1888.] Intensity of Coronal Light during Solar Eclipse. 



393 



the distance from the snn's limb. Probably the brightest part of the 

 corona was about 15 times brighter than the surface of the full 

 moon, or 37,000 times fainter than the surface of the sun. 



The instruments employed by the authors in the measurement of 

 the coronal light on the occasion of the solar eclipse of August 28-29, 

 1886, were three in number. The first was constructed to measure 

 the comparative brightness of the corona at different distances from 

 the moon's limb. The second was designed to measure the total 

 brightness of the corona, excluding as far as possible the sky effect. 

 The third was intended to measure the brightness of the sky in the 

 direction of the eclipsed sun. Tn all three methods the principle of 

 the Bunsen photometric method was adopted, and in each the com- 

 parison-light was a small glow-lamp previously standardised by a 

 method already described by one of the authors in conjunction with 

 General Festing. In the first two methods the photometer-screen was 

 fixed, the intensity of the comparison-light being adjusted by one of 

 Yarley's carbon resistances : in the third the glow-lamp was main- 

 tained at a constant brightness, the position of the screen being 

 adjusted along a graduated photometer bar, as in the ordinary 

 Bunsen method. Full details of the construction of the several 

 pieces of apparatus will be given in the full paper. 



The observations during the eclipse were made at Hog Island — 

 a small islet at the south end of Grenada, in lat. 12° 0' W. and long. 

 61° 43' 45" W., with the assistance of Captain Archer and Lieutenants 

 Douglas and Bairnsfather of H.M.S. " Fantome." The duration of 

 totality at the place of observation was about 230 seconds, but 

 measurements were possible only during 160 seconds, at the expira- 

 tion of which time the corona was clouded over. A careful discus- 

 sion of the three sets of measurements renders it almost certain that 

 the corona was partially obscured by haze during the last 100 seconds 

 that it was actually visible. Selecting the observations made during 

 the first minute, which are perfectly concordant, the authors obtain 

 six measurements of the photometric intensity of the coronal light at 

 varying distances from the sun's limb, from which they are able to 

 deduce a first approximation to the law which connects the intensity 

 of the light with the distance from the limb. 



The observations w r ith the integrating apparatus made inde- 

 pendently by Lieutenants Douglas and Bairnsfather, agree very 

 closely. It appears from their measurements that the total light of 

 the corona in the 1886 eclipse was — 



Douglas 0*0123 standard candle. 



Bairnsfather "0125 „ 



Mean 



at a distance of 1 foot. 



-0124 



