Page:Popular Science Monthly Volume 58.djvu/222

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Scientific Literature.


It is often supposed by readers of popular articles on astronomical photography that the introduction of the methods of 'the new astronomy' has done away, once for all, with the difficulties of the old. The photographic plate has taken the place of the observer's eye and the personal equation is supposed to have been abolished. Those who work in astronomical photography are the first to extol the merits of the new methods. But they are fully aware of difficulties peculiar to them which must be treated very much as if they were errors peculiar to an observer. The plate has its own personal equation. It is impossible to overestimate the benefit to eclipse observations, for example, that has resulted from the introduction of photography as a means of registering the forms and details of the solar corona. Yet the photographic plate has serious failings of its own. Some of them have lately been done away with by a device invented by Mr. Charles Burckhalter, Director of the Chabot Observatory, in Oakland, California; and it is the purpose of this paragraph to exhibit the advance made by Mr. Burckhalter's methods.

The solar corona is very bright near the edge of the sun's disc and fades away gradually till at a distance of some 80 to 100 minutes its brilliancy is about the same as that of the sky background. If a photograph is taken with a very short exposure, only the brighter parts of the corona are registered on the plate. The fainter portions do not appear at all. If a photograph is taken with an exposure sufficiently long to record the fainter portions, all the inner regions of the corona are much overexposed, and all detail is lost near the sun's edge. By the ordinary methods, then, the corona, as a whole, cannot be exhibited on any single plate. Each exposure is suitable for registering one region, and only one. The corona must be studied on a series of negatives of varying exposures.

Mr. Burckhalter has devised and tried at two eclipses (the India eclipse of 1898 and the Georgia eclipse of 1900) a simple plan which has worked very well. He uses an ordinary photographic telescope and plate, but in front of the plate he places a rapidly revolving shield or diaphragm, cut to such a shape that different portions of the corona have different exposures. At the Georgia eclipse, for example, one of his negatives was exposed for eight seconds, but it was, at the same time, screened from the light so that the equivalent exposure at the sun's edge was only 4–100 of a second; at 4' from the sun's edge, Os.32; at 8', 0s.80; at 12', 1s.38; at 16', 1s.76; at 24', 2s.40; at 34', 3s.20; at 44', 4s.00; at 64', 5s.60; at 94' and at all greater distances, 8s.00. The resulting negative is extremely fine, and it exhibits the corona as it has never before been seen on a single plate. The bright inner corona and prominences are shown in their true form and brilliancy alongside of the faint polar rays and the delicate masses of the outer coronal extensions. Those who are especially interested should consult Mr. Burckhalter's report (illustrated) in the Publications of the Astronomical Society of the Pacific, No. 75, for October, 1900. The advance over previous work of the same kind is so marked that it is to be hoped that this method will be adopted at the Sumatra eclipse of May, 1901.



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