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XIT. On the Determination of the Photometric Intensity of the Coronal Light during 
the Solar Eclipse of April 1 6th, 1893. 
By Captain W. de W. Abney, C.B., R.E., F.P.S., and T. E. Thorpe, LL.D., F.R.S. 
Received April 14,—Read April 30, 1896. 
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 E>ecember 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. Eor an account of the 
principle of these methods, as well as for the description of the instruments them¬ 
selves, 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 Testing (‘Phil. Trans.,’ 1886, ‘ Proc. Boy. Soc.,’ 1887, 43). In the 
case of the equatorial photometer, a telescope by Simms, lent by the Astronomer- 
Boyal, 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 O'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 
MDCCCXCVI.—A. 3 K 5.9.96. 
