograph, and in order to get as many as thirty points visible round each of the photographic stations, the work of the triangulation would have to be vastly increased ; to get fifteen clearly visible stations is quite difficult enough. Hence we are driven to use a wide angle lens, notwithstanding its disadvantages. Not only must many of the stations themselves be clearly distinguishable, but the ground in front must be com manded, so that every unfixed point must be visible from two of these photographic stations. How if there are no hills or natural elevations? This question has received no satisfactory answer, and it may be assumed, we think, that the plan would fail utterly in a flat country; even in a fairly hilly country, if much cut up with hedges, trees, and woods, it will be very difficult to obtain all the detail without an immense number of photographs. But even if the country is all that could be desired, the difficulties to be encountered by the photographer will be very great. The surveyor can work in nearly all weathers, and if the photographer is to help quicken the work, ho must be prepared to go out whenever required. But it must not blow too hard, or in the exposed situations in which the stations must necessarily be placed the camera is sure to shake; on the other hand, if it is a hot, still, hazy, summer day he will find it very difficult to photograph any dis tant object. It must bo a bright day to get a good picture, but bright sunshine will make it almost impossible to take a photograph in the direction of the sun ; if the sun is very high this difficulty will not be so much felt; and in any case it can be overcome by visiting the position at intervals of some hours ; but how about the rapidity of the process in that case ? There must be no mist, no haze, no smoke ; a very little smoke drifting to the lee side of a town or manufactory makes it curiously difficult to obtain a photograph of the distance. These are the chief difficulties that the photographer has to contend with—difficulties enough to render any work of this sort impossible during an English winter. But let us suppose that he has had exceptional luck, and has succeeded in obtaining a perfect series of panoramic views. Ho hands prints from these negatives to the surveyor whose duty it would be to com plete the map from these materials; the photographer had a difficult job, but we think the surveyor will have a still •worse time of it. Supposing that there is a river winding in front of our conveniently situated bills, and that the first effort is to map such a readily recognised object as this. It is not enough to seo the river on two photographs, but we must be able to recognise the same spot on the bank, or bend in the stream, in both pictures ; but from two points of view the same point may present a totally different appearance, and it is only those who have actually tried to recognise an object of any sort in different pictures that will be able to thoroughly appreciate the difficulty. The same difficulty will be found in plotting almost every natural object, and in consequence it will be found almost impossible to complete the map by this means. In all methods of surveying, as far as we are aware, the surveyor visits almost every point which is to be marked on the map, but in this place an attempt is made to put in the detail from some little distance, and the consideration of this fundamental difference would alone be enough to condemn the plan. We have only been considering this one plan of photo graphic survey in which the ordinary camera is used. There is the Chevalier plane-table, no doubt known to many, which accomplishes much, while other mechanical contrivances have been proposed by which the panorama is so taken that angles can at once be read off, thus rendering the preliminary triangulation unnecessary. This of course removes some of the objections that we have pointed out, but our readers will readily see that many of the difficulties remain in full force; strong enough, we con sider, to prevent any plan of photographic aid to surveying being practically useful. The practical test of any system is the only one ofmuch value, and until wo see amap which has been produced with photographic assistance we must be excused from thinking that all such plans are failures. The “ Topic ” for next week will be “ Mounting Photo graphs,’" by Payne Jennings. COLD EMULSIFICATION. BY H. BRUYBRE. There seems to have been but little attention paid to obtaining a sensitive gelatine emulsion by the means of cold emulsifying. Considering the ease of preparation, the avoidance of those evils, attributed, perhaps erroneously, to a prolonged heating of the gelatine, these advantages com bined with the facility of obtaining different degrees of rapidity, and with the certainty of easily reproducing them, seem to indicate that this method is deserving of more attention than is at present bestowed upon it. The only objection the writer believed might arise, would be in a want of sensitiveness, but he was somewhat gratified to fine that after six days’ emulsifying the plates vied in rapiditd with the quickest in the market. Perhaps the time necessary for the attainment of this sensitive condition might be corny mercially objectionable. A cold emulsification may b- formed either in a diluted glycerin, or in a solution of gumo arabic ; the writer prefers the latter. Subjoined is a bu- slightly altered formula of Mawdsley’s :— Distilled water ... ... ... 4 ounces Gum-arabic ’... 120 grains Ammonium bromide ... ... 80 ,, Place the ammonium bromide carefully in a stoppered bottle, pour on the water, then add the gum, agitating occasionally till dissolved. No fear need be entertained of the gum bromide solution becoming acid, having kept several batches of emulsion for a lengthened period, without perceiving any decomposition. This seemed to verify a sus picion that all the haloid salts are anti-putrescent, the cheapness of the sodium chloride determinating its use. When the gum is dissolved, 125 grains silver nitrate are dissolved in two ounces of distilled water, and added "by degrees to the gum solution, shaking vigorously between each addition. This formula gives a little over one grain of bromide per ounce in excess. Of course this last operation must be performed by a perfectly non-actinic light. No more trouble need be taken with the emulsion, except to most jealously guard it from the weakest actinic ray.
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