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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1299, July 27, 1883
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Zeitschrift
The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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July 27, 1883.] TRE PHOTOGRAPHIC NEWS. 471 able to determine the relative brightness of the two stars which form the double f Urs Majoris. But the work at Harvard University was to do more than this. The stars which Prof. Bond examined were close together. Prof. Pickering wished to compare stars far removed from each other. For this purpose the ordinary method of stellar photography, by which photographs are taken at the foci of large telescopes, would not suffice. These photographs only comprise a small region of but one or two degrees in diameter. A different method was therefore employed in the Harvard observations. V wholly different form to the ordinary equatorial telescope was used. It is not unusual to construct photographic cameras to take pictures of buildings which subtend to an angle of 60° or even 90° ; but when applied to the stars, however, the images at the edges are very poor, and only very small apertures can be used. It has, however, been found that some of the best lenses for pictures can be obtained covering a circle of 29° diameter with out serious distortion, and at the same time large apertures can be used, thus reducing the time of exposure. In order to still further this work, Mr. W. H. Pickering investigated the sensi tiveness of various photographic plates, and obtained some so sensitive that stars of the fifth and sixth magnitude have been photographed without using clockwork, they forming dots or making lines as their images pass across the photographic plate, the length of these lines depending, of course, upon the time during which the plate is exposed. If the plate be exposed dur ing ten seconds, a distinct dot is obtained, whilst an exposure of thirty seconds causes a short line to be formed. The plates used at Harvard Observatory are six by eight inches. They are divided into six equal parts, each part being in turn exposed. By this means six regions of the heavens, each about 15° square, may be photographed on one plate ; and by a variation in the dot and line system employed, sometimes taking the dot and sometimes the line first, three pictures may be taken on a single division of one of the plates without giving rise to any confusion. Instead of simply six, therefore, eighteen photographs are taken on one of these plates, so that on a single plate a portion of the heavens of more than three hours’ right ascension, and extending from 30° S. to 60° N., may be included. Since each portion of the plate covers a region of about 15°, the camera mounting has a series of notches or stops, by which it may be instantly moved through that amount either of right ascension or declination. When photographing, the following is the exact method em ployed. The first exposure takes the region between 30° and 15° south declination, and between one hour and a-half and half-an- hour west of the meridian. First, the plate is exposed for ten seconds, and each star records itself by a dot. The plate is then covered for ten seconds; next it is exposed for a period of thirty seconds, and each star makes a line on the plate. By means of the clamping arrangement, to which we have referred, the plate is then moved through one hour in right ascension. This takes up the remaining few seconds of the minute, so that the taking of the next photograph begins with the first second of another minute. The camera is then on the meridian. The same part of the plate is again exposed, and in order to distinguish this series of stars from those first photographed, this time the plate is exposed first during thirty seconds, and then during ten, so that the result is a line followed by a dot. This gives the second series. But the same portion of the plate may be again used. The remaining ten seconds of the second minute, like those of the first, are spent in moving the camera through another hour of right ascension. Then a fresh exposure is made for thirty seconds, a line simply being obtained without a dot, and this completes the series. The first class of images is in dots and lines, the second in lines and dots, the third is recognized by the presence of lines alone. The thirty seconds which remain of the third minute are employed in exposing a second portion of the plate, and changing the position of the camera, which now takes in the region from 15° S. to the equator. The same process is then gone through again, three exposures as before being made in three different positions of right ascension. By continuing this process, taking three photographs on each of the six portions into which the plate is divided, the whole region included between the declinations of — 30° and + 60°, and between three hours of right ascension, one and a-half hours on each side of the meridian, being one-eighth of the whole heavens, excluding the circumpolar stars, will be photographed on one plate, the whole operation occupying but eighteen minutes. With regard to those stars in the vicinity of the Pole, some other method will have to be adopted. Thus much for one branch of the work— and an important branch—carried on at Harvard Observatory. Another portion of their work consists in the preparation of a photographic map of the entire heavens. The method just described, in which clockwork is dispensed with, only enables those stars whose magnitude is not less than five or six to be photographed, and stars of a less magnitude than this must, of course, be included in a map of the heavens. The camera in this work, therefore, is driven by clockwork. By this means stars of the eighth magnitude record their images on the photographic plate, and as many as 200 are visible in the paper print within a circle of 5° in diameter. A photograph taken in this way of a portion of the constellation of Orion, besides showing the three stars of the Belt and the Sword-Handle, gives an interesting picture of the nebula. With reference to the question of the colours of stars, it is interesting to note the faintness of a Orionis in the photographs. To the eye its brilliancy is almost as great as that of B, whilst in the photograph it is not more prominent than A. The reason is to be found in the colour of a. It is a red star, and conse quently makes but little impression on the photographic, plate. Again, in the constellation Cetus, the three stars which are brightest to the eye are a, y, and <r. A, which is the brightest of the three, has close to it a very faint companion, scarcely visi ble to the naked eye, its magnitude being given as 6-8, whilst that of a is 2’7. This is the appearance of this part of that constellation as seen by the eye. A photograph of this region was taken at Harvard with the result that the small star is seen in the photograph nearly as bright as a, it being only three- tenths of a magnitude less. The colour of these stars again explains this, a being of a reddish tint, whilst the small star is of a deep blue colour, and being so, the rays which flow from it have a greater influence on the photographic plate. A com parison of the number of stars seen in the photograph of Orion, with the number in the photometric catalogue, further illustrates this effect of colour. In that part of this constellation included between 5° north and 5° south declination, and 75° to 90° of right ascension, sixteen stars were common to photograph and catalogue ; a like number, being either too small in magnitude or too red in colour, although catalogued, remain unrecorded on the photographic plate; whilst five others seen in the photograph are not given in the catalogue. A reduction has been made of the results given by the plates of different makers, with the view of ascertaining the value of the deviation. In two of such plates the average deviation was 0'21 of a magnitude, and in two measurements of the same plate, it was found to be 0'07 of a magnitude. It is obvious, from this account of the work at Harvard, that star photography is entering into a new phase, and one which will entirely replace the present system of eye-observations, for the reason that, whilst the eye is so variable, photographic plates may now be obtained doing their work with almost definite wave-lengths of light. The constant record of the plate must in time, therefore, be preferred to observation by the variable eye. At the same time, as photography advances, if it be considered necessary to obtain photographic star maps to record the obser vations of the average eye, there will be no difficulty in this being done.—Nature. Sinks. We regret to announce the death at Calcutta, on the 20th ult., of Mr. W. E. Batho, a name well-known to the readers of this journal as an active and energetic worker in many branches of photography. For the past three years he had been working with Messrs. Westfield and Company, and had but recently joined Major Waterhouse’s staff as head assistant at the Calcutta Ordnance Office. In the Millheilungen, Herr Obernetter gives a very simple method of recovering the silver from waste bromide emulsion. The latter is collected in a vessel with a solu tion of calcined soda (this can be present in excess), and from time to time grape sugar is added. The accumula tion of waste material may go on in this way for weeks,
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