Suche löschen...
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
- Digitalisat
- SLUB Dresden
- Lizenz-/Rechtehinweis
- Public Domain Mark 1.0
- URN
- urn:nbn:de:bsz:14-db-id1780948042-186800009
- PURL
- http://digital.slub-dresden.de/id1780948042-18680000
- OAI-Identifier
- oai:de:slub-dresden:db:id-1780948042-18680000
- Sammlungen
- Fotografie
- LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
- Strukturtyp
- Band
- Parlamentsperiode
- -
- Wahlperiode
- -
- Bandzählung
- No. 517, July 31, 1868
- Digitalisat
- SLUB Dresden
- Strukturtyp
- Ausgabe
- Parlamentsperiode
- -
- Wahlperiode
- -
-
Zeitschrift
The photographic news
-
Band
Band 12.1868
-
- Titelblatt Titelblatt I
- Kapitel Preface III
- Ausgabe Ausgabe 1
- Ausgabe Ausgabe 13
- Ausgabe Ausgabe 25
- Ausgabe Ausgabe 37
- Ausgabe Ausgabe 49
- Ausgabe Ausgabe 61
- Ausgabe Ausgabe 73
- Ausgabe Ausgabe 85
- Ausgabe Ausgabe 97
- Ausgabe Ausgabe 109
- Ausgabe Ausgabe 121
- Ausgabe Ausgabe 133
- Ausgabe Ausgabe 145
- Ausgabe Ausgabe 157
- Ausgabe Ausgabe 169
- Ausgabe Ausgabe 181
- Ausgabe Ausgabe 193
- Ausgabe Ausgabe 205
- Ausgabe Ausgabe 217
- Ausgabe Ausgabe 229
- Ausgabe Ausgabe 241
- Ausgabe Ausgabe 253
- Ausgabe Ausgabe 265
- Ausgabe Ausgabe 277
- Ausgabe Ausgabe 289
- Ausgabe Ausgabe 301
- Ausgabe Ausgabe 313
- Ausgabe Ausgabe 325
- Ausgabe Ausgabe 337
- Ausgabe Ausgabe 349
- Ausgabe Ausgabe 361
- Ausgabe Ausgabe 373
- Ausgabe Ausgabe 385
- Ausgabe Ausgabe 397
- Ausgabe Ausgabe 409
- Ausgabe Ausgabe 421
- Ausgabe Ausgabe 433
- Ausgabe Ausgabe 445
- Ausgabe Ausgabe 457
- Ausgabe Ausgabe 469
- Ausgabe Ausgabe 481
- Ausgabe Ausgabe 493
- Ausgabe Ausgabe 505
- Ausgabe Ausgabe 517
- Ausgabe Ausgabe 529
- Ausgabe Ausgabe 541
- Ausgabe Ausgabe 553
- Ausgabe Ausgabe 565
- Ausgabe Ausgabe 577
- Ausgabe Ausgabe 589
- Ausgabe Ausgabe 601
- Ausgabe Ausgabe 613
- Register The Index To Volume XII 619
-
Band
Band 12.1868
-
- Titel
- The photographic news
- Autor
- Links
- Downloads
- Einzelseite als Bild herunterladen (JPG)
-
Volltext Seite (XML)
July 31, 1868.] THE PHOTOGRAPHIC NEWS. 369 the intensity of one sot of the components of white light would give all the information wo want; just as in an analysis of a definite, chemical compound, the chemist is satisfied with an estimation of one or two constituents only, and calculates the others. Method based upon the foregoing considerations would supply us with what may be termed an absolute photometer, the indi cation of which would bo always the same for the same amount of illumination, requiring no standard light for comparison ; and pending the development of experiments which the writer is prosecuting in this direction, he has been led to devise a new, and, as he believes, a valuable form of relative photo meter. A relative photometer is one in which the observer has only to determine the relative illuminating powers of two sources of light, one of which is kept as uniform as possible, the other being the light whose intensity is to bo determined. It is therefore evident that the groat thing to bo aimed at is an absolutely uniform source of light. In the ordinary process of photometry the standard used is a candle, defined by Act of Parliament as a “ sperm candle of six to the pound, burning at the rate of 120 grains per hour.” This is the standard from which estimates of the value of illuminating gas are deduced, hence the terms “ 12-candle gas,” “ 14-candle gas,” &c. In his work on “ Gas Manipulation,” Mr. Sugg gives a very good ac count of the difficulties which stand in the way of obtaining uniform results with the Act of Parliament candle. A true sperm candle is made from a mixture of refined sperm with a small proportion of wax, to give it a certain toughness, the pure sperm itself being extremely brittle. The wick is of the best cotton, made up into throe cords and plaited. The number of strands in each of the three cords composing the wick of a six- to-the-pouud candle is seventeen, although Mr. Sugg says there does not appear to bo any fixed rule, some candles having more and others less, according to the quality of the sperm. Sperm candles are made to burn nt the rate of one inch per hour, and the cup should be clean, smooth, and dry. The wick should be curved slightly at the top, the red tip just showing through the flame, and consuming away without requiring snuffing. To obtain these results the tightness of the plaiting and size of the wick require careful attention ; and as the quality of the sperm differs in richness or hardness, so must the plaiting and number of strands. A variety of modifying circumstances thus tend to affect the illuminating power of a standard sperm candle. These difficulties, however, are small, compared with those which have resulted from the substitution of paraffine, &c., for part of the sperm ; and Mr. Sugg points out that candles can bo made with such combinations of stearine, wax, or sperm, and paraffine as to possess all the characteristics of sperm candles, and yet bo superior to them in illuminating power; while, on the other hand, candles made from the same materials other wise combined are inferior. When, in addition to this, it is found that candles containing paraffine require wicks more tightly plaited and with fewer strands than those suitable for the true sperm candle, our readers will bo enabled to judge of the almost unsurmountable difficulties which beset the present system of photometry. But assuming that the true parliamentary sperm candle is obtained, made from the proper materials, and burning at the specified rate, its illuminating power will be'found to vary with the temperature of the place where it has been kept, the time which has elapsed since it was made, and the temperature of the room wherein the experiment is tried. The Rev. W. K. Bowditch, in his work on “ The Analysis, Purification, &c., of Coal Gas,” enters at some length into the question of test-candles, and emphatically condemns them as light measures ; one experiment quoted by this author showed that the same gas was reported to be 14’63 or 17’36 candle gas, according to the way the experiment was conducted. The present writer has taken some pains to devise a source el light which should be at the same time fairly uniform in its results, would not vary by keeping, and would be capable of ac curate imitation at any time and in any part of the world by Were description. The absence of these conditions seems to be one of the greatest objections to the sperm candle. It would bo impossible for an observer on the continent, ten or twenty years hence, from a written description of the sperm candle now employed, to make a standard which would bring his photo metric results into relation with those obtained here. Without presuming to say positively that ho has satisfactorily solved all itticulties, the writer believes that ho has advanced some dis tance in the right direction, and pointed out the road fo further improvement. Before deciding upon a standard light, experiments wore made to ascertain whether the electric current could be made available. Through a coil of platinum wire, so as to render it brightly incandescent, a powerful galvanic current was passed ; and its strength was kept as constant as possible by a thick wire galvanometer and rheostat. To prevent the cooling action of air-currents the incandescent coil was surrounded with glass ; and it was hoped that by employing the same kind of battery and by varying the resistance so as to keep the galvanometer needle at the same deflection, uniform results could be obtained. In.practice, however, it was found that many things interfered with the uniformity of the results, and the light being much feebler than it was advisable to work with, this plan was deemed not sufficiently promising, and it was abandoned. The method ultimately decided upon is the following:—Alcohol of sp. gr. 0'805, and pure benzole boiling at 819 C., are mixed together in the proportion of 5 volumes of the former and 1 of the latter. This burning fluid can be accurately imitated from description at any future time and in any country, and if a lamp could be devised equally simple and invariable, the light which it would yield would, it is presumed, be invariable. This difficulty the writer has attempted to overcome in the following manner. A glass lamp is taken of about 2 ounces capacity, the aper ture in the neck being 0’25 inch diameter ; another aperture at the side allows the liquid fuel to be introduced, and by a well- known laboratory device the level of the fluid in the lamp can be kept uniform. The wick-holder consists of a platinum tube 1’81 inches long and 0’125 inch internal diameter. The bottom of this is closed with a flat plug of platinum, apertures being left in the sides to allow free access of spirit. A small platinum cup, ’5 inch diameter and T inch deep, is soldered round the outside of the tube 0’5 inch from the top, answering the three fold purpose of keeping the wick-holder at a proper height in the lamp, preventing evaporation of the liquid and keepingout dust. The wick consists of 52 pieces of hard-drawn plati num wire, each 0’01 inch in diameter and 2 inches long, per fectly straight and tightly pushed down into the platinum holder until only OT inch projects above the tube. The height of the burning fluid in the lamp must be sufficient to cover the bottom of the wick-holder: it answers best to keep it always at the uniform distance of 1’75 inches from the top of the platinum wick ; a slight variation of level, however, has not been found to influence the light to an extent appreciable by our present means ot photometry. The lamp having the reservoir of spirit thus arranged, the platinum wires parallel and their projecting ends level, a light is applied, and the flame instantly appears, forming a perfectly-shaped cono 1’25 inches in height, the point rate of maximum brilliancy being 0’56 inch from the top of the wick. The extremity of the flame is perfectly sharp, without any tendency to smoke ; without flicker or movement of any kind, it burns, when protected from currents of air, at a uniform rate of 136 grains of liquid per hour. The temperature should be about 60° F., although moderate variations on either side exert no perceptible influence. Bearing in mind Dr. Frankland’s observations on the direct increase in the light of a candle with the atmospheric pressure, accurate observations ought only to be taken at one height of the barometer. To avoid the incon venience and delay which this would occasion, a table of cor rections should be constructed for each 0’1 inch variation of barometric pressure. There is no doubt that this flame is very much more uniform than that of the sperm candle sold for photometric purposes. Tested against a candle, considerable variations in relative illu minating power have been observed ; but on placing two of these lamps in opposition no such variations have been detected. The same candle has been used, and the experiments have been repeated at wide intervals, using all usual precautions to ensure uniformity. The results are thus shown to be due to variations in the candle and not in the lamp. It is expected that whoever may bo inclined to adopt the kind of lamp here suggested will find not only that its uni formity may be relied upon, but that, by following accurately the description and dimensions here laid down, each observer will possess a lamp of equivalent and convertible photometric value, so that results may not only be strictly comparable between themselves, but within slight limits of accuracy comparable with those obtained by other experimentalists. The dimensions of wick, &c., here laid down are not intended to fix the standard. Persons engaged in photometry as an important branch of their
- Aktuelle Seite (TXT)
- METS Datei (XML)
- IIIF Manifest (JSON)