[ SfiS ] 
XL On the Determination of the Photometric Intensity of the Coronal Light (hiring 
the Solar Eclipse of August 28-29, 1886. 
By Captain W. de W. Abney, C.B., R.E., F.R.S., and T. E. Thobpe, Ph.D., E.R.S. 
Eeceived February 7,—Read February 14, 1889. 
Introduetion. 
Although it has long been suspected tliat the amount of light emitted by the 
corona, as seen at various Solar Eclipses, may vary witliin comparatively wide limits, 
no attempts to measure its intensity appear to have been made prior to the Eclipse of 
December 22, 1870. On that occasion Professor Pickering employed an arrange¬ 
ment constructed on the principle of Bunsen’s photometer. It consisted of a box 
9 inches wide, 18 inches high, and 6 feet long, within which a standard candle could 
be moved backwards and forwards by means of a rod. One end of the box was 
covered with a piece of thin white paper, on which was a greased spot about half an 
inch in diameter. The box was adjusted so that the rays from the corona were 
normal to the plane of the paper, and the lighted candle was moved backwards and 
forwards within the box until tire grease-spot was no longer visible. From a number 
of observations made during the period of totality of this eclipse, Mr. Waldo 0. Ptoss, 
acting under Mr. Pickering’s direction, found that the standard candle had to bo 
placed at distances varying from 14 "4 to 21 inches from the paper liefore the visibility 
of the greased sjiot was reduced to a minimum. (‘ U.S. Coast Survey Reports,’ 1870, 
p. 172.) The observations were much interrupted by clouds, and are also probably 
affected by irregularities in the rate of the burning of the candle. The mean of all the 
readings was 18'5 inches : hence the light of the corona in 1870 was apparently equal 
to 0’42 of a standard candle at a distance of 1 foot. 
A precisely similar arrangement was used by Dr. J. C. Smith during the Solar 
Eclipse of July 29, 1 878. Dr. Smith, observing at Virginia City, Montana, U.S., 
found from eight observations that the candle had to be placed at a distance of 
51|- inches from the screen before the minimum of visibility of the greased spot was 
obtained. 
During the same eclipse. Professor John W. Langley made observations on the 
intensity of the coronal light as seen from the summit of Pike’s Peak, Colorado, by 
means of a photometric arrangement suggested by Professor S. P. Langley, and 
intended to measure the relative distribution of light in the corona. The idea was, 
first, to draw an outline of the corona; second, to measure the light of the corona at 
8a2 9.11.so. 
