The present paper may be regarded as a continuation and elaboration of previous papers.
A description of the instruments and methods is given in these papers, and it is not proposed to repeat it here. It will be sufficient to state that the light of the night sky was analysed by appropriate colour filters into three spectral regious. The central one is designed to transmit the yellow-green light of the auroral line λ 5577 with as little else as may be. The others transmit red and blue regions of the spectrum to the exclusion of this line. The light transmitted is equated to an artificial self-luminous source, consisting of potassium uranyl sulphate, which is fluorescent under the radioactivity of the contained uranium. Neutral glasses have densities (log
10
opacity) of 0.1, 0.2, 0.3, . . . , etc.―from one glass to the next is an increase of one unit on the scale of brightness adopted.
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