XI. On the determination of the photometric intensity of the coronal light during the solar eclipse of August 28-29, 1886

Abstract

Although it has long been suspected that the amount of light emitted by the corona, as seen at various Solar Eclipses, may vary within 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 arrangement 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 the grease-spot was no longer visible. From a number of observations made during the period of totality of this eclipse, Mr. Waldo O. Ross, acting under Mr. Pickering’s direction, found that the standard candle had to be placed at distances varying from 14·4 to 21 inches from the paper before the visibility of the greased spot 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, 1878. Dr. Smith, observing at Virginia City, Montana, U. S., found from eight observations that the candle had to be placed at a distance of 51¼ inches from the screen before the minimum of visibility of the greased spot was obtained.