A direct method has been developed for measuring the reflection factors of substances at incandescent temperatures. Curves are given showing the variation in the reflection factor of tungsten for wave-lengths from 0.67{μ} to 4.0{μ} and for temperatures up to 2067^∘{}K.The reflection factor of tungsten was found to increase with temperature for wave-lengths less than about I.27{μ} and to decrease with temperature for wave-lengths longer than this. The transition from the approximately constant positive temperature coefficient in the visible to the approximately constant negative coefficient in the infra-red takes place in the rather narrow region lying between about 0.7{μ} and 2.0{μ}. The numerical values of the reflection factor agree fairly well with Worthton's values in the visible and with the theoretical equation first applied to long wave-lengths by Hagen and Rubens, beyond 2{μ} in the infra-red.
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Weniger et al. (1919) studied this question.