In the Introduction to our paper “On the Determination of the Photometric Intensity of the Coronal Light during the Solar Eclipse of August 28th, 1886,” which the Society did us the honour to print in the ‘Philosophical Transactions’ (A, 1889, p. 363), we gave an account of the attempts which had been made from time to time since the eclipse of December 22, 1870, the first occasion on which such measurements were made, to ascertain the amount of light emitted by the corona. So far as we know no other attempt of the kind has been made since the date of our last paper. We may therefore at once pass to the description of the methods adopted on the present occasion. The methods, as well as the instruments, used by us for the measurement of the coronal light during the eclipse of April 16th, 1893, were substantially the same as those employed in Grenada during the eclipse of August 28th, 1886, with certain modifications suggested by our experience on that occasion. For an account of the principle of these methods, as well as for the description of the instruments themselves, we may refer to the paper above cited. It will suffice here to say that one instrument was designed to measure the comparative brightness of the corona at different distances from the moon’s limb, whilst a second was arranged to measure the total brightness of the corona, excluding as far as possible the sky effect. The first instrument, from the mode in which it was constructed, will be called the equatorial photometer; the second will be termed the integrating photometer. In both cases the principle of photometry adopted was that of Bunsen, the intensity of the coronal light being compared with that of a glow-lamp, according to the method of Abney and Festing (‘Phil. Trans.,’ 1886, ‘ Proc. Boy. Soc.,’ 1887, 43). In the case of the equatorial photometer, a telescope by Simms, lent by the Astronomer-Royal, was employed. It had an aperture of 6 inches and the object-glass had a focal length of 78 inches, forming an image of the moon 0.76 inch in diameter. The image was received on a circular white screen contained in the photometer-box and placed in the focus of the object-glass. In the centre of the screen was traced a circle of the diameter of the image of the moon, and during the observation the moons disc was made to fall exactly within the circle.
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