Abstract

The Electrical Testing Laboratories of New York desiring to supplement the work in the photometry of eclipse phenomena which it had done through three expeditions at the time of the 1925 eclipse, designed apparatus and methods and sent an expedition to Lancaster, New Hampshire to observe the eclipse of August, 1932. In preparing for this expedition features connected with the photometry of eclipse phenomena were considered anew and are discussed. The use of the term “brightness” with reference to the solar corona is criticized as misleading and not in accordance with photometric usage. The quantity to be measured is the illumination produced by the corona. Discussion is made of erroneous and misleading values for corona illumination which have been published and which refer to illumination produced not by the corona alone but by the corona together with a large area of sky and which have no especial scientific significance. Specially designed photometers are described for measuring the coronal light, the horizontal illumination and sky brightness. These have filters to give a color match in the photometric field as closely as the color to be encountered can be predicted. They have easily manipulated neutral screens to cover the range of intensities. The coronal photometer has an optical system whereby the field included is accurately defined and a “homemade” heliostat to keep the sun in position. There is also a photoelectric photometer for corona measurement consisting of a vacuum photoelectric cell with colored filters correcting its response to the eye sensitivity curve. This acts through an amplifier with an FP-54 tube and is fitted with an optical system for delimiting the field. Special recording arrangements are described. Cloudiness at Lancaster prevented any measurements of the corona light to be obtained. It was found that the total illumination on a horizontal plane at the time of totality was 0.35 footcandle.

© 1933 Optical Society of America

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