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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. 1283, April 6, 1883
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Zeitschrift
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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- Titel
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APRIL 6, 1883. | THE PHOTOGRAPHIC NEWS. 215 the use of photography in the transformation of designs executed in a special manner on certain paper in chemical photo-engraviug. I announced the approaching publica tion of a manual of design suitable for typography. At the end of the Congress, I heard with great satisfaction the Minister for Public Instruction proclaim me Officer for Public Instruction. This national recompense for my modest work is another proof of tha interest attached to the progress and popularity of photographic processes. Leon Vidal. NOTES ON CELESTIAL PHOTOGRAPHY. BY K. C. JOHNSON, F.R.A.S.* GEEAT expectations were formed, on the discovery of photo graphy, that faithful autographic records would be obtained by its means of the wonders which are revealed by the telescope ; but, excepting as regards the two most conspicuous of the celestial bodies—the sun and the moon—these expectations have not yet been realised. The best photographs of the moon—such as have been taken by Draper, De la Rue, Rutherford, and Ellery—come far short of representing all the detail which can be perceived with high telescopic powers. Those who have not access to a large tele scope which is capable of bearing powers of from 500 to 800 diameters can judge for themselves of the truth of this assertion by comparing any particular spot on the moon, as shown by the best photograph (for instance, the well-known crater Copernicus, or the lunar Appenines), with the drawing of the same spot given in Nasmyth and Carpenter’s monograph on our satellite. These remarks apply also to the sun, excepting that an extra ordinary reticulated structure has recently been photographed by M. Janssen, of Paris. This reticulation has not been perceived by eye observation, and its nature and cause have yet to be dis covered. The principal reason why photography has failed in this res pect is because too much has been expected from it. A longer or shorter interval of time is necessary for light to act upon the plate, and during this interval countless disturbances are taking place in a cylindrical column of air some fifty miles in length, and having a diameter equal to that of the object-glass or speculum employed. It is only on those rare occasions when the air is in a state of almost absolute serenity that it is possible to obtain a fine photograph. Besides this, in photographing the stars, either singly or in groups, so long an exposure is generally required that the telescope has to be driven by a clock which will make it accurately follow (or rather counteract) the diurnal motion for a space of one or two hours. Notwithstanding the great improvement in electric and other controls, this is still a most difficult thing to accomplish. By the discovery of the rapid gelatine process, a great impetus has been given to astronomical photography. The nebula in Orion has been photographed by Dr. Henry Draper, of New York, in 1880-82, with an achromatic telescope of eleven inches aperture, and gelatine plates. Exposures respectively of 51, 104, and 137 minutes were given. While a good representation of this irregular nebulosity has been obtained, the most striking result is that with the longest exposure, the images of stars which are almost the minima visibilia with this telescope are recorded ; and the legitimate conclusion is arrived at “ that it is not un reasonable to hope that, by still further prolonging the expo sure, and by still further study of photographic processes, stars and details (of the nebul) entirely invisible to the eye may be obtained.! The late Mr. C. E. Burton, also, last year photographed the images of Mizar and its .faint companion in ten seconds, the stars being about one-tenth of an inch apart, j As the interval between these stars is only fourteen seconds of arc, this gives promise of a useful application of photography for the measure ment of double stars, or the charting of stellar groups or clusters. Mr. David Gill, the energetic Director of the Royal Observa tory at the Cape of Good Hope, has recently sent home some wonderful photographs of thegreat comet of last autumn. With a portrait lens of two and a-half inches aperture and eleven inches focus, and a camera strapped to the tube of the equatorial, • Read before the Liverpool Amateur Photographic Association, t Monthly Notiet, Royal Astronomical Society, vol. xlii., page 367. 4 Hid., vol. xlii. page 424. and with exposures varying between thirty minutes and tw ° hours, the best photographs ever taken of a comet have been secured. But these photographs are chiefly remarkable from the fact that an immense number of stars of all magnitudes are accurately depicted on one plate. These stars extend over a considerable extent of the heavens, and point out the possibility of accurately charting at one operation a space of about twenty degrees square. It is plain that if this can be successfully accom plished, an exposure of two, or even three, hours, will be worth attempting. On examination of a negative on which are the images of a great number of stars, it will be seen that these images are all circles of nearly the same size, and that the various magnitudes are shown by the intensity of the discs. If the driving-clock of the telescope has not been most accurate in its rate the discs are slightly oval, and this is nearly always the case. It is not on this account much more difficult to measure their respective distances, and there is then no fear that an accidental spot on the film may be mistaken for a star. As might have been anticipated, from the variety of colour which exists among the stars, the photographic magnitudes frequently differ from the apparent ones, and from this fact great possibilities of the value of photographic charting are at once opened up. We know that our optic nerves are insensible to rays having vibrations of less than about 325 billions per second. These are the ultra-red rays ; and conversely, that we cannot see rays beyond the ultra-violet end of the spectrum, which have vibrations exceeding in number about 763 billions per second. Yet both of these groups of rays can be photo graphed on plates which are sensitised with appropriate salts ; and it seems quite likely that, by the use of negatives so pre pared, the existence of stars which will for ever be invisible to the eye of man may be ascertained, and the autographic records may be the means of elucidating some of the mysteries of the motions of binary systems, or of finding out the position which is occupied by a great central sun. The difficulty of detecting evidence of change or of motion in nebulae, and in the richer clusters in which thousands of stars are aggregated, is so great that the attempt to solve it has been scarcely made. In these two classes of celestial objects the application of photography may lead to discoveries of the supremest interest. A NEW PLATE-CARRIER. At one of the last meetings of the Photographic Society Mr. Warnerke exhibited a plate-carrier he had devised for taking any sized plate less than the camera was made for: instead of having to carry many carriers of different sizes, his apparatus would take from plates one inch and upwards ; it would hold a glass disc, as also irregular plates, or any plate which, becoming broken, would not go into an ordinary carrier ; all these could be utilized in the one he exhibited. It consisted of a square frame, fitting the camera; on the top and bottom pieces slots were made, in which two bars of wood moved, either straight or slanting, so that any sized or irregular-shaped plate could be placed upon these two bars, the whole being kept in position (as usual) by the slide to the lid pressing upon it. Pear-tree wood was recommended for making the apparatus from. Mr. Warnerke also stated that he used black velvet attached to the lid, which he considered the best preventive against light being reflected from the back.
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