The spectrum of the source is photographed through a neutral wedge placed immediately in front of a photographic plate. The curved boundary of the spectrum varies in height, the variation depending upon the distribution of energy in the source - i.e., upon its temperature. It is assumed that the source radiates as a black body or a grey body, but with certain limitations the method is applicable to bodies which are neither black or grey.
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I. Griffith (1925) studied this question.
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