before. I next smoked the surface of the walls well, and found that the amount of radiation coming from the aperture was then sensibly reduced. It is also possible that changes in the condition of the surface of the platinum strip may effect its emissivity, and in fact it is very doubtful whether it is possible to determine with any degree of accuracy what the emissivity of bright platinum is, relatively to lamp
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black. In the original memoir we took Rosetti's estimate of 35 per cent, as the most probable value for this quantity, but as our former estimate of the solar temperature depends greatly on this factor, to which so much uncertainty attaches, I thought it would be a distinct advance to abolish entirely the platinum strip as a source of radiation, and to substitute in its place a uniformly heated enclosure which would radiate as an absolutely " black body."
In 1895 Mr. Lanchester pointed out to me that such an enclosure would be a theoretically perfect radiator ; while Lummer, Paschen and others have shown that the law connecting temperature and
