16 



Determination of Coronal Light during Eclipse. 



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 Bctnsen, 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 photo- 

 meter 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, 



