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Dr. W. E. Wilson. 



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 



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 



