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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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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. 1720, August 21, 1891
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The photographic news
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August 21, 1891.] THE PHOTOGRAPHIC NEWS. 595 of energy, while many subjects within the purlieus of the survey remain unrepresented. The western portion of the county, too, is quite unworked. Sufficient, however, is shown to indicate that great energy and skill have been devoted to the task, and to promote the completion of the survey in a satisfactory manner. It has occurred to the adjudicators whether it would be thought advisable by the committee to award the silver medal withheld to the best selection of views illustrating the castles immediately round Cardiff, viz., Cardiff, St. Fagan’s, and Castell Coch. In such case they would recommend the five series exhibited by Mr. Dighton, which, being confined to the castles named, does not conform to either of the clauses described in the schedule. (Signed per pro.) W. H. Johnston. T. H. Thomas. MEETING OF THE BRITISH ASSOCIATION AT CARDIFF. In his inaugural address as President of the British Association, which is now meeting at Cardiff, Dr. Wm. Huggins, the astronomer, said: — The new power which modern photography has put into the hands of the astronomer is great, and has led already, within the last few years, to new acquisitions of knowledge of vast importance, and the recent great suc cesses in astronomical photography are not due to greater skill, nor, to any great extent, to superior instruments, but to the very great advantages which the modern gelatine dry plate possesses for use in the observatory over the methods of Daguerre, and even over the wet collodion film on glass, which, though a great advance on the silver plate, went but a little way towards putting into the hands of the astronomer a photographic surface adapted fully to his wants. The modern silver-bromide gelatine plate, except for its grained texture, meets the needs of the astronomer at all points. It possesses extreme sensitiveness; it is always ready for use ; it can be placed in any position ; it can be exposed for hours; lastly, it does not need immediate deve lopment, and for this reason can be exposed again to the same object on succeeding nights, so as to make up by several instalments, as the weather may permit, the total time of exposure which is deemed necessary. Without the assistance of photography, however greatly the resources of genius might overcome the optical and mechanical difficulties of constructing large telescopes, the astronomer would have to depend in the last resource upon his eye. Now, we cannot by the force of continued looking bring into view an object too feebly luminous to be seen at the first and keenest moment of vision ; but the feeblest light which falls upon the plate is not lost, but is taken in and stored up continuously. Each hour the plate gathers up 3,600 times the light-energy which it received during the first second. It is by this power of accumulation that the photographic plate may be said to increase, almost without limit, though not in separating power, the optical means at the disposal of the astronomer for the discovery or the observation of faint objects. Two principal directions may be pointed out in which photography is of great service to the astronomer. It enables him, within the comparatively short time of a sin gle exposure, to secure permanently with great exactness the relative positions of hundreds or even of thousands of stars; or the minute features of nebulae or other objects ; or the phenomena of a passing eclipse, a task which, by means of the eye and hand, could only be accomplished, if done at all, after a very great expenditure of time and labour. Photography puts it in the power of the astro nomer to accomplish in the short span of his own life, and so enter into their fruition, great works which otherwise must have been passed on by him as an heritage of labour to succeeding generations. The second great service which photography renders is not simply an aid to the powers the astronomer already possesses. On the contrary, the plate, by recording light-waves which are both too small and too large to excite vision in the eye, brings him into a new region of knowledge, such as the infra-red and the ultra-violet parts of the spectrum, which must have remained for ever un known but for artificial help. The present year will be memorable in astronomical history for the practical beginning of the photographic chart and catalogue of the heavens, which took their origin in an international conference which met in Paris in 1887, . by the invitation of M. 1’Admiral Mouchez, Director of the Paris Observatory. The decisions of the Conference in their final form provide for the construction of a great photographic chart of the heavens, with exposures corresponding to forty minutes’ exposure at Paris, which, it is expected, will reach down to stars of about the fourteenth magnitude. As each plate is to be limited to four square degrees, and as each star, to avoid possible errors, is to appear on two plates, over 22,000 photographs will be required. With an exposure sufficiently long for the faintest stars to impress themselves upon the plate, the accumulating action still goes on for the brighter stars, producing a great enlargement of their images from optical and photo graphic causes. The remarkable successes of astronomical photography, which depend upon the plate’s power of accumulation of a very feeble light acting continuously through an exposure of several hours, are worthy to be regarded as a new revelation. The first chapter opened when, in 1880, Dr. Henry Draper obtained a picture of the nebula of Orion ; but a more important advance was made in 1883, when Dr. Common, by his photographs, brought to our know ledge details and extensions of this nebula hitherto un known. A further disclosure took place in 1885, when the Brothers Henry showed for the first time, in great detail, the spiral nebulosity issuing from the bright star Maia, of the Pleiades, and, shortly afterwards, nebulous streams about the other stars of this group. In 1886 Mr. Roberts, by means of a photograph to which three hours’ exposure had been given, showed the whole background of this group to be nebulous. In the following year Mr. Roberts more than doubled for us the great extension of the nebula region which surrounds the trapezium in the constellation of Orion. By his photographs of the great nebula in Andromeda he has shown the true significance of the dark canals which had been seen by the eye. They are, in reality, spaces between successive rings of bright matter, which appeared nearly straight owing to the inclination in which they lie relatively to us. These bright rings surround an undefined central luminous mass. Some recent photographs by Mr. Russell show that the great rift in the Milky Way in Argus, which to the eye is void of stars, is, in reality, uniformly covered with them. The heavens are richly, but very irregularly, inwrought with stars. The brighter stars cluster into well-known groups upon a background formed of an enlacement of streams and convoluted windings and intertwined spirals
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