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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1270, January 5, 1883
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The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
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Band
Band 27.1883
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JANUARY 5, 1883. THE PHOTOGRAPHIC NEWS. 11 A MECHANICAL AID IN DEVELOPING. BY H. MANNERS. All workers of dry plates know how plates stick to the bottom of the developing and washing trays and hooks and other things of the same kind for lifting are of little use. The following will enable the plates to be lifted with ease and certainty. In your trays (say 10 by 10) have two small holes drilled about one-eighth of an inch from top, one on each side, about one inch from the end. Pass a piece of strong linen thread or fine twine through hole on the side and tie it. Measure down side and along bottom of tray, and pass out of th e hole on the opposite side, and to this end of thread fasten a small boot-button. When plate is in tray the thread passes underneath one end of it, and by giving the button a pull the thread tightens, and lifts the plate up so that it may be easily laid hold of with the other hand. This will be found very handy to many. PHOTOGRAPHING THE CORONA.* Problems of the highest interest in the physios of our sun are connected, doubtless, with the varying forms which the coronal light is known to assume ; but these would seem to admit of solution only on the condition of its being possible to study the corona continuously, and so to be able to confront its changes with the other variable phenomena which the sun presents. “Unless some means be found,” says Professor C. A. Young, « for bringing out the structures round the sun which are hidden by the glare of our atmosphere, the progress of our knowledge must be very slow, for the corona is visible only about eight days in a century, in the aggregate, and then only over narrow stripes on the earth’s surface, and but from one to five minutes ata time by any one observer ” (The Sun, p. 239). The spectroscopic method of viewing the solar prominences fails, because a large part of the coronal light gives a continuous spectrum. The successful photograph of the spectrum of corona taken in Egypt, with an instrument provided with a slit, under the superintendence of Professor Schuster during the solar eclipse of May 17, 1882, shows that the coronal light, as a whole—that is, the part which gives a continuous spectrum, as well as the other part of the light which may be resolved into bright lines—is very strong in the region of the spectrum extend ing from about G to H. It appeared to me, therefore, very probable that, by making exclusive use of this portion of the spectrum, it might be possible, under certain conditions, about to be described, to photograph the corona without an eclipse. In the years 1866-68 I tried screens of coloured glasses and other absorptive media, by which I was able to isolate certain portions of the spectrum with the hope of seeing dii ectly, with out the use of the prism, the solar prominences (Monthly Notices, vol. xxviii. p. 88, and vol. xxix. p. 4). I was unsuccessful, for the reason that I was not able by any glasses or other media to isolate so very restricted a portion of the spectrum as is repre sented by a bright line. This cause of unsuitableness of this method for the prominences which give bright lines only, re commends it as very promising for the corona. If by screens of coloured glass or other absorptive media the region of the spec trum between G and H could be isolated, then the coronal light, which is here very strong, would have to contend only with a similar range of refrangibility of the light scattered from the terrestrial atmosphere. It appeared to me by no means im probable that under these conditions the corona would be able so far to hold its own against the atmospheric glare, that the parts of the sky immediately about the sun where the corona Was present would be in a sensible degree brighter than the adjoining parts where the atmospheric light alone was present. It was obvious, however, that in our climate and low down on the earth s surface, even with the aid of suitable screens, the addition of the coronal light behind would be able to increase but in a very small degree the illumination of the sky at those places where it was present. There was also a serious drawback from the circumstance that, although this region of the spec trum falls just within the range of vision, the sensitiveness of * « On a Method of Photographing the Solar Corona without an Eclipse.” Paper read at the Royal Society by William Huggins, D.C.L., LL.D., F.R.S., December 21, the eye for very small differences of illumination in this region near its limit of power is much less than in more favourable parts of the spectrum—at least such is the case with my own eyes. There was also another consideration of importance. The corona is an object of very complex form, and full of details de pending on small differences of illumination, so that even if it could be glimpsed by the eye, it could scarcely be expected that observations of a sufficiently precise character could be made to permit of the detection of the more ordinary changes which are doubtlessly taking place in it. These considerations induced me not to attempt eye-observa tions, but from the first to use photography, which possesses extreme sensitiveness in the discrimination of minute differences of illumination, and also the enormous advantage of furnishing a permanent record from an instantaneous exposure of the most complex forms. I have satisfied myself by some laboratory ex periments that, under suitable conditions of exposure and deve lopment, a photographic plate can be made to record minute differences of illumination existing in different parts of a bright object, such as a sheet of drawing paper, which are so subtle as to be at the very limit of the power of recognition of a trained eye, and even, as it appeared to me, those which surpass that limit. My first attempts at photographing the corona were made with photographic lenses ; but uncertainty as to the state of correction of their chromatic aberration for this part of the spectrum, as well as some other probable sources of error which I wished to avoid, led me to make use of a reflecting telescope of the Newtonian form. The telescope is by Short, with speculum of 6 inches diameter, and about 3} feet focal length. A small photographic camera was fastened on the side of the telescope tube, and the image of the sun after reflection by the small plane speculum was brought to focus on the ground glass. The absorptive media were placed immediately in front of the sensi tive film, as in that position they would produce the least optical disturbance. Before the end of the telescope was fixed a shutter of adjustable rapidity, which reduced the aperture to two inches. This was connected with the telescope tube by a short tube of black velvet, for the purpose of preventing vibra tions from the moving shutter reaching the telescope. On account of the shortness of exposures it was not necessary to give motion to the telescope. It was now necessary to find an absorptive medium which would limit the light received by the plate to the portion of the spectrum from about G to H. There is a violet (pot) glass made, which practically does this. I had a number of pieces of this glass ground and polished on the surfaces. Three or four of these could be used together, castor-oil being placed between the pieces to diminish the reflection of light at their surfaces. Some inconvenience was found from small imperfections within the glass, and it would be desirable in any future experiments to have a larger supply of this glass, from which more perfect pieces might be selected. In my later experiments I used a strong and newly-made solution of potassic permanganate, in a glass cell with carefully polished sides. This may be considered as restricting the light to the desired range of wave-length, since light transmitted by this substance in the less refrangible parts of the spectrum does not affect the photographic plates. Different times of exposure were given, from so short an ex posure that the sun itself was rightly exposed, to much more prolonged exposures, in which not only the sun itself was photo graphically reversed, but also the part of the plates extending for a little distance from the sun’s limb. Gelatine plates were used, which were backed with a solution of asphaltum in benzole. After several trials I satisfied myself that an appearance peculiarly coronal in its outline and character was to be seen in all the plates. I was, however, very desirous of trying some modifications of the methods described, with the hope of ob taining a photographic image of the corona of greater distinct ness, in consequence of being in more marked contrast with the atmospheric illumination. Our climate is very unpropitious for such observations, as very few intervals, even of short duration, occur in which the atmo spheric glare immediately about the sun is not very great. Under these circumstances I think it is advisable to describe the results I have obtained without further delay. The investigation was commenced at the end of May of last year, and the photographs were obtained between June and September 28.
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