Page:Proceedings of the Royal Society of London Vol 60.djvu/24

16 p. 363, with certain slight modifications suggested by their experience on that occasion. Two sets of observations were made: the first with a photometer equatorially mounted, and designed to measure the comparative brightness of the corona at different distances from the moon’s limb, and the second with an instrument arranged to measure the total brightness of the corona, excluding as far as possible the sky effect. In both cases the principle of photometry was that of Bunsen, the intensity of the coronal light being compared with that of a standard glow-lamp, according to the method of Abney and Festing.

The measurements with the equatorial photometer were made by Dr. Thorpe, assisted by Mr. P. L. Gray, B.Sc., those with the second or integrating instrument were made by Mr. Jas. Forbes, jun., assisted by Mr. Willoughby, of H.M.S. "Alecto."

The mean of ten concordant readings with the integrating photometer reduced to values of light intensity and expressed in Siemens’ units was 0·026.

The measurements with the equatorial photometer show that the visual brightness of the corona of the 1893 eclipse varied within comparatively wide limits, and that, at all events close to the moon’s limb, there was marked variation in local intensity. If the several values taken in the direction of the poles and equator are grouped as in the former paper (loc. cit.), they are found to afford a curve almost identical in character with that already given, showing that the diminution in intensity from the moon’s limb outwards is less rapid than accords with the law of inverse squares.

The results are as follows:—

These numbers would appear to show that the actual brightness of the corona was probably not very dissimilar at the two eclipses, the slight apparent diminution observed during the 1893 eclipse being,