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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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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. 1698, March 20, 1891
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The photographic news
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March 20, 1891.] THE PHOTOGRAPHIC NEWS. 221 THE PHOTOGRAPHIC CHART OF THE HEAVENS. BY WILLIAM E. PLUMMER, M.A., F.R.A.S. The proposal of Admiral Mouchez, the director of the Paris Observatory, to photograph the entire sky has been so long before the public, and so much has been written and said about it, that some hopeful persons may be under the impression that considerable strides have been made towards its accomplishment. Such sanguine persons may be surprised to learn that the scheme is not yet even commenced as a whole, but is still in its initiatory stage. The art—or, at least, the practice—of photography is now reduced to such simple and mechanical processes that it is feared this bold assertion, without any explanation, will suggest to many that astronomers are either very incom petent to the task, or very lukewarm in the undertaking. Such a suspicion would, however, be very unjust. Many difficulties and delays have supervened between the conception of the project and the putting it into execution. Possibly, if all the difficulties that have been successfully met had been present in the minds of the projectors, the scheme would never have entered the field of practical astronomy. As a matter of fact, some eighteen or twenty observatories have responded eagerly to the invitation of Admiral Mouchez; but the directors of these observatories have, in most cases, had to wait till callous and indifferent governments have voted the inevitable money grants. All have had to obtain the necessary instrumental appliances from a few competent artists, who required considerable time for the completion of the contracts. In order to ensure uniformity of action, many details have had to be settled by general conferences of all the participating astronomers, who cannot, of course, be frequently called together; but it is hoped that the next conference, summoned for this month, will be the last, for all the astronomers who have given their adhesion to the scheme will report that they are in possession of their instruments, and have made some progress in acquiring a knowledge of their peculiarities and facility in their use. The principal optical difficulty, it goes without saying, is the construction of the lens. Photographers are so accustomed to make pictures covering wide angles that they will probably not appreciate the difficulties that opticians have encountered in the construction of a lens, which should give a sufficiently accurate field of two degrees in diameter. This is the same thing as photo graphing a picture of approximately six inches square at a distance of fifteen feet, and it would be thought a very poor lens that could not satisfactorily accomplish this task. But astronomers are exacting in their demands; they may not reduce the aperture of their lens (13 inches) by any “ stops,” and they have to submit the resulting pictures to considerable magnification, when many defects which would otherwise escape notice become apparent. It is then found that the images of stars, though perfectly round in the centre of the plate, become very elliptical near the edges. The size of the plate, it may be mentioned, is about six inches square, the focal length being about eleven feet; but to “cover” a six-inch plate is not generally thought a very severe trial in photographic manipulation. We have said that the image of a star in the centre of the plate is round, but when seen under a microscope of moderate power the image does not present the appearance of a simple, round black disc, varying in size with the brilliancy of the star and the time of exposure, but rather exhibits itself as the condensation of many minute black points, very dense and compressed towards the centre in moderately bright stars, but towards the periphery the condensation is much less marked, and the total area covered by the star image is generally ill defined. In faint stars the condensation is scarcely noticeable; a larger or smaller number of particles of the silver salt are decomposed, but these particles are not necessarily in contact, and, therefore, complete blackness does not result. A more or less nebulous appearance, possibly a three-hundredth of an inch in diameter, therefore accom panies the images of faint stars on the negative, and, since the distance between these images has to be measured to about the ten-thousandth of an inch, it will be readily seen how important it is to procure small, hard, black discs. A peculiarity in stellar negatives, which is not yet fully explained, is the considerable spread of these images. In an ordinary telescope a faint star is a very minute point, approaching the geometrical definition, but on a sensitised film after long exposure, the size of the image is of a very measurable extent; and the rate of increase is peculiar. Allowing for some irregularities, the diameter may be said to increase as the fourth root of the time of exposure ; that is to say, if, after an exposure of a minute, a star has a certain diameter, then, on the expo sure being continued for sixteen minutes, it will be found that the diameter has doubled. This spread of the image may arise from two causes, either operating as an irradia tion from the centre of disturbance ; but whether this irradiation is produced by scattered reflection among the separate particles of bromide of silver, or is due to a spe cies of fluorescence, or some similar property in the sensi tive film, is undecided. Be that as it may, a plate after development exhibits a number of these small black discs, differing in size and intensity according to the brightness of the stars, and therefore giving, in addition to their relative positions, the photographic magnitude of the stars themselves. These negatives can be printed on either silver or bromide paper, and thus a picture of the heavens of great accuiacy is always at the command of astron omers, who, to some extent, may, therefore, hope to con tinue their studies irrespective of clouds and bad weather, while to successive ages is secured a picture accurately representing that portion of the sky at the period at which that negative was taken. The measurement of these plates is, however, not a simple business, for over and above the distortion likely to be introduced from the necessary imperfection of the object glass and the employment of a plane plate, there is the additional difficulty that the film, in some of the pro cesses of developing, fixing, washing, and preserving, may shift locally on the plate itself, carrying with it the images impressed, and so giving a faulty representation of the relative positions of the stars. This difficulty has been met in an ingenious manner. On a plate—the size of those to be subsequently employed—a film of silver has been deposited, and on this film a network of lines about one-fifth of an inch distant is ruled at right-angles, in such a manner as to remove the opaque silver and leave the glass exposed. Before a plate intended to obtain a picture of the stars is exposed in the telescope, this re ticulated plate is placed against the film, and, by means of parallel light, a latent picture of the reticulations is left on the sensitised surface. The silver, of course, prevents the light affecting the other parts of the film; there fore, after exposure, in developing, this network picture
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