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
• Digitalisat: SLUB Dresden
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-188000001
• PURL: http://digital.slub-dresden.de/id1780948042-18800000
• OAI: oai:de:slub-dresden:db:id-1780948042-18800000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Bemerkung: Exemplar unvollständig: Seite 1-82 in der Vorlage nicht vorhanden
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Bemerkung: Paginierfehler: S. VI-VII als S. VIII-IX, S. XI-XII als S. X, XX gezählt.
• Strukturtyp: Ausgabe

April 23, 1880.] THE PHOTOGRAPHIC NEWS. 195 their mark oa the delicate pictures. Every negative is edged with black paint, for the double purpose of giving the prints a white margin—especially agreeable in the case of unmounted prints, and Mr. Bedford issues all in this condition—and to economise the toning bath. The deep black edges in an ordinary print, Mr. Bedford avows, run away with as much gold as the picture itself. Finally, the painting of the margin facilitates the marking of the negative with its number. The cracking of the negative film is rarely seen now, and this is attributed by our host to the circumstance that the collodion is better than it used to be. It is the latter, and not the varnish, that is at fault. Mr. Bedford has had negatives closely packed together, and almost unused, which on unpacking have shown defects of this nature, while others freely stacked in boxes have exhibited nothing ’of the kind. An ingenious method of improving the skies in negatives and softening the horizon line, and perhaps adding detail to a foreground, is adopted by Mr. Bedford. His former plan, as many of our readers know, was to cover certain portions of the negative with tracing paper, and work with pencil, stump, or brush upon this. Tracing paper, however, gets yellow and opaque in time, and in any case shows a very decided outline. But by grinding the reverse face of the negative—Mr. Bedford, like a careful photographer, always employs patent plate—by means of a glass muller and emery powder, a surface is secured upon which work of any description can be done. A few free strokes with a brush dipped in Indian ink, or with a pencil, add fre quently to the value of a negative, while the mere grinding of the glass behind the horizon line, whether sea or ridge of hills, tends to soften this portion of the plate consider ably. In a word, there are very few photographers who take such extreme care over their printing as the Messrs. Bedford. The ‘‘At Home” for next week will be "Messrs. W. and D. Downey at Ebury Street.” PRINCIPLES OF OPTICS INVOLVED IN LANTERN CONSTRUCTION; AND ON A NEW ENLARGING LENS, ESPECIALLY DESIGNED FOR USE WITH THE MAGIC LANTERN. BY J. H. DALLMEYER, F.R.A.S., ETC.* To photography must bo assigned a considerable share in pro curing an enlarged field of usefulness for the lantern—the in strument said to have been invented so long ago as the thirteenth century, by Roger Bacon. It is no longer the toy, as in bygone ages, for the raising of spectres, &c., but is daily gaining in favour with the public lecturer, who has discovered in it a ready means for the diffusion of knowledge. However, even in its present state it leaves much to bo desired, for who has not witnessed tie often contorted and ill-defined pictures thrown upon the screen ? The practical application of the law of conjugate foci consti tutes the principle of lantern construction, viz., the object or slide to be enlarged is placed in one of the conjugate foci of the lens, and the imago or screen in the other ; thus, for a larger image, the screen must be removed to a greater distance from, and the slide brought nearer to, the lens, and vice versa. To obtain an enlarged picture of suflicient brilliancy, a lantern comprises three essential elements, viz. 1. A brilliant source of light. The illumination of the picture diminishes in the proportion of the square of the distance of screen or the size of enlarge ment; thus, if at. distance 1 the illumination equals! candle, then at distance 50 it is only 286, of that amount; in other words, if at distance 50 the picture is to appear as bright as it is at distance 1, we require 2,500 caudles in place of 1 for the source of light. A case in point when a 3-inch picture is en larged to 12 feet by means of a 6-inch focus lens, without a condenser. * Read before the Photographic Society of Great Britain. 2. A condenser to concentrate this light upon the picture or slide. 3. The lantern “ front,” or objective, to project an enlarged image upon the screen. Years ago my attention was called to the necessity for a better lantern-lens or objective, by one well known in the annals of photography. I refer to Mr. Latimer Clarke, the inventor of a stereoscopic camera known by his name, who employed the lantern for the purpose of amusing and instructing his children. I well remember being struck by the ingenuity of this gentle man in providing a plaster of Paris screen, made concave, to suit the curvature of image produced by his lens. Though suc cessful, the dimensions of such a screen were necessarily limited; and hence his request for a better lens to project a well-defined image upon a flat screen of larger dimensions. The problem presented for solution is by no means an easy | one, and had it not been for a further incentive to the same I end by another gentleman, still better known in photographic annals—the Rev. F. Hardwich, formerly Professor of Photo- ! graphic Chemistry at King’s College, the subject would, pro- I bably, have remained in abeyance ; at least as far as I am । concerned. The reverend gentleman, as far back as the year 1876, wrote me a first letter urging the importance of an im- ; proved lantern for public lecturers. In subsequent letters, in I response to"my application, he kindly furnished me with all the desiderata ; and this has enabled me to complete my task. 1 think I shall best meet the wishes of my audience if I give some of the valuable hints obtained, in Mr. Hardwich’s own words, arranged under three heads. First— The source of light, or the radiant, as I shall term it. “ The process I use is the oxy-hydrogen or mixed gases; the perforation of the lime which shows the greatest intensity is about the size of a split pea. Then there is an outer circle of less luminosity, &c. I do not like to risk anything nearer than 2] in. to 3 in., if the thickness of the glass is considerable. “ I never had a fracture, but I take precautions, for the sake of the lime cylinders as well as the condenser. First, I turn on the hydrogen only, for three or four minutes, then enough oxygen to make the flame burn red, for another three or four minutes ; and after that the full amount of oxygen required for incandescence.” Second.—The Condenser. “ The one I use does not bring the whole of the light to a focus. The lighted disc should be as white as possible. Quantity as well as quality of light is wanted in a new condenser. I have obtained the best results from the American or symmetrical condenser of 4 in. diameter and 2 in. solar focus (3 in. equivalent), but I fully expect that you will go beyond it.” Third.—The Lens, or Objective. “ For lecture halls, a G in. (equivalent focus) is a good size. The field should be flat, or nearly so. The practical point is a brilliantly lighted picture of great size. A 3 in. should be enlarged to, say, 12 feet. “ I cannot get beyond 8} feet; enlarged beyond that the pictures are not brilliant, especially at the edges, &c., &c. I noticed that in showing a spectrum on the screen to illustrate the rain bow, the blue appeared green, and all the upper violet shades were destroyed.” Mr. Hardwich then recounts the difficulty experienced by him in procuring a good French lens, remarking that all the quarter-plate lenses sent to him to choose from were defective in flatness of field, the outsides being quite out of focus. He then became possessed of a half-plate French lens, which gave aflat field, and admitted of much more latitude in the adjust ment of the lime. An alteration in its position, further from, or nearer to, higher or lower, not sensibly affecting the disc, as compared with the quarter-plate lenses referred to. Tested for distortion upon a square border, it proved to be of the hour glass description, and the picture looked as though it were pro jected upon a slightly concave surface. He then refers to the advice so often given, viz., “ to pick out a French lens with a flat field,” concluding, “ and that is precisely the weak point, as far as my experience goes.” With this ample summary of requirements before you, I pro ceed to make a few remarks in the same order. First, the source of light, or radiant. Theoretically, this should be a point - at any rate, it should be as small as possible, but intensely white or luminous. The nearest approach to such a source is the lime-light, or the oxy-hydrogen light, referred to above; but even this is not a point. How much inferior, then, are the various kinds of lamps or lamp-flames used for the