Other
Scientific paper
Jun 1871
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1871natur...4..160w&link_type=abstract
Nature, Volume 4, Issue 87, pp. 160 (1871).
Other
Scientific paper
IN his letter published in NATURE on the 1st of June, and to which Mr. Brothers courteously replies in your issue of the 15th, the writer briefly touched upon four different points bearing on the value of the eclipse photographs. Those points are :-1st, The possibility in a comparatively cloudless sky of a luminosity akin to that represented under the name of the corona on page 370 of the number of NATURE issued on March 9, but caused only by moisture in our atmosphere, as illustrated by his instance of what he termed a lunar halo; 2ndly, The presence of a luminosity on what he apprehends should have been the dark disc of the moon, as represented in the photograph of the American observers at Cadiz; 3rdly, The indifferent definition of the published photograph; and 4thly, The evidence as presented by the photograph of the identity of the coronal rifts. Referring to the first of these points, Mr. Brothers ``fails altogether to see the connection between the solar corona and a lunar halo.'' If the term halo, as applied to any appearance pertaining to the moon, is confined to the ring of light so frequently to be observed apparently surrounding the lunar disc, the writer would substitute the word ``luminosity'' for halo. The appearance he alluded to resembles Mr. Brothers's woodcuts of the corona already mentioned, more than anything else to which he can compare it, and in common phraseology may be described as a patch of light surrounding the apparent position of the moon, extending from it to a distance varying from about one degree to two or two and a-half, and having an irregular or rifted outline. The phenomenon in question was observed by the writer when the atmosphere was in such a condition that no trace of cloud whatever was visible for a distance round the moon of some thirty or forty degrees. He mentions this merely to show that even when no visible condensation of moisture is present, an appearance-attributable to nothing but atmospheric moisture, and analogous to what is termed the solar corona-is not to be regarded as out of the common, and nothing to be wondered at. Touching the second point, Mr. Brothers would seem to be of opinion that a solar corona may be seen even when the dark disc of the moon intervenes between it and the observer's eye; for he says of the luminosity in question that if caused by our atmosphere it would extend all round and all over that disc. The point at issue here is a very simple optical question, in the discussion of which space would be merely wasted, and in reference to which the writer would simply reiterate the opinion he has already expressed, that the luminous appearance as seen extending on to the disc of the moon in the Cadiz photograph, is (if it were visible outside the camera) attributable to nothing but the influence of the terrestrial, or of a lunar atmosphere. Whilst speaking of the American observer's picture, he would remark in answer to two observations by Mr. Brothers, first, that he is not in ``possession of exclusive information'' concerning the circumstances of its production; and secondly, that he does not assume that it was taken under conditions less favourable than those prevailing at Syracuse. He does, however, assume that that photograph either represents only the phenomenon to which the instrument used in its production was directed, or that it represents something in addition to that phenomenon. If its representation is confined to the phenomenon, then upon the grounds already shown, he considers that what is called the coronal light in the Cadiz picture not only may be, but most certainly is, in part at any rate, merely the result of atmospheric moisture. If, however, the American observers were unfortunate enough to represent in their photograph a luminosity not belonging to the eclipse at all, then he considers that what did belong to the eclipse is not distinguishable in their picture from what did not. In short, whether the Cadiz picture does or does not represent only what it should do, the writer is of opinion that any evidence it can afford respecting the identity of the coronal rifts must be other than satisfactorily conclusive.
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